21.Anesthesia technologies - Colleges

Anesthesia
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ANESTHESIA
PREPARED BY
Hatem Al-Rashdan
Hesham Al-salim
Abdullah Al-Monayee
SUPERVISED BY
Dr.Bassim Odah
FIRST EDITION
1425-2004
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1. INTRODUCTION
1.1 Meaning of Anesthesia:
The word anesthesia came from the Greeks and actually means "without feeling." So we
can define anesthesia as a state of insensibility to most external stimuli, such as pain.
Before the discovery and application of methods for. achieving general anesthesia,
surgeons were judged primarily by speed. The best could amputate a leg in less than 45
seconds, assisted by several strong men to restrain the patient.
The general anesthesia usually implies obliteration of consciousness, elimination of
recall, abolition of pain and paralysis of musculature (muscular relaxation).
1.2 History of anesthesia:
Events pertaining to Anesthesia and ventilation:
. Discovery of oxygen 1770s
. Nitrous oxide as "laughing gas" 1808
. Discovery of morphine 1800s
. Non-anesthetic use of ether & chloroform 1800s
. First general anesthesia (with ether) 1846
. Main blood groups: transfusion of blood 1900s
. Aseptic 1900s . First intravenous anesthesia 1932
. Neuromuscular blocking agents 1942-
. Manually controlled breathing 1940s
. Modern inhaled anesthetics 1956 –
1.3 Need for anesthesia
Surgical methods of treatment consists mainly of operations which are normally
carried out under some of anesthesia . Anesthesia serves the following two functions.
- It ensures that the patient does not feel pain and minimizes patient
discomfort ;and
- It provides the surgeon with favorable conditions for the work .
When anesthesia is given so that the patient loses consciousness, it is called general
anesthesia. In general anesthesia, the anesthetic agent is administrated to the body so that
it reaches the brain via the blood stream . The usual method is inhalation anesthesia in
which gaseous anesthetic agents are introduced via the lungs. Examples of such agents
are directly ether, chloroform, halothane, cyclopropane and nitrous oxide. During
anesthesia not only is the anesthetic administered in the required amount but also oxygen
. Any excess carbon dioxide is also eliminated. In the superficial stages of an anesthesia,
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the patient can breath for himself – spontaneous ventilation. At a greater depth of
anesthesia ,it may be necessary to support the patient artificial ventilation known as
controlled ventilation .
1.4 Major element of Anesthesia System
• The primary and secondary sources gases O2, air, N2O2 vacuum, as
scavenging , and possible Co2 and helium).
• The gas blending and vaporization systems.
• The breathing circuit ( including methods of manual and mechanical
ventilation .
1.5 Major Elements:
• The excess gas scavenging system that minimizes potential pollution of the
operating room by anesthetic gases.
• Instruments and equipment to monitor the function of the anesthesia delivery
system .
1.6 Major Gases:
• Most inhaled anesthesia agents are purchased as liquids and then vaporized in a
device within the anesthesia delivery system
• Some anesthetic agents are administered intravenously with the aid of various
types of infusion pumps or infused directly into compartment within the spine .
• Balanced general anesthetic : inhalation agent + intravenous analgesic drug.
• Primary sources O2, air, N2O2 and possible Co2 and helium they are supplied from
a hospital distribution system through gas columns , or wall outlets.
• Secondary sources , vacuum , gas scavenging. These are hung on the anesthesia
delivery system .
1.6.1 Oxygen :
• Prolong exposure to high concentration of O2 may result in toxic effects within
the lung that decrease diffusion of gas into and out of blood .
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• O2, is usually supplied to the hospital in liquid form and enters the hospital piping
system as a gas.
• The 2 nd source of O2 , within an anesthesia delivery system in one or more
cylinders filled with gaseous O2
1.6.2 Air :
• . Air ( 78%, N2,2% O2 . 0.9% Air, 0.1% other gases )
• The primary use of air during anesthesia is as a diluents to decrease the inspired
oxygen concentration.
• The primary source of medical are is special compressor . Dryers are employed to
rid the compressed air of water prior to distribution thought the hospital .
• Cylinder is a secondary source .
1.6.3 Nitrous Oxide
• Breathing more than 85% , may be fatal
• It is not anesthetic rather , it is analgesic
• It is used for enhancing the speed of induction and emergence from anesthesia,
and decreasing the concentration requirements of potent inhalation anesthetics .
• Primary supply is cylinders , secondary id from cylinders on the anesthesia
machine .
1.6.4 Carbon Dioxide :
• It is administrate to stimulate respiration that was depressed by anesthetic agents
and to cause increased blood flow in otherwise compromised vasculature during
some surgical procedures .
• Primary is cylinders and secondary is on the anesthesia machine
1.6.5 Helium :
• It used to enhance flow through small orifice , as asthma, airway , trauma, or
tracheal stenosis.
• It isles dens than other gases and thus , helps in the case of turbulent flow.
• It has a long specific heat relative to other anesthetic gases and therefore, can
carry the heat from laser surgery out of the airway more effectively than air ,
oxygen or N2O
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2. Delivery of anesthesia
The anesthesia delivery system consists of an anesthesia machine, a patient breathing
circuit , a ventilator and air way equipment .
The machine comprises a gas supply – delivery unit and an anesthetic vaporizer . The
breathing system consists of a closed loop of breathing tubing , containing two uni-
directional breathing valves and an Adjustable Pressure Limiting valve , a CO2 absorber ,
a means for venting excess gases ( scavenging ) humidifier , and collapsible reservoir
bag.
The airway management equipment includes the mask and end tracheal tube. Which
interface the patient with breathing circuit?
3. ANAESTHESIA MACHINE DESCRIBTION
An anesthesia machine is a device which is used to deliver precisely known but
variable gas mixture including anesthetic and lif- sustaining gases to the patient's
respiratory system . Generally, a variable concentration gas mixture of oxygen, nitrous
oxide and anesthetic vapor like ether or halothane is obtained from the machine and is
made to flow through the breathing circuit to the patient . It is composed of two
subsystems ( fig. 7 ).
- the gas supply – delivery unit , which consists of tubing and
flowmetres interconnected in parallel. And
- The anesthetic vaporizer, which is used to produce an anaesthetic
vapour from a volatile liquid.
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FIG 7
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3.1 GENERAL ANESTHESIA PRACTICE
Usually, there are four phases in al techniques of general anesthesia: induction,
maintenance, emergence and recovery. The patient will undergo a pre-operative
preparation procedure before being commenced to these phases.
During pre-operative preparation, the patient will be given a certain drug which is called
"premedication. "
The reasons for this pre-medication are:
1. To raise the pain threshold.
2. To reduce reflex central nervous activity (e.g. salivation, vagus nurve).
3. To sedate.
4. To counteract unwanted side effects caused by the anesthetic agent.
5. Facilitation of the induction of anesthesia.
6. Reduction of salivary and bronchial secretions.
An adequate premedication is an essential factor in all forms of anesthesia. In a well
premedicated patient, the dosage of anesthetic agent can be kept to a minimum and the
disadvantages of otherwise large amounts of agent can be avoided. In balanced general
anesthesia, the patient will be given an induction agent by intravenous short-acting
barbiturate. The patient is thus put to sleep quickly and pleasantly. In some certain cases
(pediatrics), the patient will be induced to anesthesia by inhaling a mixture of oxygen and
nitrous oxide by face mask and slowly inducing the inhalational agent in small
increments.
After the patient is properly positioned on the operating table, the operation may
commence. When the operation is concluded, the depth of anesthesia should be lightened
and the patient should be allowed to emerge. The phase of maintenance refers to the
period beginning with the onset of surgical anesthesia and ending with emergence. The
anesthesia is maintained by administering a mixture of oxygen and nitrous oxide
supplemented by a volatile inhalation agent, for example, halothane. Sometimes, a
neuroleptic agent (a drug which provides mental detachment from patient surroundings)
is given to the patient which, besides its sedative effect, also counteracts the nausea
which can accompany large doses of analgesics. How soon the maintenance phase is
reached varies greatly and depends, among other things, upon the patient's general
conditions and the premedication drugs received.. The agent's solubility in various tissues
are of importance as for example, it takes longer to anesthetise an obese patient because
the anesthetic agent is highly soluble in fatty tissue (taking longer to become saturated)
and for the same reason, recovery takes longer in the obese patient. The muscular
relaxation is of importance at this phase in order to allow the surgeon to operate freely,
but the patient's tubation, if required, should be carried out before giving muscle
relaxants.
Muscle relaxation in general anesthesia can be achieved by deeply anesthetising the
patient (by increasing the agent concentration), but it is much more common nowadays to
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give muscle relaxants and the risk of exessively deep anesthesia are then avoided. The
muscle relaxants block the normal transmission between the nerve ending and the muscle.
Because these muscle relaxants paralyze the respiratory muscles, the anesthetist must
give artificial respiration to the patient with the aid of a ventilator. Also, because of the
use of this muscle relaxant during general anesthesia, the patient is often intubated to
provide a safe airway and maintain adequate ventilation, while for shorter operations and
when muscle relaxants are not used, anesthetic gases are administered via a face mask.
At the end of the surgical anesthesia and as soon as neuromuscular function returns and
ventilation is adequate, the administration of nitrous oxide and volatile agent is
discontinued and the patient is allowed to emerge from the stage of surgical anesthesia. If
muscle relaxant has been given, it is important not to let the patient regain consciousness
before neuromuscular function returns as the experience of being awake but paralyzed is
extremely unpleasant. The patient will be allowed to breath pure oxygen at high flow for
at least 2 minutes before he is allowed to breath room air. This is to flush out the large
amount of nitrous oxide leaving pulmonary capillary blood during this phase into the
alveolar and prevent diffusion hypoxia. The patient then is to be admitted to a post
anesthesia recovery area for continuing observation and care.
3.2Anesthesia Drugs:
» Pre medication
. 30-60 minutes before starting anesthesia
. To remove fear and anxiety
. Examples:
. Benzodiazepines, Barbiturates and Opioids
» Pre Induction drugs
. Vagolytics are administered prior to induction to
. Protect the heart from of anesthetic agents' depressant effects but may cause
unwanted changes in heart rhythm
. Reduce airway and gastric secretions and sweating
. Includes:
. Atropine
. Glycopyrrolate
» Induction
. To put patient to sleep
. Done using:
. Inhalational anesthetics with Children or
. intravenous anesthetics with adults (thiopental and Propofol)
. Intravenous analgesics (for pain removal) and NMBAs begins simultaneously with
anesthetics
. Inhalational anesthetic agents are: Halothane, Enflurane, Isoflurane,
Sevoflurane and Desflurane together with N2
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4.Purpose of anesthesia units:
Anesthesia units dispense a mixture of gases and vapors and vary the proportions to
control a patient's level of consciousness and/or analgesia during surgical procedures.
Basically, anesthesia units perform the following four functions:
. Blend gas mixtures that can include (besides 02) an anesthetic vapor, nitrous oxide
(N20), other medical gases, and air
. Facilitate spontaneous, controlled, or assisted ventilation with these gas mixtures
. Reduce, if not eliminate, anesthesia-related risks to the patient and clinical staff
The patient is anesthetized by inspiring a mixture of 02, the vapor of a volatile liquid
halogenated hydrocarbon anesthetic, and, if necessary, N20 and other gases. Because
normal breathing is routinely depressed by anesthetic agents and by muscle relaxants
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administered in conjunction with them, respiratory assistance - either with an automatic
ventilator or by manual compression of the reservoir bag - is usually necessary to deliver
the breathing gas to the patient.
5.Principles of operation :
An anesthesia system comprises four basic subsystems: a gas supply and control circuit, a
breathing and ventilation circuit, a scavenging system, and a set of system function and
breathing circuit monitors (e.g., inspired 02 concentration, breathing circuit integrity).
Also included in some anesthesia systems are a number of monitors and alarms that
indicate levels and variations of several physiologic variables and parameters associated
with cardiopulmonary function and/or gas and agent concentrations in breathed-gas
mixtures. Manufacturers typically offer a minimum combination of monitors, alarms, and
other features that customers must purchase to meet standards and ensure patient safety.
To meet the minimum standard of care in the United States, anesthesia machines must
monitor 02 concentration, airway pressure, and either the volume of expired gas (V exp)
or the concentration of expired CO2. Stand-alone monitors may be used to track other
essential variables such as electrocardiogram, temperature, and blood pressure.
5.1-Gas supply and Control :
Because 02 and N20 are used in large quantities, they are usually drawn from the
hospital's central gas supplies.
valve, and a regulator that lowers the pressure to approximately 45 pounds per square
inch (psi). There is no need for a separate regulator when the central gas supply is used
because the pressure is already at about 50 psi.
Most anesthesia machines have an 02 supply failure device and alarm that protect the
patient from inadequate 02 supply. If the 02 supply pressure drops below about 25 to 30
psi, the unit decreases or shuts off the flow of the other gases and activates an alarm.
The flow of each gas in a continuous-flow unit is controlled by a valve and indicated by a
flowmeter. The flowmeter can be a purely mechanical arrangement, with a flow tube in
which a bobbin moves up and down depending on the flow, or it can be an electronic
sensor with an LCD (liquid crystal display). After the gases pass through the control
valve and flowmeter, enter the low-pressure system, and, if required, pass through a
vaporizer, they are administered to the patient. On machines sold in the United States, the
N20 and 02 flow controls are interlocked so that the proportion of 02 to N20 can never
fall below a minimum value (nominal 0.25) to produce a hypoxic breathing mixture. An
02 monitor that is located on the inspiratory side of the breathing circuit analyzes gas
sampled from the Y-piece of the patient's breathing circuit and displays 02 concentration
in volume percent. 02 monitors should sound an alarm if the 02 concentration falls below
the preset limitIf the flow of anesthetic gases to the patient must be stopped for any
reason, an 02 flush valve can be activated to provide a large flow of central-source 02 to
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purge the breathing circuit of anesthetic vapors. The 02-flush flow bypasses the
flowmeters and vaporizers.
In some units, the anesthetic gas flow momentarily shuts off.
5.2-Vaporizers :
Because the inhaled anesthetic agents, with the exception ofN20, exist as liquids at room
temperature and sea-level ambient pressure, they must be evaporated by a vaporizer.
Vaporizers add a controlled amount of anesthetic vapor to the gas mixture. Some
anesthesia units can accommodate up to three vaporizers. Most units have a lockout
mechanism that prevents the use of more than one vaporizer at a time.
There are several types of vaporizers, including variable bypass (conventional), heated
blender, measured flow, and draw-over. Variable bypass vaporizers can be either
mechanically or electronically controlled.
Variable bypass and heated blender vaporizers are concentration calibrated and thus can
deliver a preselected concentration of vapor under varying conditions.
In a variable bypass vaporizer, such as one used for enflurane, isoflurane, halothane, or
sevoflurane, a shunt valve divides the gas mixture entering the vaporizer into two
streams; the larger stream passes directly to the outlet of the vaporizer, while the smaller
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stream is diverted through an internal chamber in which vapor fills the space over the
relatively volatile liquid anesthetic. The vapor mixes with the gas ofthe smaller stream,
which then rejoins the larger stream as it exits the vaporizer. In a mechanically controlled
variable bypass vaporizer, a bimetallic thermal sensor that regulates the shunt valve to
divert more or less gas through the chamber compensates for temperature changes that
affect the equilibrium vapor pressure above the liquid. Each variable bypass vaporizer is
specifically designed and calibrated for a particular liquid anesthetic.
The heated blender vaporizer was introduced for use with the anesthetic agent desflurane.
In this type of vaporizer, desflurane is heated in a sump chamber. A stream of vapor
under pressure flows out of the sump and blends with the background gas stream flowing
through the vaporizer. Desflurane concentration is controlled by an adjustable, feedbackcontrolled
metering valve in the vapor stream.
Measured-flow vaporizers (also known as copper kettle or flowmeter-controlled) are not
concentration.
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A few anesthesia units now have a liquid-injector type of vaporizer. This vaporizer is
electronically controlled and injects the liquid anesthetic agent directly into the stream of
gases.
5.3 Ventilation :
Manual ventilation, which requires that an operator manually squeeze the reservoir bag
for each patient breath, can be tiring during long procedures and can compete with other
tasks; therefore, an automatic ventilator is often used to mechanically deliver breaths to
the patient. These ventilators, which have a minimal number of control settings and are
usually electronically controlled and pneumatically powered, use a bellows in place of the
manually compressed reservoir bag. The ventilator forces the anesthesia gas mixture into
the patient's breathing circuit and lungs and, in a circle breathing system, receive
exhaled breath from the patient as well as fresh gas. The anesthetist can vary the volume
of a single breath (tidal volume) and the ventilation rate, either directly by setting them
on the ventilator or indirectly by adjusting parameters such as the duration of inspiration,
the inspiratory flow, and the ratio of inspiratory to expiratory time. The ventilatory
pattern is adjusted to the varying needs of the patient. For patients with special respiratory
support needs, a more sophisticated ventilator with capabilities similar to those used in
critical care applications may be required.
Minute ventilation, the total volume inspired or expired during one minute, can be
evaluated as the product of the expired tidal volume and the ventilation rate. It requires
careful monitoring, not only because it is physiologically important to the patient, but
also because it can indicate malfunctions of the ventilation delivery system (e.g., leaks in
the breathing circuit).
The expired tidal volume can be measured with a flowmeter, with a spirometer, or with a
sensor placed in the expiratory circuit. Some anesthesia ventilators can also limit the peak
inspiratory pressure, slow the rate of exhalation, provide ventilation only when the patient
is not making inspiratory efforts, and maintain a positive airway pressure during the
expiratory phase of the breath (positive end-expiratory pressure [PEEPD]).
5.4-Breathing circuits
Most anesthesia systems are continuous-flow systems, which provide a continuous supply
of 02 and anesthetic gases. There are two basic types of breathing circuits used in these
systems: the circle system and the T-piece system, each of which can assume various
configurations. (A common configuration of the T-piece system is the Bain modification
of the Mapleson D system.) A higher proportion of anesthetic gases is rebreathed in the
circle system, which uses check valves to force gas to flow in a loop and returns expired
gases (minus the CO2), plus fresh gas, to the patient. In the T -piece circuit, most of the
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exhaled gas is vented out of the system, and the portion rebreathed depends on the freshgas
flow rate.
In the circle system, fresh gas from the anesthesia machine enters the inspiratory limb of
the breathing circuit and mixes with gas in the system before the resulting mixture flows
through a one-way valve to the patient. Expired gas flows from the patient through a
second (expiratory) limb of the circuit, passing another one-way valve, into either a
reservoir bag or a ventilator bellows. When positive pressure is generated in the system,
either by a manual squeeze of the reservoir bag or by compression of the bellows by a
mechanical ventilator, collected gas that does not escape via an adjustable pressurelimiting
(APL) valve to the scavenging system is driven through a CO2 absorption
Figure 2-4: continous flow anesthesia system
canister or LIOH or Amsorb and back to the patient. The canister contains either soda
lime or barium hydroxide lime that removes CO2 from the rebreathed gases. In circle
breathing systems, a fresh-gas flow of 1 L/min or less is typically considered low-flow
anesthesia (4 to 10 L/min is typically considered the usual fresh-gas flow rate). A freshgas
flow of 0.5 L/min is generally considered minimal-flow anesthesia. In situations in
which the cost of anesthetic agents is high, low-flow anesthesia may be the preferred
option.
Machines with a T-piece design have corrugated tubing in which fresh gas and some
expired gas mix before entering the patient at each inhalation. Partial rebreathing is
controlled by the supply rate of fresh gas, and the exhaled anesthetic mixture leaves the
circuit through an APL valve. Elimination of rebreathed CO2 depends on fresh-gas flow
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and occurs in direct proportion to that flow. This system, although adaptable to a variety
of anesthetic procedures, is used most often in pediatric anesthesia.
Circle systems offer advantages over T-piece systems in that they conserve a greater
proportion of the anesthetic gases and conserve body heat and moisture from the patient.
The advantages of T-piece systems include a lower circuit compliance, easier circuit
sterilization, and a less complex design requiring fewer valves and no CO2 absorber
(although one can be used with it).
Because excess pressure imposed on the patient's lungs can cause serious lung damage,
either an APL valve or a valve in the ventilator allows excess gas to escape when a preset
pressure is exceeded. There are two types of APL valves: spring-loaded and needle
valves. The spring tension in spring-loaded APL valves can be adjusted to control the
pressure at which the valve will open. At lower pressures, the valve is closed.
The pressure in the breathing system maintained by the needle valve depends on the flow
through the valve.
Therefore, when the valve is not fully closed, gas will always leak from the system. The
minimum exhaust pressure required to refill a ventilator bellows is usually 1 to 2 cm
H2O; for maximum pressure, both types of valve are fully closed. Because many APL
valves do not have calibrated markings, the anesthetist must adjust them empirically to
give a desired peak inspired pressure. Circle systems and T-piece systems also include a
pressure gauge for monitoring circuit pressure and setting the APL valve. An
electronically controlled, settable, and calibrated APL valve is available on some
anesthesia machines.
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Figure 2-5 breathing circuits
5.5-Scavenging system :
A scavenging system captures and exhausts waste gases to minimize the exposure of the
operating room staff to harmful anesthetic agents. Scavenging systems remove gas by a
vacuum, a passive exhaust system, or both. Vacuum scavengers use the suction from an
operating room vacuum wall outlet or a dedicated vacuum system. To prevent positive or
negative pressure in the vacuum system from affecting the pressure in the patient circuit,
manifold-type vacuum scavengers use one or more positive or negative pressure-relief
valves in an interface with the anesthesia system. In contrast, open-type vacuum
scavengers have vacuum ports that are open to the atmosphere through some type of
reservoir; such units do not require valves for pressure relief.
Passive-exhaust scavengers can vent into a hospital ventilation system (if the system is
the nonrecirculating type) or, preferably, into a dedicated exhaust system. The slight
pressure of the waste-gas discharge from the anesthesia machine forces gas through
largebore tubing and into the disposal system or directly into the atmosphere.
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5.6 Monitors and alarms :
Anesthesia systems incorporate a set of equipmentrelated monitors, including those for
airway pressure, expiratory volume, and inspired 02 concentration.
They can also include exhaled anesthetic agent monitors, such as those for CO2
concentration, N20 concentration, and agent concentration, or physiologic monitors such
as those for blood 02 saturation by pulse oximetry, electrocardiogram, invasive and
noninvasive blood pressure, and temperature.
Anesthesia systems are typically configured with respect to their monitors in one of two
ways: as modular systems or as preconfigured systems. In the modular approach, an
anesthesia machine with a basic set of equipment monitors (usually airway pressure,
inspired 02 concentration, and expired volume) is used as a physical platform for the
system. Additional physiologic monitors, individually or in a monitoring system (with its
own display and alarms), along with other devices as needed, are obtained separately and
added to the system. The preconfigured approach involves a more completely integrated,
manufacturer-assembled system that already includes all physiologic and equipment
monitors and displays in a turnkey unit.
Some units may have methods of integrating, analyzing, displaying, and recording the
information generated by the monitors' sensors and alarms.
Microprocessors have been incorporated into the systems to implement these functions.
Stand-alone microprocessor-controlled data collection and display units have been used
to integrate modular anesthesia systems.
These units can also be used as part of an anesthesia information management system
(AIMS).
Integration of the information and alarms from each of the monitors into a single display
has become very important. An integrated display gives the anesthetist a single point of
reference for a wide variety of equipment and physiologic information. Anesthesia
machines that lack integrated alarms can sometimes cause confusion among anesthetists
and operating room teams by sounding numerous alarms simultaneously. In an integrated
system of information and alarms, visual alarm messages appear on a central display;
furthermore, audible and visual alarms are prioritized so that the more urgent alarm
sounds and visual signals are associated with the more vital monitored variables.
An anesthesia workstation is designed to centralize system control and to integrate the
display of information. This involves continuous acquisition, recording, and presentation
on a central display of selected monitored physiologic and equipment variables (in real
time or using historical trends) along with limit settings and the status of all alarms, plus
explanatory messages.
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Several models exist to predict the level of wakefulness in anesthetized patients, such as
the Ramsay Scale and the Modified Observer's Assessment of Alertness/Sedation Scale.
However, in lieu of a direct method of monitoring brain activity during surgery, users
may rely on indirect means of assessing consciousness, such as blood pressure and vital
signs.
According to proponents, one indirect method, the Bispectral Index (BIS), or
Physiometrix's Patient State Index, measures the effectiveness of painkilling agents while
ignoring the sedative and paralytic elements that constitute a significant portion of
anesthetic agents. Some anesthesia units may incorporate this technology as an additional
tool to monitor the patient.
BIS monitors use a metered scale (0 to 100) to indicate the degree of patient wakefulness
based on collected and processed data. A digital meter indicates the number on the scale
that corresponds to the patient's degree of wakefulness, with a higher number
representing a higher degree of consciousness and awareness of sensation despite the
presence of anesthetic agents.
5.7 Monitoring Function
• Delivery of hypoxic gas mixture to the patient . This can be detected using
oxygen analyzer in the breathing circuit.
• Inability to adequately ventilate the lungs by not producing positive pressure in
the patient’s lung inadequate volume of gas or improper breathing circuit
connections . This can be monitored by pressure in the breathing circuit , the
volume exhaled from the patient’s lung , and the amount of exhaled carbon
dioxide.
• Delivery of an overdose for an inhalational anesthetic agent which could be
detected by agent – specific gas analyzer.
• Devices can monitor these events are : Pressure monitor, oxygen monitor ,
volume monitor CO2 monitor , and spectrometers.
Never monitor the anesthesia delivery system performance through the patient’s
physiological response .
Breathing Circuits :
• It is used to avoid and a adequate volume of a controlled concentration of fresh
gas to the patient during inspiration and to carry the exhaled gases away from the
patient during exhalation .
• Open circuit , no re breathing of any gases and to CO2 absorber or valves.
• Closed circuit : indicating the presence of CO2 absorber an some re breathing of
other gases.
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6. Mechanical Ventilation
• Volume ventilation; the volume of gas delivered to the patient remains constant
regardless of pressure that is required .
• It is the most popular , since volume delivered remains constant despite changes
in lung compliance .
• Pressure ventilation; the ventilator provides whatever volume to the patient that is
required to produce some desired pressure in the breathing circuit.
• It is useful when compliance losses in the breathing circuit are high relative to the
volume delivered to the lung.
• Humidification reduces heat loss and maintains the integrity of the cilia that line
the airway and promote the removal of mucus and particulate matter from the
lungs .
• Humidification of dry breathing gases can be accomplished simple passive heat
and moisture exchangers inserted into the breathing circuit at the level of the
endotracheal tube connectors.
• Humidification can also be accomplished by electronic humidifiers that heat a
reservoir filled with water and also heat a wire in the gas delivery tube to prevent
rain – out of the water before it reaches the patient
• Electronic safety measures must be included in these active devices due to the
potential for burning the patient and the fire hazard .
7.Gas Scavenging Systems
• It is used to reduce or eliminate the potential hazard to employees who work in
the environment.
• Usually , more gas is administered to the breathing circuit than is required by the
patient , resulting in the need to remove excess gas from the circuit .
• The system must be able to collect gas from all components of the breathing
circuit valves, ventilators , gas monitors , etc. without affecting the pressure and
gas flow to the patient.
• Open interface a simple design that require a large physical space for the reservoir
volume.
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• Closed interface , with an expandable reservoir bag, which must include relief
valves for handling the case of no scavenging flow and great excess of scavenging
flow
• Trace gas analysis must be performed.
8. Gas Flow Through Two – Gas Anesthesia Machine
Fig. 1 illustrates gas flow and mixing as well as functional components encountered
in a typical two – gas anesthesia machine . On examining the gas flow through the
machine one must realized that oxygen and nitrous oxide may be supplied from either
the wall supply or from nitrous oxide may be supplied from either the wall supply or
from cylinders through the hanger yoke assembly . Thereby providing the assurance of
always having a full cylinder available . When suing the wall supply as the source of gas .
one must be certain to remember to keep to all cylinders closed . it the cylinder pressure
regulator is to reduce the pressure to something higher than the wall supply , then the
check valve in the wall supply pipeline inlet will close . It this occurs the cylinders will
be the only source of gas and they may become depleted without the user’s knowledge .
9.Power outlet to ventilator
This pathway provides a mean of supplying pneumatic power to a ventilator . There is
a valve that remains closed unless the proper connector is attached thereby depressing the
valve off its seat and allowing oxygen to flow into the pneumatics of the ventilator . One
must be sure to understand that this oxygen never enters the ventilator bellows or patient
circuit , it only provides a patient circuit , it only provides a pneumatic source to drive the
ventilator. The oxygen flow required by the pneumatics of the ventilator may be
considerable ; therefore , when using the cylinders as the oxygen source , the cylinders
may become depleted sooner than expected.
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FIG 1
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Anesthesia
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10.Oxygen Supply Failure Alarm and the pressure Sensor Shutoff Valve
These devices are both safety components designed to warn the used of a drop in
oxygen pressure and to protect the patient from a hypoxic mixture of gases when oxygen
pressure is lost . The oxygen supply failure alarm is a pressure sensitive device designed
to produce a loud audible alarm whenever the oxygen pressure drops below 5% of
normal . When the machine is in use and the oxygen pressure is lost , the alarm should
sound for at least 7 seconds , thus providing time for the user to either turn on the backup
cylinder or change to a full one . In Fig. 1 this device is located upstream of the second
stage regulator and therefore alarms when the oxygen pressure reaches 20 to 30 psi .
Once oxygen pressure has been elevated above the alarm setting , the nose should cease .
The user must realize that this alarm is not activated when pressure of any gas other than
oxygen is lost ; therefore the oxygen concentration in the fresh gas flow may change
without warning to the anesthetist .
The pressure sensor shutoff valve is also referred to by a variety of other name ; Fail
Safe , Oxygen Failure Safety Valve , and Oxygen supply pressure . Failure Device (
fig.2). This device , as the name implies , is sensitive to oxygen pressure within the
anesthesia machine and is required by the ANSI machine standard . The function of this
valve is to stop the flow of all gases other than oxygen whenever the oxygen supply
pressure drops below 50% of normal . Usually the oxygen failure alarm is set slightly
above the pressure sensor shutoff valve . thus the user is alerted before the drop in gas
flow actually occurs . The user should understand that these safety devices are limited in
their ability to protect the patient from a hypoxic mixture of gas , therefore , an oxygen
analyzer should always be used in the patient circuit .
The gas then travels through pressure reducing valves to the flow control knobs where
flow is controlled through the flow meters . The gases mix with each other after
ascending the flow meters and pass through the calibrated vaporizer \, where anesthetic
agent is added to the mixture . The gas mixture then exits the machine via the machine
outlet to enter the patient circuit
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11.THE FUNCTION OF THE MAJOR COMPONENTS
11.1Wall versus Cylinder Supply of Gases :
Gas to the machine may be supplied from either the wall supply ( the built – in gas
system within the hospital ) or the cylinders attached to the hanger yoke assembly .Wall
supply pressure is typically 50 psi , a full cylinder of oxygen is 2.200 psi and nitrous
oxide is 750 psi . Therefore , distal to the hanger, yoke assembly is a cylinder pressure
regulator that reduces the pressure from the cylinder to that of the wall source ( 40 to 60
psi ) . The double yoke assembly ( Fig .1 ) allows two cylinder of the same gas to be
mounted to the anesthesia machine. Usually the cylinders are used only as a backup to the
wall supply or if the machine is used when there is no piped - in gas . In this double
yoke system the gauge will indicate the pressure in the fullest tank when both cylinders
are turned on . Therefore , each tank must be tested separately to be certain that each is
full . there is a check valve inside each yoke assembly which eliminates the possibility of
the cylinder of higher pressure emptying into the lower one . There is also a check valve
at the wall supply pipeline inlet which blocks gas flow from the machine into the wall
supply pipeline inlet which blocks gas flow from the machine into the wall supply when
cylinders are being used a a gas source . When a cylinder position is unoccupied a block
supplied by the manufacturer should be in place to eliminate the possible leakage of gas .
It is recommended that when using cylinders as the gas supply , only one cylinder be
turned on at a time in a double – yoke assembly , thereby providing the The Oxygen
Flush Valve
When this valve is depressed , oxygen flows at a high rate ( 35 to 75 Ipm ) directly
into the common gas line near the machine outlet ( fig.1) . it used to rapidly fill the
patient circuit . This flush valve is most often used when the anesthetist is manually
ventilating the patient, and the bag is not inflating as fast as needed ( usually due to
leakage around the mask ) .
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Anesthesia
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Fig 2
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Anesthesia
==============================================================
Since this valve shuts only 100% oxygen into the system, one must realize that if used
excessively during an inhalation induction, the process may be delayed due to the dilution
of anesthetic agent . If the valve malfunctions and remains in the open position , or the
operator depresses it for an extended period of time , the possibility exists of over-
inflating the patient's lungs.
Second stage pressure Regulator
This pressure regulator is used in the system to assure that a constant pressure of
gas is delivered to the flow meters ( fig.1) if this valve were not in place , every time a
fluctuation in delivery pressure occurred , the flow through the flow meters would
change . This regulator steps down the pressure from 40 to 0 psi to -16 psi . In the case of
nitrous oxide there may or may not be a second stage regulator present . If there is not ,
then the pressure directed to the flow control valve at the flow meters is -∼50 psi
Flowmeters
The pressure of oxygen at this point is -∼16 psi and that of nitrous oxide is -∼50
psi . Each individual glass flowmeter is ground and individually calibrated with its float .
For this reason , floats and tubes are not interchangeable . The flowmeters are tapered in
the grinding process and the diameter at the upper end is slightly larger than tat the lower
end ( fig. 3) . With this design , the amount of gas flow around the float increases as the
float is raised in the flowmeter . The flow control knob is attached to a needle valve .
Turning the knob clockwise increase the flow and counterclockwise decrease it . The
most recent anesthesia machines are designed to provide, whenever the machines are "
ON" . a minimum oxygen flow of approximately 300 ml/min , even when the control
knob is in the extreme counterclockwise position . The safety feature is designed to
provide a flow of oxygen equal to the rate of minimum oxygen consumption the ANSI
machine standard require that the oxygen flowmeter be on the extreme right of the bank
of flowmeters . The flow control knobs are color coded for easy identification; for
example , green for oxygen and blue for nitrous , oxide . In addition , the oxygen flow
control knob has a unique " fluted . which is different from the others. This distinctly –
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Anesthesia
==============================================================
shaped knob is designed to help the anesthetist positively identify the oxygen control by
touch .
The accuracy of flowmeters has been examined by servral investigators . The
result indicated that flowmeters mayb be inaccurate at settings between0 to 14 L/min.
Therefore , an oxygen analyzer should always be used to avoid delivering a hypoxic
mixture . The flow meters on Drager's Narkomed 2 A are certified to be accurate within
+ 3% of full scale , at 20oc and 760 mm hg barometric pressure. Similarly standards of
calibration are performed by all manufacturers . Nonetheless. inaccuracies have been
found in some flowmeters and therefore , an oxygen analyzer should be used in order to
avoid delivering a hypoxic mixture , particularly at low flows.
Recent anesthesia machines features low and high range oxygen flowmeters
which connect in series and are controlled by a single knob ( Fig. 4). The low flowmeter
is calibrated from 100 to 1,000 ml/min, and the high flowmeter is calibrated from 1 to 12
or 5 L/min . An old design of tandem flowmeters had separate knobs , for low and high
flow control, but ,for obvious safety reasons , the one- knob design is more desirable.
Newer machine designs also incorporated devices which alarm or will not allow hypoxic
mixtures of oxygen and nitrous oxide .
11.4 Vaporizer
Calibrated vaporizer(s) are located downstream from the flowmeters ( fig.1).
After the flowmeters, oxygen and nitrous oxide become thoroughly mixed as they flow
through the common gas line approaching the vaporizer. The fresh gas enters the
vaporizer through the fresh gas inlet, and soon thereafter a bypass mechanism directs a
portion of the stream into the vaporizing chamber where it becomes saturated with
anesthetic vapor ( fig.5) . The bypass mechanism is functional over a wide range of
flows; typically newer vaporizers demonstrate linear output with flows ranging form 5 to
15 liters . There also exists a temperature compensating device which functions to
increase the flow into the vaporizing chamber at low temperatures and to decrease it at
higher ones . The fresh – gas that was diverted around the vaporizing chamber mixes with
the saturated gas in the mixing chamber and travels out through the fresh – gas outlet.
Fig. Technological advances in agent – specific vaporizers have made them reliable ,
accurate , and easy to use . Vaporizers on newer anesthesia machines are connected in
series via a specially designed manifold known as and exclusion system . This makes it
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Anesthesia
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possible to have only one vaporizer on at a time . However, there are many older
machines in use today that do not have this exclusion system in place; therefore, the
possibility exists of administering more than one volatile anesthetic to the patient.
Fig.4
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12.1 Factors Affecting Vaporizer Output
The use of free– standing vaporizers brings into consideration several common
problems . It is possible in some instances to reverse the fresh gas connections, thereby
reversing the flow of gas through the vaporizer; an arrangement that has been reported to
increase the output drastically . This free- standing arrangement also increase the possibly
of tipping the vaporizer. When a vaporizers tipped beyond 54 o , the possibly exist of
liquid agent spilling into areas other than the vaporizing chamber . With the volatile
anesthetic agent in other areas of the vaporizer, the output may become unpredictably
altered . Drager recommends that when their vaporizer is tipped beyond the allowable
limits , it should be flushed out by running 10L/min of gas with the concentration dial on
the highest setting for a period of up to 30minutes . The Cyprane Tec 4 vaporizer is
designed with a baffling system that allows the vaporizer to be tipped 180o . It is
recommended , however , that if a vaporizer has been tipped , it's output should be
verified by an agent analyzer , or flushed out with high flows , before it is put back in use
.
Increased vaporizer output can occur with pressure fluctuation in the patient circuit .
This phenomenon is referred to as the " pumping effect . " The pressure fluctuations are
reflected back into the vaporizing chamber of the vaporizer and may cause a significant
increase in the anesthetic concentration delivered . The effect , however, has been greatly
diminished with the more recent deigns of vaporizers and machines. Newer designs
incorporate unidirectional check valves and internal modifications in the vaporizer that
eliminate the pressure fluctuations from reaching the vaporizing chamber.
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Fig. 5
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Anesthesia
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12.2 Vaporizer filling Mechanism
To eliminate the possibility of filing a vaporizer with the wrong liquid agent, an agent
– specific keyed filling developed (Figs . 6A.B) This device is color coded to match the
corresponding anesthetic agent, and also has slots which align with the notches on the
agent and also has slots which align with the notches on the collar of the agent bottle .
This method of filling and emptying is slower than pouring directly from the bottle into
the vaporizer filler port . These safety devices have lost their popularity with many
anesthetists because, in addition to being slower, they may also have a tendency to leak .
It is impotent to always place the vaporizer concentration kel in the “ off “ position
before filling it or the liquid agent will spurt out when the filling port is opened .
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Fig 6
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Anesthesia
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12.2.1- Gas Supply System
Gases are provided to the anaesthesia machine from either a pressurized hospital
central supply or small storage cylinders attached to the machine .
Centralized supply : Centralized supply systems consist of bulk or cylinder storage
for main and reserve supply , control equipment including valves and pressure regulators,
a distribution pipeline , and numerous supply outlets. The system is so designed and
operated that the necessary supply of gases is always a available is always available .
The gas supplied by the hospital is regulated and maintained at the wall outlet. Gases are
supplied to the anaesthesia machine inlet from the central system via a flexible hose
connected to the operating room wall outlet .In order to prevent interchanging the gas
supply wall outlet with the incorrect anaesthesia machine inlet , for example , nitrous
oxide for oxygen , non interchangeable connectors are used at each end of the hose . The
two types of non interchangeable connections most commonly used are the diameter
index Safety System ( DISS) and non – interchangeable quick couplers . Each type of
connecton incorporates a male and female end that is specially designed for each type of
gas. In addition to the connective design , colour – coded hoses for each specific gas are
utilized .
12.2.2- Gas Cylinders :
A second gas supply source is the cylinders located in yokes attached to the
anaesthesia machine . This supply can be utilized as either he main source when a central
gas supply dies not exist , or a reserve when central gas supply is available .
Yoke : Each anaesthesia machine has at least one yoke for an oxygen cylinder but
most are provided with two . In addition to oxygen, most machine designs include a
nitrous oxide yoke . In order to prevent incorrect placement of a tank into the wring yoke
, two pins located in the yoke must fit into corresponding holes drilled into the tank neck
. The placement of these pins and corresponding holes is unique for each gas . This
identification system , which is referred to as the Tin Index Safety system , has been
standardized to prevent the accidental fitting of a wrong cylinder to the yoke .
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12.2.3- Pressure Gauge :
Pressure gauge are attached to the cylinders to indicate the contents to the gases in the
cylinders . For oxygen , the operating range of the gauge is 0 to 150 kg/cm 2 . Whenever
the new oxygen cylinder is hooked up and taken in line , the indicator should be above
this mark . With the gradual usage of the gas, the reading would drop gradually, when
the indicator show that the pressure has fallen below the minimum level of acceptance ,
the cylinder should be refilled . If for any reason , the pressure gauge shows a reading
above 150kg/cm2 during use, the cylinder should be disconnected immediately and
replaced.
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12.2.4- Pressure Regulator :
Machine pressure regulators reduce cylinder gas pressure to 275 kpa (40 psi ) before
the gas flows through the machine . The regulators has one high – pressure inlet, one high
– cylinder through anon – return valve . the no – return valve prevents the flow into an
empty cylinder or back into the central piping system and also enables it's removal and
replacement when the reserve cylinder is turned on without interrupting the supply of gas
.
12.2.5- Fail Safe System :
From the supply , the gas flows into the inlet the inlet of the anaesthesia machine and
is directed through the pressure safety system towards the flow delivery unit. The
pressure safety system will not allow nitrous oxide to flow unless an oxygen supply
pressure exists in the machine . The fail – safe system consists of a master pressure
regulator valve located in the oxygen supply line. From the master regulator , a reference
pressure is provided to the salve regulator valve controlling the pressure and flow of the
nitrous oxide line . When sufficient oxygen pressure of 275 kpa is present in the master
regulator , the reference pressure enables the slave regulator valve to open and for
nitrous oxide to flow . Unfortunately, pure nitrous oxide can be delivered with only
oxygen supply pressure present , oxygen flow is not required.
Regulation now require oxygen – nitrous oxide ratio safeguards, which need a
minimum continuous low flow of oxygen varying from 200 to 300 mL/min , as indicated
by the low – flow Rota meter. In newly designed machines , ingenious mechanical
devices prevent the delivery of gas mixtures with an oxygen concentration below a low
limit. Oxygen – nitrous oxide ratios vary from 25 : 75 to 30 : 70 depending on the
manufacturer .
12.2.6- Gas Delivery Units :
From the fail – safe system , the gas is directed to the flow delivery unit. Two
methods have been used to accomplish delivery and control the gas mixture ; gas
proportioning and gas mixing .
In a gas proportioning system , the delivered concentration of each gas constituent is
the function of a pre- determined . precisely controlled ratio of proportionality which is
independent of the total gas flow . For example , for desired mixture of 70% nitrous oxide
and 30% oxygen , the metered ratio mass delivery will always be 7.3 regardless of the
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Anesthesia
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total flow rate . Concentration is only a function of the proportional relationship between
constituents. It dose not rely on setting individual gas flows . An oxygen – nitrous oxide
bleeder used in a manner similar to the oxygen air blenders commonly used with
mechanical ventilator performs this function.
Most current anaesthesia machines use gas mixing . In this technique , the flow rate of
each constituent is independently controlled and measured by a delivery unit consisting
of a needle valve and a rotameter. The needle valve functions as a flow controller and a
means of turning the gas on and off. The rotameter is a variable orifice flowmeter and
consists of a transparent tube with a tapered internal diameter and a floating bobbin flow
indicator .
During the administration of anaesthesia, it may be necessary to fill the patient
breathing circuit with oxygen at a rate higher than what the gas delivery unit can supply .
For example , such a situation exist any time the patient is disconnected to the breathing
circuit . This higher flow of oxygen is supplied via the oxygen flush valve and line . The
oxygen flush system provides a high flow ranging from 35 to 75 L/min at a high pressure
, directly into the patient breathing circuit.
Each has a specific delivery unit . These units are connected in parallel and exhaust
into common manifold prior to leaving the machine . The final concentration and total
flow determined by mixing the component flows are dependent functions and subject to
the accuracy of the control and measurement equipment .
12.2.7- Vapour Delivery :
The various liquids that posses anaesthetic properties are too potent ( strong ) to be
used as pure vapours . They are thus diluted in a carrier gas such as air and/or oxygen , or
nitrous oxide and oxygen . The device that allows vapourization of the liquid anaesthetic
agent and it’s subsequent admixture with a carrier gas for administration to a patient is
called a vapourizer. Vaporizers thus producte an accurate gaseous concentration from a
volatile liquid anaesthetic . The anaesthetic vapour can then be safely added to the
previously metered oxygen and nitrous oxide as the mixture leaves the mixing manifold.
Vapourizers are available in one of the two basic designs;the flowmetre controlled or
the concentration – calibrated . In either device , the anaesthetic vapours are picked up
from the vapourizer by a carrier gas consisting either of pure oxygen or an oxygen –
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Anesthesia
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nitrous oxide mixture that bubbles through or passes over the liquid . The liquid surface
areas to gas interface is designed to ensure the most efficient vapourization process . As
aresult of vapourization a drop in liquid temperature is produced . As the liquid
temperature decreases , a thermal gradient is established between the liquid and the
surroundings. This results in a decrese in the quantity of the vapour produced . In order
to maintain the performance of the vapourizer , the temperature drop is minimized or
prevented by the incorporation of a thermal source . This is achived by using a water
bath or surrounding the vaporizing liquid with a heating element . These devices may
also control the temperature of the carrier gas entering the vapourizer . The materials
selected for vapourizer construction require both a high specific heat and high thermal
conductivity . Material with high specific heats will change temperature more slowly and
maintain an appropriate thermal inertia. The higher the thermal conductivity , the higher
the conduction of heat from the surroundings . Because of its availability and lower cost ,
copper has been one of the most common materials used . Although not ideal, copper has
a moderate specific heat and a high thermal conductivity. Early vapourizers were
accordingly called “ copper kettle”.
In order to proved a stable and predictable concentration of anaesthetic vapour , the
vapourizer include a suitable method of obtaining calibrated dilution of vapour to avoid
administration of too powerful volatile anaesthetic agents to the patient . This can be
dome by several means and the vapourizers are accordingly classified into various
categories discussed below .
12.2.8-Variable Bypass Vapourizer :
Here the carrier gas flow from the flowmeter is split into two streams in a known
ratio one stream which is called “ chamber flow “ , flows cover the liquid agent while the
final concentration can be controlled by varying the splitting ratio between the vapourizer
gas and the bypass using an adjustable valve (fig. 8)
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Anesthesia
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Fig. 8
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Anesthesia
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The splitting ratio of the two flows depends on the ratio of resistances to their flow ,
which is controlled by the concentration control dial and the automatic temperature
compensation vavle.
Usually , less than 20% of the gas becomes enriched – saturated with vapour and
more than 80% is bypassed , to rejoin at the vapourizer outlet . The output of current
varialble bypass vapourizers is relatively constant over the range of fresh gas flows from
approximately 250 mL/min to 15L/min. The output of vaporizers in linear at the ambient
temperature ( 2o – 35 o C) due to automatic temperature compensating devices that
increase carrier gas flow as the liquid volatile agent temperature decrease . Also, they are
composed of metals with high specific heat and thermal conductivity . Check valves are
provided to prevent back pressure effect on the vapourizer from the breathing circuit due
to positive pressure ventilation.
Measured – flow Vapourizers : In these devices , the anaesthetic agent is healed to a
temperature above the boiling point and is then metred into the fresh gas flow . (fig. 9)
Various anaesthetic gent have widely different potencies and physical properties and
hence require vapourizers constructed specifically for each agent . They are thus agent –
specific . They are only calibrated for a singl gas , usually with keyed filters that decrease
the likelihood of filing the vaporizer with the wrong agent.
:
Vapourizer are provided with various safety related inter – locks which ensure tha
- Only one vapourizer is turned on;
- Gas enters only the on which is on
- Trace vapour output is minimized when the vapourizer is off .
- Vapourizers are locked into the gas circuit, thus ensuring that they are sealed
correctly ; and
- Other important safety features are followed including keyed filters and
secured mounting to minimize tipping ( tilting ) which may obstruct the
working of the valves.
12.2.9- Delivery System :
Patient Breathing System : the function of a patient breathing system is to deliver
anaesthetic and respiratory gases to and from the patient . It describe both the mode of
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Anesthesia
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operation and the apparatus by which inhalation agents are delivered to the patient. The
breathing system may be :
. Re -breathing Type: This refers to re – breathing of some or all of
the previously exhaled gases , including carbon dioxide and water
vapour .
. Non- re- breathing Type : In this a fresh gas supply is delivered to
the patient and re – breathing of previously exhaled gases is
prevented . Usually , non – rebreathing type systems are applied in
practice . This is achieved by using : - Uni – directional ( circle )
system , and
• Bi – directional ( to – and – fro ) system .
Fig. 10
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Anesthesia
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Fig. 10 shows the principle of a non – re -breathing system which uses uni –
directional non – breathing valve . Fresh gas entering the inspiratory part is either sucked
in by the patient's inspiratory effort or blown in during controlled that when it is open to
admit inspiratory gas, it does not permit the flow from the expiratory part to get through it
. When the patient exhales , the reverse happens , as the inspiratory valve is occluded and
the expiratory valve is opened to allow expiratory gases to escape . The inspiratory
system usually includes a rubber bag of two – liter capacity which acts as a reservoir for
fresh gas . The reservoir bag is refilled with fresh gas during the expiratory phase . It can
also be compressed normally to provide assisted or controlled ventilation . The fresh gas
supply is linked to a length of corrugated breathing hose ( minimum length – 10 cm with
an internal volume of 550 ml). This represents slightly morethan the average tidal volume
in an aneaesthetized adult breathing spontaneously . this is , in turn , connected to a
variable tension, spring – loaded flap value for venting off exhaled gases . This valve is
located as close to the patient as possible as possible , and is called an APl ( adjustable
pressure limiting ) valve . The APL valve works as pop – off valve to ensure that the
patient is not subjected to the surges in the gas supply . When the pas encounters
resistance from the patient , the excess gas pops out . The arrangement is shown in fig 11
. In this case , carbon dioxide elimination is achieved by the flushing action of the fresh
gas introduced with the breathing system , rather than by separation . Obviously , this
systemretains the potential for re – breathing of carbon dioxide when the fresh gas flow
rates are reduced .
Fig. 11
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Anesthesia
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Circle System : The circle is the most popular breathing circuit and is a closed loop of
large – bore , low – pressure tubing divided into an inspiratory and an expiratory lim.
Contained within this loop are two uni – directional valves , as CO2 absorber , circuit gas
venting ( scavenging ) , an adjustable pressure – limiting valve, reservoir bag, and airway
management equipment including masks and endotracheal tubes . The patient is
connected to absorber by two corrugated hoses , one inspiratory and the other expiratory .
Fresh gas is introduced proximal to a uni-directional inspiratory valve . During
inspiration , gas move through the absorber from either the reservoir bag or ventilator
bellows and inspiratory valve into the inspiratory lim of the circuit . The pressure
difference between the inspiratory and expiratory limbs keeps the uni – directional
expiratory valve closed . During exhalation , the pressure differential reverses . The
inspiratory valve closes and the expiratory valve opens, allowing the exhaled gas to flow
into the reservoir bag or ventilator bellows and bsorber . The APL or the pop – off valve
enables the anaesthetist to control the circuit volume and pressure by the regulation of gas
venting from the circuit . Circuit exhaust is either carried into the room or collected by a
gas scavenging system .
Uni- directional breathing valves are available in several design . This disk valve is
the most common in modern systems . This valve consists of only one movable part , a
flat disk. The disk is made of either plastic or metal and is held against the valve seat by
either gravity or a mechanical spring. The valves are placed in transparent devices so that
their action may be observed.
The APL is designed to regulate circuit pressures by manually adjusting the spring
tension against a disk. When circuit pressure overcomes the valve resistance, the disk is
lifted from its seat and gas is allowed to exhaust from the circuit. The circuit volume and
pressures throughout the delivery of anaesthesia are continuously observed so that the
APL the APL valve can be appropriately adjusted.
The absorber contains a carbon dioxide absorbent ( soda lime)in a closed container.
Soda lime is used in the form of granules so that they have volume and large surface area
.thus, the expiated air remains in contact with the coda lime for a relatively long period of
timely, increasing the efficiency of absorption. Granules of an optimum size are selected
as, too large a size leads to poor contact and poor absorption, while too small a size clogs
the soda lime bed and causes resistance to the gas flow. The exhaled gas is made to flow
thorough the absorber where the CO2 is removed. The remaining gas mixed with fresh
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Anesthesia
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gas flowing from the machine and re- breathed via the inspiratory limb. When the soda
lime gats consumed its color changes from pink to yellowish.
The reservoir or breathing bag is highly complainant with an easily expanded volume.
The bag allows the accumulation of gas during exhalation so that a reservoir is available
for the next inspiration. It provides a means for visually monitoring the spontaneous
breathing pattern of the patient and buffers increases in breading circuit pressure. The bag
also provides a means that can be used to manually ventilate the patient. Either passive or
active scavenging systems are utilized in removing the circuit exhaust. In passive
scavenging, anaesthetic waste gases are vented directly into the existing room ventilation
systems. The tubing, connects either the ventilator or the APL valve exhaust port of the
breathing circuit to the hospital ventilation system. In active methods, the anesthetic
circuit connects directly to a high flow vacuum system via appropriate interface.
12.2.10- Humidification
Dry gases supplied by the anaesthesia machine may cause clinically significant
desiccation of mucus. This may contribute to retention of secretion and the mucus flow
may cease. Lung compliance will consequently fall. Therefore, air or anaesthetic gases
need to be humidified.
Absolute humidity: this is the maximum mass of water vapour which can be carried
by given volume of air (mg/L). this quantity is predominantly determined by temperature.
Warm sir can carry much more moisture.
Relative Humidity ( RH ) : This is the percentage of the amount of humidity present
in a sample; as compared to the absolute humidity possible at the sample temperature . It
is ideal to provide gases at body temperature and 100%, RH to the patient ‘ airway > the
humidification measures that are commonly employed include heated airway humidifiers
, nebulizers and heat and moisture exchangers.
§ In the heated humidifiers , the air passes over the surface of the
heted water and vapourization takes place . the temperature of the
water is thermostatically controlled preferably , tow thermostats in
series are used , so that if one thermostat fails , the other would
still cut off the electric supply before dangerous temperature is
reached . The temperature sensor is usually placed near the patient
–end of the delivery tube so as ensure the maximum deficiency .
§ Nebulizers aure used to supply moisture in the form of deroplets .
A jet of air or gases may be used to entrain water drawn from a
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Anesthesia
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reservoir fig. 12.. as the water enters the jet, it is broken up into a
large number of droplets . Nebulizers based on this principle are
also used in some ventilators . In ultrasonic nebulizers, water is
broken into droplets by continuous bombardment of ultrasound
energy which vigorously vibrates the water .
§ Heat and moisture exchangers are based on the principle of
conserving the patient’s own heat and moisture without external
energy or water supply .
Fig 12
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Anesthesia
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§ The fibre – packed cartridges contain a moisture – absorbent
material that absorbs the patient’s exhaled water and heat. During
inspiration dry, inspired circuit gas flows through the warm,
moisturized absorbent where it is warmed and humidified . Fibre
cartridge are not as efficient at warning as humidfiers . However,
they do significantly retard patient heat loss.
For protection of patients from infection , clean or sterile disposable breathing
circuits and bacterial filters have been advocated and widely used to reduce post –
operative respiratory infections
Although simple in design, breathing circuits can be source of may problems. The
most common and serous problem is the potential for disconnection at any of several
locations. Numerous investigators have shown that 10 – 15% of preventable mishaps
result directly from airway leaks and disconnects . The cause for leaks and disconnects
include poor fit due to incorrect size , incorrect shape taper connections , inappropriate
fabrication of materials, thermal expansion , broken fittings and the absence of a locking
device .
12.2.11- Ventilators :
An integral component of the anaesthetic delivery system is the ventilator . The
ventilator provides a positive force for transporting respiratory and anaesthetic gases into
an apneic patient . The ventilators prove positive pressure ventilation at a controlled
minute volume ( Tidal volume, Rate ) They operate either electronically or mechanically
with pneumatic or electric power source.
Most of the currently used ventilators consist of a bellows contained within another
housing . The bellows communicate directly with the breathing circuit and causes a pre-
selected volume of gas to flow into the patient . The flow of gas into circuit results from
collapsing the ventilator bellows by pressurizing the surrounding gas volume contained
within the bellows housing.
The ventilator is either located within the mainframe of the anaesthesia machine or is
attached as an accessory unit .The outlet of the ventilator connects directly to the patient
breathing circuit of the anaesthetic delivery system at the location and in place of the
breathing reservoir bag. The ventilator thus functions as controller for both ventilation
and circuit gas supply by replacing the functions of the reservoir bag and APL valve .
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12.2.12- Patient Circuit :
The patient circuit consists of black corrugated anti – static rubber tube , a chrome
plated tube fitting ( T joint ) , a tow litre re- breathing bag, a Heidrink valve and a face
mask with an elbow fitting. The face mask is designed to fit the patient’s face perfectly
without any leaks and yet to exert the minimum of pressure which might depress the
jaws and cause respiratory obstruction.
12.2.13- Electronics In the Anaesthetic Machine
Any delivery system is expected to meet accurately and safely , the patient’s varying
requirements for respiratory and anaesthetic gases. The system must be able to monitor
the function of the delivery system itself and the effect of the anaesthesai on the
patient.Also, during the entire procedure the machine performance should not only be
monitored and controlled , but it’s status should be continually assessed and recorded. In
order to meet these requirement , the impact of electronics on the design and functioning
of the anaesthetic machine has been phenomenal.
The totally pneumatic anaesthetic machine stillhas many merits, which include it’s
being easy to understand and easy to maintain, as also its cheaper cost and reliability .
However , certain problems are encountered which directly affect the performance and
safety of a pneumatic anaesthetic machine . This is overcome through the utilization of
newer technologies and automation of instrumentation and anaesthetic delivery .
Microprocessor – based anaesthetic equipment facilitates improvements in :
§ Gas supply and proportioning systems;
§ Breathing circuits;
§ Gas scavenging and humidification devices ; and
§ Ventilators .
The use of microprocessor technology allows us to fully integrate control and safety
functions and protects the patient from gas supply failure, electrical supply failure,
hypoxic mixtures disconnections, vapourizer function , excessive airway pressure exhaled
minute volume outside pre – set limits , oxygen or volatile agents outside the pre- set
limit , end – tidal CO2 outside present limits and technical failure. All abnormal
conditions cause an alarm to appear on the monitor panel , which also displays the nature
of the fault.
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Computer application with appropriate data processing inputs and outputs,
automation and integration of machine functions and record – keeping are increasingly
becoming possible . Ergonomically designed machines with easy to read and interpret
display systems are also being commonly used.
Anaesthesia has a profound effect upon all physiological systems . Most of these
effects are deleterious, and therefore , it is important to know how the human body is
affected by a anaesthesia . With a view to increasing patient safety and achieving a good
degree of risk management , all systems affected by anaesthetic drugs must be monitored
. This is done by using monitoring equipment with visible and audible alarms .
Fig. 13
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Since individual responses to a particular dose of a anaesthetic vary considerably , it
advisable to measured the effect of the anaesthetic on a patient’s level of consciousness .
One method to do so is to measure . Auditory Evoked Potentials ( AEP ) , which is a
neuro – physiological indicator of the changes in the level of consciousness during
anaesthesia . This is an electrical signal contained within the EEG which is obtained by
delivering an auditory stimulus to the patient’s acoustic nerve . The fast extraction of the
complex AEP signal , the brain’s response to the auditory stimulus of the acoustic nerve
is obtained by mapping the signal and establishing an index which is developed as a
graphic curve and a single number on the monitor screen fig 13 .This index which is
calculated from a proprietary mathematical modeling method , quantifies the level of
anaesthesia. For example , typically , if this index is higher than 60, the patient is awake ,
and decreases in line with decreasing level of consciousness ( loss conscious typically
occurring below 30) . Re - usable head – phones apply stimulation to the acoustic nerve
to obtain AEP, which is then measured by a set of three disposable electrodes, two
electrodes are applied in the forhead and one behind the ear to estimate the level of
consciousness in a fast and non – invasive manner .
13.Maintenance and Calibration
Vaporizers should b drained of the liquid agent periodically to rid it of any
contaminants that may be present . This drained liquid should be properly labeled and
discarded . Halothane contain 0.01 percent thymol which is used to improve the stability
of halothane . Thymol tends to accumulate in the vaporizer with time because it’s vapor
pressure is much lower than that of halothane . High concentration of thymol may effect
some of the internal components of the vaporizer thereby altering its accuracy . The
increased thymol concentration may be noticed because it gives a brown tint to the liquid
in the vaporizer .
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13.1Anaesthetic apparatus: (Narkomed machines)
13.1.1The pneumatic piping system:
13.1.2Oxygen gas circuit
Because oxygen is the primary gas for all Narkomed machines, we will begin our
exploration of the pneumatic circuitry by tracing the flow of gas through the oxygen
circuit The internal pneumatic circuit plays an important role in patient safety.
Figure 14: The pneumatic circuit for a Narkomed two gas anesthesia system. The oxygen
circuit is on the right.
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13.1.3Cylinder Gas Supply Enters The Anesthesia System
Oxygen from the E cylinder enters the anesthesia system through the yoke assembly,
passing through the yoke check valve (Figure 15). The yoke check valve is a one-way
valve that allows gas to enter the anesthesia system from the yoke, but does not allow gas
to exit the anesthesia system through the yoke. As gas enters the yoke check valve, it
forces the ball in the valve away seat. The seat. The gas flows around the ball and exits
the yoke check valve through two holes in the side of the copper tubing connector. If the
E cylinder is absent or empty, the gas supplied by the hospital piping system flows in the
opposite direction (Figure 16). This gas flow forces the ball against the O-ring seat,
sealing the entry port of the yoke assembly and retaining the hospital pipeline gas within
the anesthesia system.
Figure 15:
Yoke check valve assembly - gas is flowing from the E cylinder through the yoke.
Figure 16:
Yoke check valve assembly - gas is flowing from the hospital pipeline gas supply toward
the yoke.
Pressure Reducing regulator
1 Gas enters the anesthesia system from an E cylinder, through the yoke at a high
pressure (typically ranging from 750 psi to 2200 psi). This pressure must be reduced for
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the gas circuits of the anesthesia system. The pressure reducing regulator accomplishes
this task in two phases. Figure (17) identifies the components of the regulator.
Figure 17: pressure reducing regulator.
13.1.5Phase 1
High pressure gas flows from the yoke check valve to the inlet port of the regulator and
enters the high pressure chamber (Figure 18). High pressure gas then flows from the high
pressure outlet port to the cylinder pressure gauge. High pressure gas will remain trapped
in the high pressure chamber until some adjustment is made to the main spring.
Figure 18: high pressure gas enters to the regulator.
Once the pressure control is set, it compresses the main spring that in turn moves the
diaphragm. The diaphragm forces the nozzle away from the seat, allowing high pressure
gas to flow into the low pressure chamber.
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13.1.6Phase 2
As the high pressure gas flows into the low pressure chamber, the diaphragm is forced
backwards and the main spring is compressed (Figure 19). This allows the small spring
behind the nozzle to move it toward the seat. When the gas pressure in the low pressure
chamber equals the tension of the main spring, the nozzle closes against the seat, cutting
off the flow of high pressure gas into the low pressure chamber. The gas in the low
pressure chamber then flows through the low pressure port and into the pneumatic circuit
of the anesthesia system. As the gas leaves the low pressure chamber and the pressure
lessens, the main spring forces the diaphragm and the nozzle away from the seat, starting
the whole cycle again.
Figure 3-19: Gas pressure and spring pressure equalize.
13.1.7-Cylinder Contents Pressure Gauge
The high pressure gas that flows from the high pressure outlet port of the regulator is
piped to a Bourdon pressure gauge. The pressure reading obtained from the gauge reflects
the amount of gas remaining in the E cylinder. These gauges are used to measure gas
pressure in large units, such as psi. This type of gauge is also used to measure pipeline
gas pressure.
The gauge consists of a hollow, curved tube connected to a gear rack that meshes with a
pinion gear. A needle is mounted on the pinion gear shaft. When the gas pressure
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increases inside the tube, the tube begins to straighten. This causes the gear train to move,
which in turn rotates the needle around the face of the gauge.
13.1.8-Pipeline Gas Supply Enters the Anesthesia System
Gas supplied by the hospital piping system enters through a hose connected to the
anesthesia system by a Diameter Index Safety System (DISS) fitting. The nut and stem
assembly on the end of the hose mates to a matching DISS inlet on the anesthesia system.
The stem and the body mate via the two shoulders on the stem that match two bores in
the inlet Thus, mismatches between the shoulders and the bores will not allow the wrong
gas to be connected to a given gas inlet. The DISS connectors are designed for the
delivery of gases at less than 200 psi of pressure.
Pipeline Check Valve
Gas that enters through the DISS inlet flows to the pipeline check valve. The pipeline
check valve performs the same function as the yoke check valve. It allows gas nom the
hospital piping system to enter, but not exit, the anesthesia system. The pipeline check
valve is mounted vertically with the pipeline gas entering from the bottom. As the gas
flows upward, it lifts the piston and seal off the seat, and exits the pipeline check valve at
the top (Figure 19). If the hospital pipeline gas supply fails, the piston and seal assembly
drops onto the seat and prevents any gas supplied by the E cylinder nom escaping through
the DISS inlet (Figure 20).
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Figure 19: pipeline check valve forward flow.
Figure 20: pipeline check valve reverse flow.
Auxiliary Oxygen Flowmeter
A tee fitting in the oxygen circuit supplies gas from either the pipeline or cylinder supply
to the auxiliary oxygen flowmeter. This device can be activated whether the main switch
is in the ON or STANDBY position. It allows the operator to deliver up to 10 Ipm of
100010 oxygen to a patient, usually through a nasal cannula. This device is a convenience
feature and is seldom used in the administration of general inhalation anesthesia.
Oxygen Flush Button
Regardless of whether the gas in the oxygen circuit was supplied by the hospital piping
system or an E cylinder, a tee fitting allows oxygen to flow to the oxygen flush button at
all times. The flush button consists of a valve and a restrictor. The flow restrictor is
located in the outlet port of the valve.
When the oxygen flush button is activated, it supplies the patient breathing circuit with
100% oxygen. The flush button can be activated whether the main switch is in the ON or
STANDBY position.
When activated, the valve opens, permitting 50 psi of oxygen to be applied to the flow
restrictor resulting in an output flow of approximately 55 l/min. This flow of oxygen is
delivered to the patient breathing circuit through the fresh gas outlet.
Locking Fresh Gas Outlet
All gases flow from the coarse flowtube and enter the fresh gas circuit The fresh gas then
flows through the vaporizer bank where anesthetic agent from a single vaporizer is added
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to the fresh gas mixture. The fresh gas mixture then flows to the patient breathing circuit
through the Locking Fresh Gas Outlet.
The fresh gas outlet has a spring-loaded locking cap designed to prevent an accidental
disconnect between the fresh gas outlet and the fresh gas hose of the patient breathing
circuit The fresh gas outlet mates with the fresh gas hose through a standard 15 mm
tapered fitting.
System Power Switch
A tee fitting allows both sources of oxygen to flow to the system power switch (Figure 3-
10). With the system power switch in the STANDBY position, the valve remains closed
eliminating pneumatic power.
The leaf also switch remains closed eliminating electrical power.
Figure 21: system power switch in the STANDBY position.
As the system power switch is rotated to the ON position (Figure 22), the switch moves
into the assembly. It depresses the valve plunger, allowing oxygen to flow to the rest of
the oxygen circuit At the same time, the rotation causes the pin to move away from the
leaf switch. The switch opens and activates the electrical circuitry of the anesthesia
system. Notice that the leaf switch opens when turned ON, allowing the anesthesia
system to remain in use should the leaf switch malfunction.
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Figure 22: system power switch is on.
Oxygen Supply Pressure Alarm Switch
A tee fitting located directly downstream of the system power switch allows oxygen to
flow to the oxygen supply pressure alarm switch. This pressure switch warns the operator
of diminishing oxygen supplies. In the inactive position, oxygen pressure enters the
bellows. The bellows expand in proportion to the gas pressure in the oxygen circuit
(Figure 23). The bellows in turn compresses a spring and moves the connecting rod
toward the electrical switch, opening the contacts. The switch remains open and the
alarms are inactive as long as the pressure in the oxygen circuit remains above the set
point.
Figure 23: alarm inactivated.
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As the gas pressure in the oxygen circuit decreases, the pressure inside the bellows also
drops. As the pressure in the bellows drops, the spring causes the bellows to collapse,
allowing the connecting rod to move away from the electrical switch (Figure 24). When
the pressure falls below the set point, the electrical switch closes and the clinician gets
both an audible and a visual alarm.
Figure 24: alarm active.
Flow Control Valve
The oxygen circuit on a Narkomed anesthesia system terminates at the flow control
valve. This valve regulates the flow of oxygen supplied to the patient breathing circuit
through the fresh gas outlet. The valve consists of a threaded shaft with a tapered end that
mates with a tapered seat. When the flow control valve is closed (Figure 25), the tapered
end seals against the tapered seat and does not allow any gas to flow through the valve.
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Figure 25: flow control closed position
As the flow control valve is opened (Figure 26), the tapered end of the threaded shaft
moves away from the seat, allowing the oxygen to flow through the valve. The greater the
space between the tapered end and the tapered seat, the higher the gas flow.
Figure 26: flow control open position
Flowtubes
After establishing gas flow through the flow control valve, the gas flow must be
measured. All Narkomed anesthesia systems use two flowtubes, a fine flow tube that
measures gas in ml/minutes and a coarse flowtube that measures gas in l/minutes. The
fine and coarse flowtubes are connected in tandem to measure the flow of a given gas.
Using tandem flowtubes allows for more accurate delivery of gas throughout the entire
flow range. All flowtubes used in Narkomed anesthesia systems are permanently
calibrated.
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15.Nitrous Oxide Gas Circuit
The nitrous oxide gas circuit shares many of the same components as the oxygen circuit.
Nitrous oxide supplied by an E cylinder enters the anesthesia system through a yoke
indexed for nitrous oxide. The nitrous oxide flows through a yoke check valve into the
high pressure regulator. From the regulator, the nitrous oxide flows to the cylinder
pressure gauge and the oxygen failure protection device (Figure 27).
Nitrous oxide supplied by the hospital piping system enters the anesthesia system through
a DISS inlet and flows to the pipeline pressure gauge and the pipeline check valve.
Nitrous oxide flows through the pipeline check valve to the oxygen failure protection
device. As the nitrous oxide flows out of the oxygen failure protection device it enters a
gas proportioning device in the anesthesia system.
Figure 27: the nitrous oxide gas circuit
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15.1-Oxygen Failure Protection Device
In the event of a total failure of the oxygen supply system. it is necessary to protect the
patient against the delivery of a hypoxic gas mixture. To prevent hypoxic gas mixtures,
an Oxygen Failure protection Device (OFPD) is incorporated into all gas circuits, except
oxygen, in the anesthesia system.
In the example shown, both supply sources of nitrous oxide now into the bottom of the
OFPD. Under normal circumstances, the OFPD is activated by the oxygen supply
pressure. The oxygen enters the OFPD under pressure and forces the piston downward.
N. the piston moves down, it compresses the spring and opens the valve. When the valve
opens, the nitrous oxide flows through the OFPD.
toward the nitrous oxide flowmeters.
Figure 28: OFPD is activated
If the oxygen supply to the anesthesia system fails, the OFPDs stop the flow of all other
gases to the patient As the supply pressure of the oxygen decreases, the spring forces the
piston and seal assembly up, narrowing the valve opening and decreasing the flow of
nitrous oxide in proportion to the oxygen flow. If the oxygen pressure fails completely,
the valve closes and the nitrous oxide flow stops at the OFPD.
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15.2-Oxygen Ratio Controller
Much like the OFPO, the Oxygen Ratio Controller (ORC) is designed to prevent the
delivery of a hypoxic gas mixture to the patient breathing circuit. On a Narkomed
anesthesia system, this is achieved by controlling the ratio of nitrous oxide to oxygen.
The ORC is essentially a proportioning device that responds to pressure differentials
produced by resistors placed between the flow control valves and the fine flow tubes in
both the oxygen and nitrous oxide gas circuits.
The ORC consists primarily of two rolling diaphragms connected by a moveable piston,
which in turn controls a proportioning valve. As gas flows through the flow control
valves and through the resistors, each resistor generates a back pressure proportionate to
the amount of gas flowing through each respective flow control valve. The back pressure
of oxygen flows to the upper chamber of the ORC. The back pressure of nitrous oxide
flows to the lower chamber. Each gas exerts a pressure on its respective rolling
diaphragm.
The piston that connects these two diaphragms moves up or down in response to any
changes in back pressure from either gas, opening or closing the proportioning valve
located below the lower chamber.
The ORC is designed to prevent the delivery of less than 22 - 28% oxygen to the patient
breathing circuit.
The following illustrations show the changing positions of the various components of the
ORC in response to changes in the flow rate of oxygen or nitrous oxide.
16-Three Gas Circuits
A Narkomed anesthesia system can be equipped with a third gas circuit The third gas
circuit is very similar to the nitrous oxide circuit up to the OFPD. The OFPD is controlled
by oxygen pressure and as long as sufficient oxygen is available to the anesthesia system,
the third gas circuit will remain enabled.
The third gas is not controlled by any shut off valves or proportioning devices and it is
possible to deliver only the third gas to the patient As long as the third gas circuit is
configured for Air, it is not possible to give an hypoxic gas mixture to the patient even if
the flow of oxygen has been reduced to minimum flow.
If the third gas circuit is configured for Heliox mixtures, the yoke is configured to allow
only mixtures of less than 80% Helium. The pin index safety system allows for this
distinction ( In the event that only minimum oxygen flow is being given in conjunction
with the Heliox mixture containing at least 21 % oxygen, the patient still does not receive
a hypoxic gas mixture.
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With this piping configuration, it is possible to deliver all three gases at the same time.
Even though the third gas circuit is independent of the other gas circuits (with the
exception of the OFPD), the flow of nitrous oxide is still controlled by the flow of oxygen
via the Oxygen Ratio Controller.
Figure 29: Three gas circuit
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16.1-The 19.0 Vaporizer
The heart of any anesthesia system is the vaporizer. The vaporizer used on Narkomed
anesthesia systems is the 19.1 Vaporizer (Figure 30). The primary function of the
vaporizer is to control the rate of evaporation of a given liquid anesthetic and introduce a
precise volume percentage of that anesthetic vapor into the fresh gas stream of the
anesthesia system. The vaporizer must be able to control or counteract all physical
changes that affect the percent volume output of anesthetic vapor in the fresh gas stream.
Tracing the flow of fresh gas on its journey through the vaporizer will demonstrate how
this is accomplished.
Figure 30: The 19.n vaporizer in the “0” position.
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Figure 31: Fresh gas flow through the 19.n vaporizer.
16.2-Gas Flow Through the Vaporizer
When the concentration dial of the vaporizer is in the "0" position, the internal on/off
switch is in the "off' position. In this position, the fresh gas enters the vaporizer through
the inlet port, flows through the on/off switch, and is directed to the outlet port without
entering the interior of the vaporizer
When the concentration dial is rotated to any concentration above 0.2% a cam in the
handwheel rotates the on/off switch to the "on" position. The fresh gas flows through the
on/off switch and is directed into the interior of the vaporizer . When the fresh gas flow
enters the interior of the vaporizer, it encounters the temperature compensating bypass.
The bypass divides the fresh gas flow into two streams. The majority of the fresh gas
flows through the bypass and exits the vaporizer without combining with any anesthetic
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vapor. The remaining stream of fresh gas does not flow through the bypass, but proceeds
directly to the pressure compensator. The pressure compensator eliminates pressure
fluctuations within the vaporizing chamber. Without a pressure compensator, any
pressure fluctuations upstream or downstream of the vaporizing chamber could affect the
percent volume concentration output of the vaporizer.
The fresh gas stream flows through the pressure compensator and into the vaporizing
chamber, where it becomes partially saturated with anesthetic agent as it flows over the
wick. The fresh gas/agent mixture then flows out of the vaporizing chamber through the
concentration control cone, which is actuated by the concentration dial. As the
concentration is increased or decreased, the space between the concentration control cone
and the cone housing increases or decreases, allowing more or less fresh gas/agent
mixture to flow out of the vaporizing chamber. After the fresh gas/agent stream flows
through the concentration cone, it rejoins the fresh gas stream that passed through the
temperature compensating bypass. The reunification of the two streams of fresh gas, one
stream containing anesthetic agent and the other unchanged, results in the final percent
volume concentration of anesthetic agent that goes to the patient breathing circuit.
Temperature changes inside the vaporizing chamber (caused primarily by changes in the
rate of evaporation) are the major influence on the concentration output of the vaporizer.
The temperature compensating bypass corrects for temperature variations by altering the
ratio of gas between the two streams of fresh gas flow. If the temperature decreases in the
vaporizing chamber (due to an increase in the rate of evaporation), the bypass cools, the
brass shell contracts, and the bypass moves upward. This action further restricts the fresh
gas stream that flows through the bypass, forcing a greater proportion of the fresh gas
flow into the stream that flows to the vaporizing chamber. The extra fresh gas offsets the
lower vapor pressure (caused by the decrease in temperature) in the evaporation process,
the concentration rises, and thermostability is reestablished in the temperature
compensating bypass. The opposite occurs if the temperature increases in the vaporizing
chamber.
17.The Absorber System and Breathing Circuits
The fundamental components of the patient breathing circuit on Narkomed anesthesia
systems include a carbon dioxide absorber, a positive end-expiratory pressure valve, an
adjustable pressure limiter valve, and a scavenger system. Each component has a specific
function and safety features associated with its function.
In this chapter, we will discuss the absorber system and various types of breathing
circuits.
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17.1-Absorber Assembly
The main function of the absorber is to remove exhaled carbon dioxide from the patient
breathing circuit of the anesthesia system. The absorber assembly consists of two
canisters to hold carbon dioxide absorbent, two unidirectional flow valves, a pressure
gauge, a fresh gas connector, and a manual/automatic selector valve). The absorber
permits spontaneous, manually assisted, or mechanical ventilation of the patient, while
allowing any unused fresh gas mixture to be recirculated to the patient after the carbon
dioxide is removed.
17.2-Narkomed Absorber Circle System
The Narkomed absorber circle system allows three modes of patient ventilation:
spontaneous, manually assisted, or mechanically assisted ventilation.
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13.7-Spontaneous Ventilation
In spontaneous ventilation, patients control their own respiratory rate and tidal volume.
During spontaneous inspiration the patient inhales from the breathing circuit, creating a
partial vacuum that lifts the valve disc in the inspiratory valve, opening the valve.
Because this partial vacuum is also felt on the bottom of the expiratory valve disc, the
valve remains closed. Gas flows from the breathing bag through the absorber and is
joined by fresh gas supplied by the anesthesia system just below the inspiratory valve.
Figure 32: spontaneous inspiration in a narkomed absorber circle system.
During spontaneous exhalation, the patient exhales into the breathing circuit forcing the
exhalation valve disc from its seat, opening the valve and allowing the exhaled gases to
flow into the breathing bag. At the same time, the patient's exhalation is creating a
positive pressure on the valve disc in the inspiratory valve, forcing this valve to remain
closed. During this phase, fresh gas ftom the anesthesia system continues to flow into the
absorber, but can flow only into the breathing bag. When the pressure in the breathing
bag exceeds the preset pressure of the APL valve, excess gas flows to the scavenger
system.
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Figure 33: spontaneous inspiration in a narkomed absorber circle system.
17.4-Manually Assisted Ventilation
During manually assisted ventilation, the clinician compresses the breathing bag during
inspiration, creating a positive pressure in the absorber circle system. The positive
pressure closes the expiratory valve and opens the inspiratory valve. The gas mixture in
the breathing bag flows through the manual/automatic selector valve, the breathing
pressure gauge, and through the CO2 absorbent to the patient breathing circuit When the
gas mixture flows from top to bottom through the absorbent canisters during inhalation,
the CO2 is scrubbed from the gas mixture by the absorbent material. Some of this gas
mixture may exit to the scavenger depending upon the setting of the APL valve. The
manual/automatic selector valve prevents any gas flow to the ventilator.
Figure 34: manual ventilation breathing circuit.
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During manually assisted ventilation, the patient "exhales" into the breathing circuit
(Figure 35). The exhalation creates a positive pressure that closes the inspiratory valve
and opens the expiratory valve.
Exhaled patient gas containing carbon dioxide flows through the spiromed sensor, across
the top of the absorber canisters, past the breathing pressure gauge, through the
manual/automatic selector valve, and fills the breathing bag. Fresh gas from the
anesthesia system continues to enter the absorber and flows from bottom to top through
the absorbent canisters, mixing with the exhaled patient gas as it flows across the top of
the absorber. When the pressure in the breathing bag exceeds the preset limit of the APL
valve, exhaled gas flows through the APL valve and into the scavenger system.
Figure 35: manual ventilation breathing circuit.
17.5-Mechanically Assisted Ventilation
During mechanically assisted ventilation, the ventilator controls all factors relating to the
patient's breathing.
The manual/automatic selector valve is rotated to the auto mode, and the gas mixture now
flows to the ventilator circuit, bypassing the APL valve and the breathing bag. During
inspiration (Figure 3-26), the ventilator drive gas compresses the bellows, creating a
positive pressure within the patient breathing circuit This pressure closes the expiratory
valve and opens the inspiratory valve. The gas mixture inside the bellows flows through
the manual/automatic selector valve, past the breathing pressure gauge, from top to
bottom through the absorbent canisters, and into the patient breathing circuit The
ventilator relief valve is also pressurized by the ventilator drive gas and closes,
preventing any of the gas mixture from entering the scavenger system.
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Figure 36: inhalation during mechanical assisted ventilation mode.
During exhalation (Figure 37), the ventilator drive gas ceases to flow into the canister,
ending the positive pressure that was compressing the bellows. When the positive
pressure is ternimated, the patient can exhale into the breathing circuit This causes a
positive pressure that closes the inspiratory valve and opens the expiratory valve. Exhaled
gas flows through the spiromed sensor, across the top of the absorbent canisters, past the
breathing pressure gauge, through the manua1lautomatic selector valve, and fills the
ventilator bellows. Excess gas from the patient breathing circuit enters the scavenger
through the ventilator relief valve only after the bellows fills completely and the pressure
of the patient's exhalation lifts the ball valve.
Figure 37: exhalation inhalation during mechanical assisted ventilation mode.
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16.6-Oxygen Flush
The following illustrations demonstrate how the oxygen flush removes waste gases from
the patient breathing circuit and the absorber system. In the first phase (Figure 38),
activating the oxygen flush valve allows a flow of approximately 55 l/min of
100%oxygen into the absorber system. At this point, any exhaled gases are forced back
through the absorber toward the breathing bag.
Figure 38: phase one of the oxygen flush.
In the second phase of the oxygen flush (Figure 39), a minimal positive pressure is
created in the absorber system and the inspiratory valve opens. With the inspiratory valve
open, 100% oxygen now flows to the patient breathing circuit and the breathing bag.
Figure 39: phase two of the oxygen flush.
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During phase three of the oxygen flush (Figure 40), a positive pressure builds up in the
absorber system. the patient breathing circuit, and the breathing bag. When this pressure
exceeds the preset limit of the APL valve, gases begin to flow to the scavenger system.
Figure 40: phase three of the oxygen flush.
In the fourth and final phase of the oxygen flush (Figure 41), all exhaled gas containing
carbon dioxide and anesthetic agent is flushed from the patient breathing circuit, the
absorber system, and the breathing bag. All gases are replaced with 100% oxygen. The
APL valve continues to vent excess gas to the scavenger system while sufficient pressure
remains in the system.
Figure 41: forth phase of the oxygen flush
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18.Scavenger Systems
Scavenging is defined as the collection and removal of exhausted gases from the
operating room. Installing an efficient scavenging system is the most important step in
reducing trace gas levels in the operating room.
Scavenger systems can be divided into two general categories. One category comprises
the closed scavenger system of spring-loaded valves for positive and negative pressure
relief. The other category is the open reservoir scavenger system that relies on open ports
for positive and negative pressure relief.
18.1-Open Reservoir Scavenger System
The open reservoir scavenger system is called an "open" system because it relies on open
relief ports for positive and negative pressure relief. When the float stays between the
lines on the flowmeter, the needle valve is adjusted properly (Figure 42). The gas flow
from the scavenger to the central vacuum system is approximately 25 l/min.
Figure 42: the open reservoir scavenger system.
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17.2-Scavenger Interface for Passive Systems
The scavenger interface for passive systems (Figure 43) is called a "closed" system
because it relies on a spring-loaded valve for positive pressure relief. This system is
typically used in hospitals where scavenging is done through the air conditioning system.
A hose connects the exhaust port of the scavenger to the evacuation vent of the air
conditioning system. If the hose from the exhaust port becomes blocked, a +5 cmH20
safety relief valve activates, bleeding excessive pressure into the atmosphere.
Figure 43: a scavenger interface for passive systems.
,
17.3-MEDICAL GASES AND CYLINDERS
Gases used during the course of anesthesia include oxygen and nitrous oxide; and less
frequently, carbon dioxide, air, and helium. These chemicals must be supplied in the
compressed state so a continuous supply for a practical period of time is available. In the
compressed state, a gas can be supplied in practical quantities from any of several
different sizes of cylinders, giving the advantage of portability and the disadvantage of
limited volumes. In the compressed, liquid state, some gases (e.g., °2) can be supplied
from bulk storage tanks at great advantages in capacity and cost, with the obvious
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disadvantage that the gas must be piped throughout a building and made available at
carefully placed outlets in walls and ceilings.
Figure 44: nine different size of medical gas cylinder.
20.PREVENTIVE MAINTENANCE
The purpose of performing routine preventive maintenance on a periodic basis is
to prevent possible malfunction or breakdown and to assure optimum performance of the
equipment at all times . All anesthesia machine manufacturers offer service contracts
which cover preventive maintenance and emergency repairs . Current recommendations
are that every anesthesial machine should be serviced every 3 to 4 months .Dr. Clayton
Petty. In this book The Anesthesia Machine , recommends that this service should be
carried out only by factory – authorized representative . and not by hospital biomedical
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electronic technicians or anesthesia technicians . Having only factory – trained
representative service the anesthesia machines provides additional support for the
anesthetist and the hospital as liability would be shared by the manufacturer if an
equipment – related anesthetic mishap occurred . Documentation of the service
performed on each anesthesia machine , including tests performed and parts used , should
be kept on file in the department . In addition to the preventive maintenance records , the
department may find it desirable to keep track of modification and complaints associated
with use of the machine .
Fig. 6
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Preventive maintenance must be followed by cleaning and thorough operational
checks by anesthesia support personnel and anesthesiologist before each use the FDA has
produced an anesthesia apparatus checkout procedure intended to be performed before
delivering anesthesia . It is expected that this procedure be modified to comply with
differences in equipment design and variations in clinical practice . These modification
should be consistent with the procedures and precautions listed in the manufacturer’s
operator’s manual as well as have the acceptance of peer review. Dr. Petty has suggested
a short daily anesthesia machine checklist that may be incorporated into the patient’s
anesthetic record.
21.Anesthesia technologies:
21.1-Obsolete anesthesia technologies:
Anesthesia equipment does not become obsolete due to the wearing out of components,
but as a result of changes in medical procedures, innovations regarding efforts to increase
safety, and changes in the education, training, and experience of personnel responsible for
the administration of anesthesia.
Some of the resones for replacing anesthesia machines with newer technology:
• Safety machines with better safety for the patient.
• Main Switch Interface Many anesthesia machines produced after 1980 incorporate
a main switch which provides pneumatic and/or electrical signals which may be
utilized to control the activation of monitors and alarms. Monitors and alarms
which are not interfaced to the main switch have a certain risk of not being
activated intentionally or as an oversight. Replacement of anesthesia machines not
providing this feature (main switch interface that automatically turns on monitors
and alarms) should be seriously considered. Integrated anesthesia workstations or
systems composed of equipment and monitors provided by different
manufacturers should be capable of electronic communication between the
different components of the system
• Hypoxic Fresh Gas Flow Safeguard Most anesthesia machines produced after
1980 do not permit the administration of a fresh gas mixture with a nominal
oxygen content of less than 25% because of the incorporation of devices like the
ORMC¨ and ORC¨ by North American Drager or LINK 25¨ by Ohmeda. Oxygen
pressure failure protection devices (so-called "fail-safe" systems), on the other
hand, have an extremely limited safety potential (activation only by failure of
oxygen pressure) and are not a substitute for the described devices which
specifically safeguard against hypoxic fresh gas mixtures. Anesthesia machines
not incorporating such a fresh gas ratio protection device or not capable of
accepting an in-field installation of one may be serious candidates for
replacement.
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• Hanging Bellows Ventilator Until the early part of the 1980s, the majority of
anesthesia machines used in the United States incorporated ventilators with
bellows which descended during the expiratory phase.2 While these designs have
certain advantages over bellows that ascend during expiration, alarm devices
utilizing pressure monitoring to detect circuit disconnects can easily be fooled by
the design; furthermore, monitoring for expiratory flow to help detect a
disconnect is not possible. In addition to this, the bellows will continue its up and
down motion during disconnection from the patient, which may fool the operator
into believing that the patient is adequately ventilated. Anesthesia machines
incorporating a ventilator with hanging bellows are serious candidates for
replacement in the event that the manufacturers do not have a conversion kit
available to modify the ventilator and eliminate the hanging bellows in favor of
the more recent opposite design.
• Fresh Gas Hose Locking Device a 15mm slip fit connection has been standardized
for the connector of the fresh gas hose to the anesthesia machine. This standard
not only actually can encourage the accidental disconnection of the fresh gas hose
from the machine, but also invites the utilization of a cheap plastic connector,
normally used with endotracheal tubes. Manufacturers of anesthesia machines
have gone through relatively expensive efforts to make the unsafe fresh gas hose
coupling (based on a 15mm connector) safe by incorporating an additional
locking device. An anesthesia machine which does not incorporate a locking
device for the 15mm fresh gas hose connector is not a candidate for replacement
based on this feature alone, but a locking device should be installed to eliminate
accidental disconnect of the fresh gas hose if there are not enough other features
to induce machine replacement.
• Ventilation The 1980s saw significant progress in the development of ventilators
utilized during the administration of anesthesia. Features such as PEEP, peak
pressure limit, extended respiratory frequency ranges, and clearly marked controls
for frequency and inspiratory-expiratory face time ratios were incorporated. The
decision whether the performance of older style ventilators satisfies the need of an
institution or not depends very much on the procedures performed with the
equipment.
• Automated Record Keeping Many anesthesia machines produced after the mid
1980s permit data acquisition through electronic interfaces directly from the
equipment. In the event that an institution plans to introduce automatic record
keeping or other means of data management, there may be advantages to
replacing old equipment not having the features required for data management
instead of attempting to modify old equipment.
• Vaporizers In recent history, new volatile anesthetic agents introduced into the
market require the installation of new vaporizers into the anesthesia system.
While older anesthesia machines may have had vaporizers mounted in parallel or
vaporizers mounted in tandem (permitting the simultaneous administration of
more than one agent), newer standards require that only one vaporizer can be
activated at any given time. The mounting of a vaporizer downstream of the fresh
gas outlet is extremely dangerous for many reasons and should not be done at any
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time. Anesthesia machines which do not permit the installation of a new vaporizer
for a new volatile anesthetic may need to be replaced simply for this shortcoming
21.2-State of the art technologies:
Narkomed Anesthesia Workstation 6400
The Narkomed 6400 is the latest enhancement to the Narkomed 6000 Series product line,
representing the evolution of Dräger's well-established Narkomed technology.
The Narkomed 6400 anesthesia workstation provides an intelligent approach to
integration. Innovative technologies are combined to provide true clinical benefits to the
patient and user. State of the art computer technology integrates hemodynamic and
respiratory monitoring onto a single color touch screen display. The open interface allows
the export of monitored data for communication with anesthesia information systems,
such as the Dräger Saturn system.
At the heart of the Narkomed 6400 is the DIVAN ventilator. This electrically driven
piston ventilator has set the standard for anesthesia ventilation around the world.
Features
• Full ECG monitoring of up to seven leads
• ST Segment Analysis on all leads
• Four Invasive Blood Pressures
• Pulse Oximetry
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• Dual site Temperature Measurement
• Thermodilution Cardiac Output Measurement
• Non-invasive Blood Pressure Measurement
• Output for communication with defibrillators and intra-aortic balloon pumps
FabiusTiro
Anesthesia Workstation
Developed to be used in facilities where space is a premium Fabius Tiro combines the
latest ventilation and gas delivery technologies with an ergonomic and compact design.
Adding the ability and the functionality of the Draeger Infinity patient monitoring range
allows you to create yourself a workplace environment that is a sound investment into the
future. The open modular architecture and the software and hardware upgrade options
ensure, that your anesthesia workplace fulfills your expectations, even if you are not in
the main operating room.
Wall Mount
Offering an additional wall mount option allows for covering a broad
range of special segments and ensures that the known and well
perceived modularity within the Fabius family reaches a new dimension.
From induction rooms to alternate hospital sites up to the vast amount of
small outpatient surgery operating rooms Fabius Tiro will suite your
needs
FabiusGS
Anesthesia Workstation
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Dräger has been at the forefront of anesthesia system design for over 100 years. The
Fabius GS anesthesia system is the latest development in the evolution of anesthesia
technology. Designed for routine anesthesia practice, the Fabius GS combines the latest
ventilation and gas delivery technologies with an intuitive and familiar user interface.
Features
Fabius GS is a Simply Smarter anesthesia solution because it integrates:
• an ergonomic design of all components
• an economical and upgradable architecture
• an intelligent safety concept
• an optimized workstation structure
• a simple and intuitive user interface
• a highly advanced ventilator
• an innovative gas delivery module
21.3-Establish anesthesia machines
The most modern anesthesia machine we fond in the hospitals was the norkomed 6400
and 6000 in king fasil hospital and king fahad hospital.
In the military hospital, they have a norkomed m and norkomed GS and norkomed 4000.
The oldest anesthesia machine was in prince sulman hospital and shemacy hospital the
had a norkmed 2B and norkomed 4000.
Most of the private hospitals have the norked 4000 and norkmed 6000.
21.4-Emerging anesthesia technology:
We fond this article describing how a 2010 opreation room would look like…
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Welcome to the Operating Room at Utopia General Hospital. You have come to check
out our anesthesia machines? Here, take a look:
The anesthesia workstation consists of two components, the ventilator and the Anesthesia
Information System, or AIS. Allow me to show you some of the features of the
workstation:
21.4.1-THE AIS IS KNOWLEDGEABLE
The AIS knows who I am
At the start of the day, I show my thumbprint to the AIS, so it recognizes me. This
automatically sets the display and the alarms to my personal preferences, arranges that
messages for me are forwarded to the AIS, and sets the security level of the machine to
the staff anesthesiologist setting
The AIS knows the patients
The AIS accesses the OR Schedule for the day from the hospital network. As each patient
enters the OR, a nurse scans the barcode on the patient's ID band to confirm that we have
the correct patient, and to notify the network that the patient had entered the OR. The
PACU will scan the patient when he or she arrives in that unit, so that the family can be
paged to let them know that the surgery has been completed.The AIS now displays the
patient's relevant information: Age, weight, medications, drugs, allergies, lab results, and
notes from previous anesthetics. Abnormal findings from the preoperative questionnaire
are highlighted. New information, since I reviewed the chart at home last night, is also
highlighted. If necessary, the whole previous chart can be called up.
The AIS knows the operation and the surgeon
It shows the average duration of surgery, and data on the usual blood loss. It can show the
surgeon's anesthetic preference ("Always needs nasogastric tube" "Give cefazolin if not
allergic") and previous anesthetists' comments ("Can do this case with laryngeal mask").
The AIS knows how I am likely to give the anesthetic
For most cases, almost the entire chart can be filled in by the AIS. It knows the usual
drugs I use for this type of case, and the typical doses I would use, given the patients age,
weight and medical history. For children, I allow it to calculate the doses based on the
patient's weight. If I decide to give a different dose, based on the patient's response to
titration, then I have to edit the anesthesia chart appropriately, but often I just confirm
that I have given the dose as calculated. If I enter a drug on the chart before I give it, the
AIS will check this information against a database of doses, interactions with drugs the
patient has taken recently, and the known patient allergies. This provides a useful
safeguard. I enter information about the airway management, using menus based on my
known style of practice, and the monitors write to the anesthesia record automatically.
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21.4.2-THE AIS IS CONNECTED
The AIS is the anesthesiologist's communication center
It allows and controls access to telephone, voicemail, local e-mail and the Internet.
Instant messaging can be used to contact anesthesiologists working in other rooms in the
OR suite, or anywhere else in the world. Videoconferencing between ORs is also
possible. Of course, any other monitor on the network can be called up on any other AIS.
The AIS is connected to my P3
P3? Sorry - that's a combined Phone, Pager and Personal Digital Assistant. While I am in
the OR all my phone calls and e-mail are routed to the AIS, which intelligently filters
them. Only calls from certain numbers, or with certain security codes, get forwarded to
me in the OR. The level of filtration depends on how busy I am. Only emergency calls
get through during intubation, emergence, or if the patient is unstable. During long boring
cases with stable vital signs almost all calls are allowed to go through. If I have to leave
the OR, or if I am supervising more than one operating room, alarms on any of the AISs I
am responsible for generate a page on my P3. The alarm system uses intelligent filtering.
For example, if a resident is in the OR and the 02 Sat drops below 90, I have set the
system up so the resident has one minute to correct the problem. If the trend is not
upwards after a minute, I am alerted. If the patient is a child, or if the resident is in his
first year, I am warned earlier.
Display options
I am experimenting with a heads-up display built into my spectacles, which allows me to
view the monitor screen of any AIS from anywhere in the OR suite, projected onto the
lens of my spectacles in such a way that the screen appears to be floating in the air about
six feet in front of me.
The Operating Room table includes the BP machine and is connected to the AIS
The patient position can be controlled from the AIS, and is automatically entered in the
chart. The non-invasive blood pressure machine is built into the OR table. The patient's
BP cuff, the ECG, pulse oximeter and other monitors are plugged into the OR table which
communicates wirelessly with the AIS, so there are no cords between the patient and the
AIS.
The suction is connected to the AIS
A small monitor wraps around the suction tubing, measuring flow rates and hemoglobin
concentration in the suction fluid, dynamically calculating the blood loss and the patient's
hemoglobin concentration. These figures are displayed on the AIS and will generate
appropriate alarms.
The AIS has a built-in printer
Sometimes a paper document is still the easiest way of recording or transmitting
information. One of the main uses of the AIS printer is to give the patient a copy of the
anesthesia record.
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The AIS has a floppy drive
Floppies are still not quite extinct. Data can nearly always be transmitted over the
network. One function of the floppy drive is to produce a tamper-proof authenticated
medical record in the case of an undesired outcome. The record is put in an electronic
"envelope" so that it cannot be tampered with. In this way the AIS functions rather like
the "black box" flight data recorders on aircraft, documenting everything that happened
prior to an incident.
The Weather Camera
This, I must admit, is a bit of a gimmick. The Quality of Working Life, Recruitment and
Retention Committee approved it, as our new ORs, with real windows, will not be ready
for another couple of years. In a corner of the monitor, I can display a view of the current
weather, from one of three cameras on the hospital roof, along with temperature,
humidity and wind information. It's nice to know, especially on winter days when I arrive
before dawn and leave after sunset. The little humanoid figure is advises what to wear
when leaving the hospital. In summer it notes if sun block is needed, and in winter it
gives a wind-chill warning.
The AIS is connected to expert systems
If the AIS notes abnormal vital signs, it interprets them and suggests possible causes and
treatments. For example, if the patient develops a fever, a protocol for investigating and
treating malignant hyperthermia appears in a window on the screen.
21.4.3-THE AIS IS INTELLIGENT (IN A LIMITED FASHION)
Automatic Ventilation
Instead of setting a tidal volume and rate, mostly I just allow the AIS to tell the ventilator
to ventilate to my preferred end-tidal CO2 reading. It gives a couple of test breaths, based
on the patient's weight, then measures the compliance and decides on an optimum
respiratory pattern. It tells me what it has decided, and I press "confirm" to signal my
agreement. If anything changes by more than 10%, the AIS is programmed to alert me.
Automatic Scheduling
The AIS shows me when it anticipates that we will have finished the scheduled list. This
is based on the surgery, the surgeon, the number of nurses, cleaners and porters available,
as well as my work habits. The finish time is not always accurate, but it is close enough
to predict patient flow through PACU and the Day Surgery Unit, so that the lists can be
re-arranged or nursing staff rescheduled to avoid bottlenecks.
The Overall Physiological Score (OPS)
The OPS is a way of integrating all the available data related to the patient's status into a
single number. The AIS considers the O2 Saturation, the Blood Pressure, Heart Rate and
data from any other monitors in use to give a single number, ranging from 100% - ideal
health - to 0%, which is death. If the OPS is over 95, it is hardly necessary to check the
rest of the data - everything must be close to normal. The average OPS during the course
of an anesthetic can be used as a measure of the quality of anesthesia.
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Productivity and approval rating
This was very controversial when it first came out. The rating is based on turnover times,
adverse events such as abnormal blood pressure readings or nausea in the PACU, and
feedback from patients, surgeons and nurses. Once the data was available as part of the
electronic record, it was inevitable that administration would begin to use it to evaluate
staff and physician performance. So it was decided that everyone should be able to access
their own rating, and be able to see the scores of other members of their group, but with
no names attached. It was also decided to use the ratings only to reward top performers,
not to punish those who fared poorly. For example, the hospital is funding an allexpenses-paid
trip to the ASA Annual meeting for the top performing anesthesiologist.
The AIS is a powerful research tool
For example, if I decide I need to compare two anti-emetic agents for routine use, the AIS
can select suitable patients, find matching controls, and gather the necessary outcome
data. In fact, it becomes very easy to make almost every case part of a research protocol
The AIS does my billing for me
It knows the name and insurance details for each patient, and bills from five minutes
before the patient enters the OR till ten minutes after. I can alter these times if necessary.
If the monitor detects an arterial waveform, it bills for insertion of an arterial line. This
feature is called "plug and pay".
The AIS generates postoperative orders
It also generates the postoperative PCA orders for me to confirm, and sends a message,
with all the relevant patient details, to the Acute Pain Service anesthesiologist once the
patient is in PACU.
At present there are no vacancies for anesthesiologists at the Utopia General Hospital, but
if you are interested I could put your name down on the waiting list.
21.5-Visionary technology:
Most of the future devolpment in the anesthesia machines is on computer and workstation
part of the machine.
Developments in fast, inexpensive, small, powerful computers, wireless technology, and
the Internet are revolutionizing anesthesia in many ways including better patient
monitoring, easier, more accurate record keeping, and improved patient care through the
use of expert systems.
One of the future technology is a integrated system that is capable of new application of
artificial intelligence systems in the management of anesthetic techniques. Such systems
can be programmed to provide "smart alerts," recognizing patterns or spotting trends in
physiologic parameters during anesthesia. Through the use of artificial intelligence
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systems, defined data relationships and parameters can be tied to statistical processcontrol
charts. Anesthetic machines can thus be "taught" to respond automatically to
designated deleterious events.
anesthesia delivery will be driven by software integrated with the electronic health record
and with monitoring to assist the provider to avoid potential pitfalls. It will be updating
constantly, providing real-time integration of past and present patient history, with backup,
of course.
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REFERENCE
. Basic physics and Measurements in Anaesthesia:
§ Third Edition
§ GD Parbook
§ PD Davis
§ EO Pabrook
. Technical Manual of Anesthesiology An Introduction for:
. ECRI
. Service manual of Draeger.
. King Fahad Medical City
§ James E.Heavner
§ Craig Flinders
§ Dennis Mcmahom
§ Tim Branigan.
. Draeger medical company (Riyadh, Germany)
. Lectures with Dr.Nabeel AL-Rajeh from 414 BMT
. National guard hospital
. http://www.rpresearch.ca/?Second=anaesthesia%20general%20smokin
g&Top=General%20Anesthesia
. http://www.udmercy.edu/crna/agm/05.htm
11. http://gasnet.med.yale.edu/machine/part1.htm
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