DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

and left-to-right shunting of cardiac output, oxygen supplementation
must be regulated carefully because of the risk of further reducing
pulmonary vascular resistance and increasing pulmonary blood flow.
Metabolism. Inhalation of 100% O 2
does not produce detectable
changes in O 2
consumption, CO 2
production, respiratory quotient,
or glucose utilization.
Oxygen Administration
Oxygen is supplied as a compressed gas in steel cylinders,
and a purity of 99% is referred to as medical
grade. Most hospitals have oxygen piped from insulated
liquid oxygen containers to areas of frequent use.
For safety, oxygen cylinders and piping are color-coded
(green in the U.S.), and some form of mechanical
indexing of valve connections is used to prevent the
connection of other gases to oxygen systems. Oxygen
concentrators, which employ molecular sieve, membrane,
or electrochemical technologies, are available for
low-flow home use. Such systems produce 30-95%
oxygen depending on the flow rate.
Oxygen is delivered by inhalation except during
extracorporeal circulation, when it is dissolved directly
into the circulating blood. Only a closed delivery system
with an airtight seal to the patient’s airway and complete
separation of inspired from expired gases can precisely
control FI O2
. In all other systems, the actual delivered
FI O2
will depend on the ventilatory pattern (i.e., rate, tidal
volume, inspiratory–expiratory time ratio, and inspiratory
flow) and delivery system characteristics.
Low-Flow Systems. Low-flow systems, in which the oxygen flow is
lower than the inspiratory flow rate, have a limited ability to raise the
FI O
because they depend on entrained room air to make up the balance
of the inspired gas. The FI O
2
of these systems is extremely sensitive
to small changes in the ventilatory pattern. Nasal
2
cannulae—small, flexible prongs that sit just inside each naris—
deliver oxygen at 1-6 L/min. The nasopharynx acts as a reservoir for
storing the oxygen, and patients may breathe through either the
mouth or nose as long as the nasal passages remain patent. These
devices typically deliver 24-28% FI at 2-3 L/min. Up to 40% O 2 FIO 2
is possible at higher flow rates, although this is poorly tolerated for
more than brief periods because of mucosal drying. The simple face
mask, a clear plastic mask with side holes for clearance of expiratory
gas and inspiratory air entrainment, is used when higher concentrations
of oxygen delivered without tight control are desired. The
maximum FI O
of a face mask can be increased from around 60% at
2
6-15 L/min to > 85% by adding a 600- to 1000-mL reservoir bag. With
this partial rebreathing mask, most of the inspired volume is drawn
from the reservoir, avoiding dilution by entrainment of room air.
High-Flow Systems. The most commonly used high-flow oxygen
delivery device is the Venturi-style mask, which uses a specially
designed mask insert to entrain room air reliably in a fixed ratio and
thus provides a relatively constant FI O
at relatively high flow rates.
2
Typically, each insert is designed to operate at a specific oxygen flow
rate, and different inserts are required to change the FI . Lower
O 2
delivered FI O
values use greater entrainment ratios, resulting in
2
higher total (oxygen plus entrained air) flows to the patient, ranging
from 80 L/min for 24% FI to 40 L/min at 50% O
. While these
2 FIO 2
flow rates are much higher than those obtained with low-flow
devices, they still may be lower than the peak inspiratory flows for
patients in respiratory distress, and thus the actual delivered O 2
concentration
may be lower than the nominal value. Oxygen nebulizers,
another type of Venturi-style device, provide patients with
humidified oxygen at 35-100% FI O
at high flow rates. Finally, oxygen
blenders provide high inspired oxygen concentrations at very
2
high flow rates. These devices mix high-pressure compressed air and
oxygen to achieve any concentration of O 2
from 21-100% at flow
rates of up to 100 L/min. These same blenders are used to provide
control of FI O
for ventilators, CPAP/BiPAP machines, oxygenators,
2
and other devices with similar requirements. Again, despite the high
flows, the delivery of high FI O
to an individual patient also depends
2
on maintaining a tight-fitting seal to the airway and/or the use of
reservoirs to minimize entrainment of diluting room air.
Monitoring of Oxygenation. Monitoring and titration are required
to meet the therapeutic goal of oxygen therapy and to avoid complications
and side effects. Although cyanosis is a physical finding of
substantial clinical importance, it is not an early, sensitive, or reliable
index of oxygenation. Cyanosis appears when ~5 g/dL of deoxyhemoglobin
is present in arterial blood, representing an oxygen saturation
of ~67% when a normal amount of hemoglobin (15 g/dL) is
present. However, when anemia lowers the hemoglobin to 10 g/dL,
then cyanosis does not appear until the arterial blood saturation has
decreased to 50%. Invasive approaches for monitoring oxygenation
include intermittent laboratory analysis of arterial or mixed venous
blood gases and placement of intravascular cannulae capable of continuous
measurement of oxygen tension. The latter method, which
relies on fiber-optic oximetry, is used frequently for the continuous
measurement of mixed venous hemoglobin saturation as an index of
tissue extraction of oxygen, usually in critically ill patients.
Noninvasive monitoring of arterial oxygen saturation can be
achieved using transcutaneous pulse oximetry, in which oxygen saturation
is measured from the differential absorption of light by oxyhemoglobin
and deoxyhemoglobin and the arterial saturation
determined from the pulsatile component of this signal. Application
is simple, and calibration is not required. Pulse oximetry measures
hemoglobin saturation and not PO 2
. It is not sensitive to increases in
PO 2
that exceed levels required to saturate the blood fully. Pulse
oximetry is very useful for monitoring the adequacy of oxygenation
during procedures requiring sedation or anesthesia, rapid evaluation
and monitoring of potentially compromised patients, and titrating
oxygen therapy in situations where toxicity from oxygen or side
effects of excess oxygen are of concern. Near infrared spectroscopy
(NIRS) is a noninvasive technique being used to monitor oxygen
content in the cerebral cortex. Unlike pulse oximetry NIRS measures
all reflected light in both pulsatile arterial blood and nonpulsatile
venous blood, the primary compartment in the cerebral
vascular bed. NIRS is useful to monitor cerebral oxygenation in surgical
procedures involving cardiopulmonary bypass and circulatory
arrest (Guarracino, 2008).
Complications of Oxygen Therapy. Administration of supplemental
oxygen is not without potential complications. In addition to the
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CHAPTER 19
GENERAL ANESTHETICS AND THERAPEUTIC GASES