Aanesthetic Agents for Day Surgery - NIHR Health Technology ...

Background
The empirical study required accurate and
detailed information on the quantities of anaesthetic
used. For volatile anaesthetics, it is not
possible to measure the quantity administered
in terms of volume. The carrier gases, normally
oxygen and N 2O, pass through an agent-specific
vaporiser mounted on the anaesthetic machine
in which the liquid anaesthetic agent is vaporised
and added to the stream of gases. The combination
of gases and anaesthetic vapour is carried
to and from the patient’s lungs by a system of
tubing known as the ‘breathing system’. The
vaporisers used to administer volatile anaesthetics
are calibrated to show the percentage concentration
of vapour delivered to the breathing system
rather than the volume of liquid agent delivered
in millilitres. The anaesthetist intermittently varies
the concentration of agent delivered to the breathing
system by adjusting the dial on the vaporiser
according to the needs of the patient. The volume
of liquid agent delivered to the patient depends,
inter alia, on the flow of carrier gases passing
through the vaporiser (known as the ‘fresh gas
flow’) and the percentage concentration of
vapour set on the vaporiser dial.
The type of breathing system is relevant to the
economy with which the anaesthetic agent is
utilised. There are broadly two types. In a nonre-breathing
system, a large flow of anaesthetic
gases and vapour is carried to the patient’s lungs.
An amount of anaesthetic agent and oxygen is
taken up by the patient and the remainder is
vented to the atmosphere. In a re-breathing
system the ‘waste’ gases and vapour exhaled
by the patient are recycled continuously by
immediately directing the exhaled breath through
a low-resistance carbon dioxide-absorbing canister
and thence back to the patient. This system is
commonly described as a ‘circle system’. All that
has to be added to a circle system is the amount
of oxygen (approximately 200 ml/min) and the
amount of anaesthetic vapour actually taken up
by the patient with each breath. The predominant
advantage of reducing the fresh gas flow to a
minimum is extreme economy of use. A second
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Appendix 18
Health Technology Assessment 2002; Vol. 6: No. 30
Determining the quantity of the
volatile anaesthetic agents used
advantage is the considerable reduction in the
release of N 2O and anaesthetic halogenated
hydrocarbons into the atmosphere. A potential
disadvantage is that it is difficult rapidly to vary
the amount of anaesthetic agent available to the
patient if, for example, the severity of the surgical
stimulus suddenly increases and it becomes necessary
to quickly deliver more anaesthetic to the
patient to avoid excessively light anaesthesia.
In these circumstances it is usual to increase
the fresh gas flow for a few minutes in addition
to increasing the vaporiser setting. During the
first few minutes of anaesthesia, the uptake of
anaesthetic agent by the patient’s tissues is very
high. In order to provide sufficient anaesthetic
agent, a relatively high fresh gas flow is required
initially and, subsequently, the flow is progressively
decreased by the anaesthetist. To ensure that the
appropriate anaesthetic concentration is achieved
in the circle breathing system, the actual percentage
of vapour being delivered to the patient is
routinely measured on a breath-by-breath basis
throughout each anaesthetic using an accurate
monitor. In practice, circle systems are seldom
used in their most parsimonious ‘closed’ state and
a fresh gas flow is chosen which achieves the best
compromise between economy on the one hand
and the ability to vary the amount of anaesthetic
agent available to the patient on the other.
Previous studies have attempted to estimate the
cost of anaesthetic agents by simply measuring
the volume of liquid agent used. 22 Weighing the
vaporiser containing the volatile anaesthetic
before and after anaesthesia is the most accurate
way to calculate the quantity of volatile anaesthetic
agent used. 28 However, this process is logistically
demanding and time consuming and was considered
on these grounds to be inappropriate for
the large numbers of patients recruited to the
current study. The Dion formula was developed
and has been used in several studies to estimate
the quantity and cost of volatile anaesthetic
agents administered using machines with no
system to allow re-breathing of the vapours. 24,220
The widespread introduction of circle systems
into anaesthetic practice 246 has generated some
uncertainty in the CESA team of investigators
215