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| Journal of the New Zealand Medical Association,
16-May-2003, Vol 116 No 1174
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An audit of anaesthetic fresh-gas flow rates and volatile
anaesthetic use in a teaching hospital
Ross Kennedy and Richard French
Inhalational anaesthetic agents such as isoflurane and
sevoflurane feature frequently in the top ten drugs by cost for hospital
pharmacies. Although the high total cost of their use in part represents the
large number of patients receiving them, it is still important that
anaesthetists use these drugs in a responsible and cost-effective way.
All inhalational anaesthetic agents introduced into clinical
practice over the past 40 years are closely related. In order of introduction
into clinical practice they are halothane, enflurane, isoflurane, sevoflurane
and desflurane. The differences between these drugs are in solubility, which
affects the speed of onset and offset, degree of metabolism and some side
effects. In general, the newer agents have faster onset and offset, and less
reliance on metabolism.
The ‘circle’ type anaesthetic circuit (Figure 1)
is in common use worldwide. This circuit allows expired gases to be reused after
CO2 is removed. The anaesthetist determines the
concentration of agent they wish to deliver to the patient and sets the vapour
content and flow rate of fresh gas into the circuit to produce this
concentration in the circle circuit. The speed with which the agent
concentration in the circuit changes towards that of the fresh gas depends on
the flow rate of fresh gas into the circuit, the volume of the circuit, and the
uptake of agent by the patient. The low solubility of isoflurane and sevoflurane
in blood means that after an initial wash-in phase of five to ten minutes their
uptake decreases markedly.
Figure 1. Schematic diagram of a circle breathing
system. Fresh gas from the anaesthesia
machine enters at F and moves clockwise around the circuit. Direction of flow is
controlled by one-way valves at V. The breathing bag, B, acts as a reservoir
during spontaneous ventilation and can be used to provide positive pressure or
be replaced by a ventilator. The patient connection is at P. Carbon dioxide is
removed in the absorber at C. Excess gas is ‘lost’ at W. Other
arrangements of the components are possible.
 Since the volume of the circuit remains constant from breath
to breath, almost the same volume of gas as is added to the circuit is lost (the
actual difference will be 200–250 ml/min representing oxygen uptake, plus
a small volume representing anaesthetic vapour uptake by the patient). This
‘waste’ gas contains considerable amounts of anaesthetic agent (and
oxygen).
The anaesthetist chooses a point on the continuum between a
high fresh-gas flow rate, which results in rapid changes in gas concentrations
within the circuit and consequent significant wastage of an equivalent volume of
gas, and a low fresh-gas flow rate, which is more economical but engenders a
considerable time lag before the circuit concentration changes. The extreme
example of low flow rates is a ‘closed’ circuit in which inputs to
the circuit exactly match uptake by the patient and no gas is wasted. Although
there are particular advantages unique to a closed circuit, the main economic
advantage is seen in the reduction of flows from traditional levels of 4–6
l/min towards levels of 1–2 l/min.
The use of
very low flows, however, makes the response time of the system slow. Even at
moderately low flows of 1 l/min a given change will be only 87% complete after
16 minutes (two time constants). This delay may be shortened by initially adding
a large amount of the anaesthetic agent to the gas mixture and then reducing the
amount as the inspired concentration approaches the desired level. Technical
issues, which are now largely historical, such as flowmeter accuracy, system
leaks and the availability of accurate oxygen and anaesthetic agent monitors
have also made an impact on the use of low fresh-gas flows (FGFs). The residual
effect of these various issues means that use of low flows is still considered
by some practitioners to be difficult, prone to error, and the domain of the
enthusiast.
The use of low flows varies by region and hospital.
Christchurch would not be considered a region in which low flow use is
particularly promoted, although these techniques do have local enthusiasts.
Various techniques have been described to encourage the use of low flows,
including reporting total consumption to a whole
department,1 reporting individual
usage,2 and introducing practice
guidelines.3 These approaches often require
considerable effort and, once feedback or constant attention ceases,
practitioners may revert to old habits.1,4 Our
perception prior to undertaking this audit was that moderate to high FGF rates
are commonly used in our department.
In recent years, new anaesthetic machines have been
introduced into Christchurch. These ‘anaesthesia work stations’
possess very accurate FGF systems and have minimal gas leaks, which, combined
with sophisticated gas monitoring already in place, have markedly reduced the
technical difficulty of practising low-flow anaesthesia. These machines are
largely electronically controlled and as a result many of the various gas
settings are accurately recorded and can be collected by a computer.
The purpose of this study was to audit the actual FGF rates,
and choice and consumption of anaesthetic agents in our department. We planned
to feed back the results and reassess anaesthetic gas use as part of the
department quality assurance programme.
MethodsData were collected from the
anaesthetic machine (Datex ADU) and fed into a computer placed on the base of
the machine. The computer was contained in a slimline ‘pizza box’
case without keyboard or monitor (Macintosh LC). Data were requested from the
ADU every 10 seconds using a locally developed application. The data stream was
broken into its components and the following data were recorded: flow rates of
oxygen, nitrous oxide, and air; vapour in use and vaporiser setting; and total
FGF. The system was designed to automatically restart after any interruption of
the power supply.
Data recording took place in one theatre at Christchurch Hospital for two periods of one month during 2001. Christchurch Hospital has 11 operating theatres and the chosen theatre is used primarily for general and vascular surgical cases, both elective and acute. It is the primary out-of-hours theatre and hence is used by a wide range of specialists and trainees. The presence of ancillary computer or monitoring equipment is commonplace in this theatre and the introduction of the system used in this study did not elicit queries or comment. The specific purpose of the data collecting system was not advised to the department during the first study period. After the results from the first period had been analysed, these and similar data collected in another hospital were presented to the department. During the second period of data collection, a prominent notice on the machine announced ‘Gas Flow Audit in Progress’. At the end of the data collection period, data were transferred to another computer and run through the analysis program. This second program sorted and counted each 10-second sample. The pattern of gas use was categorised as follows: ‘machine off’ (no gas flow); ‘oxygen only’; or ‘GA’ (oxygen plus any other gas such as air, nitrous oxide, or a volatile anaesthetic). A count was kept of the number of samples in each category thus allowing the calculation of time spent in each state. Total FGF for each 10-second period was sorted into various ‘bins’ (0–0.5, 0.51–1.0, 1.01–1.5, 1.51–2, 2.01–2.5, 2.51–3, 3.01–4, 4.01–5, 5.01–6, 6.01–8, 8.01–10, >10 l/min) and a count kept of the number of samples in each bin for total gas flow and each volatile agent. This method of data processing was to allow construction of a distribution curve for overall gas usage with each volatile anaesthetic. Finally, total FGF for each 10-second period whilst in the GA state was recorded to provide cumulative totals of gas use with both isoflurane and sevoflurane, thus allowing calculation of mean gas flows with each agent. ResultsData were collected for 781 hours
(32.5 days) commencing 1 March 2001, and 826 hours (34.4 days) commencing 1
November 2001. Utilisation was similar, with more than one gas (GA) being in use
for 27% (207 hr) and 28% (232 hr) of the total time respectively. The time when
oxygen was being used alone reduced from 13% to 9% between the
periods.
Between the study periods, hours of sevoflurane use
increased, and mean flow rates with sevoflurane decreased slightly as shown in
Table 1. Overall, the mean gas flow was 2.0 l/min in March and 2.1 l/min in
November.
Table 1. Comparison of total fresh-gas flow rates (FGF)
and choice of anaesthetic agent between the two study periods
The total amount of time classified as GA increased 12%.
Correcting for this increase, sevoflurane use by time increased 10.5%. This
increase in the time sevoflurane was in use was offset by a 3.5% reduction in
FGF resulting in a net increase in consumption of only 6.6%. In contrast,
isoflurane was used for 7.7% less of the total time but, because mean gas flows
increased by 23%, total consumption increased by 13.7%.
Figure 2. Graphical presentation of cumulative
percentage of fresh-gas flows used with sevoflurane and isoflurane for the two
study periods (March 2001: open symbols;
November 2001: closed symbols)
  The impact of notification of the flow monitoring process is
best represented in a graph of the cumulative percentage of flow rates in each
bin (Figure 2). This demonstrates that when comparing sevoflurane flow rates
between the two study periods the maximum difference (15.5%) occurs in the
1.01–1.5 l/min flow category. In contrast, the isoflurane usage remains
similar until the 3.01–4 l/m bin where a 12% difference is
observed.
Statistical analysis
The statistical analysis of these data is difficult as it is debatable
whether the data points recorded are independent. It could be argued that the
flow data arising from within a single anaesthetic are not independent. However,
the use of a flow rate at any particular moment is governed by the
anaesthetist’s assessment of the clinical situation, and is not especially
dependent on the flow rate previously used. If the data points are considered to
be independent the appropriate test would be a Kolmogorov-Smirnov 2 sample
test.5 Applying this test to data where there
is a degree of dependence between data points increases the likelihood that the
null hypothesis (no difference in flow-rate usage) is falsely rejected. We
considered it would be instructive to apply the test whilst acknowledging this
caution. The Kolmogorov-Smirnov test defines the difference in proportion
between samples required to be significant at a given alpha (chance of falsely
rejecting the null hypothesis). For an alpha of 0.001 the critical difference in
proportion for the sevoflurane data is 0.017 (1.7%), which means that the
observed difference of 15.5% is highly significant if the data points are
independent. For the isoflurane data the critical difference in proportion is
0.019 (1.9%), again much less than the maximum observed difference.
DiscussionWe were pleasantly surprised by the
relatively low average gas flows from the first part of this audit. To achieve a
mean FGF of 2 l/min in a typical case requires the FGF to be 1 l/min or less
during the maintenance phase of anaesthesia. Although cost limitation is an
important reason for reducing anaesthetic gas flows, there are several other
significant reasons to minimise the use and wastage of anaesthetic agent.
Volatile anaesthetics are all halogenated hydrocarbons and are all potential
ozone-depleting agents. The agents are the product of complex chemical processes
that in themselves may be bad for the environment. Nitrous oxide is a greenhouse
gas. Although the risk of occupational exposure has not been clearly defined,
these are all drugs that in experimental conditions can do
harm.6 It is therefore important to keep
workplace exposure to a minimum and one important way of doing this is to keep
FGFs as low as possible.
The results show that our department is using these agents
responsibly. Given the need for higher FGFs at the beginning and end of a case,
an overall average of around 2 l/min means that gas flows during the maintenance
phase are around 1 l/min. Below this level there is little financial gain since
total consumption of anaesthetic vapour is the product of FGF and vaporiser
setting and at total flow rates <1 l/min the vaporiser setting needs to be
increased to maintain inspired concentration. The major cost gain is seen in
reducing gas flows from the traditional levels of 4–6 l/min down to
1–2 l/min.
McKenzie
used education and feedback of total isoflurane consumption in a department to
reduce volatile anaesthetic consumption by 65%, but the methodology of this
study did not involve the collection of FGF
rates.1 Assuming that the same anaesthetic
‘dose’ of isoflurane was being delivered it seems likely that the
pre-education gas flow rates were relatively high, as if they been low it would
possibly have proved difficult to reduce isoflurane consumption this much. Body
and others provided individualised feedback on FGFs at the midpoint of
anaesthesia.2 They saw a 26% reduction in gas
flows to 1.8 l/min. Lubarsky and others introduced a wide range of practice
guidelines, one aspect of which was the use of an FGF of 1
l/min.3 Our study also attempted to assess the
effect of an intervention on fresh-gas usage. Between the study periods, results
were discussed with the department as a group, and the second study period was
clearly advised. The subsequent analysis of the data showed that average flows
in the first period were low and this left relatively little room to move in
terms of further reducing flow rates by an educational intervention. The
cumulative graph for sevoflurane does, however, reveal an interesting change
between the two study periods. In March, 48% of observed flow rates were below
1.51 l/min, whilst in November 63% of flow rates were below this figure. When
considering isoflurane the figures are somewhat different with similar
cumulative percentages observed for the lower flow rates, with separation only
being observed at the 3.01–4 l/min flow rate. Prior to instituting the
study, the confounding effect on statistical analysis of uncertain independence
of the data points was not appreciated by the investigators. The large number of
data points collected means that if they are considered as independent the study
would be capable of detecting very small changes in practice. However, whilst we
have presented the statistical analysis as a point of interest we feel it should
be treated with some caution. In terms of clinical relevance the data shows
that, regardless of the effect of our intervention, flow rates in use are
satisfactory.
One interesting result was that there appeared to be large
periods of time when oxygen was the only gas running through the machine. It is
usual to administer 100% oxygen at the beginning and end of a case; however, a
further analysis of the data suggested that the oxygen was being left running
for long periods of time between cases. In one specific incident occurring
overnight one weekend this continued for 14 hours. We drew this issue to the
attention of anaesthetists and technicians and note the reduction in the
‘oxygen only’ time between the study periods. Although this
represents a large amount of oxygen wasted, the total cost of this waste is
surprisingly low since liquid oxygen costs less than $1 per cubic meter or 0.1
c/l.
Modern anaesthetic equipment facilitates reduced gas flows.
Continuous monitoring of inspired oxygen levels has been available for some
time; more recently, anaesthetic agent monitoring on a breath-by-breath basis
has become commonplace and has been required by the monitoring standards of the
Australian and New Zealand College of Anaesthetists for a number of years. New
types of anaesthetic machines have minimal leaks and will not actually function
with a leak of more than 100 ml/min. It is anecdotally reported that in the past
some centres tolerated anaesthetic machines with leaks of up to 1–2 l/min.
Other advantages of modern anaesthetic machines include ventilators that are
able to take account of the compliance of the breathing circuit, more
sophisticated models of ventilation, and integration of gas delivery, ventilator
monitoring and alarms.
Reduced anaesthetic gas flows can represent a significant
financial saving and may have significant local and global environmental
effects. With modern equipment they can be achieved simply and at no risk to the
patient. In addition, the use of low flows allows cost savings while still
allowing the clinician the option of using more modern (and therefore more
expensive) drugs. This audit has shown that FGFs in our hospital are at a
satisfactory level (at least as low as other published department-wide audits),
and that the more expensive agent, sevoflurane, is being used in a responsible
and economical manner.
Author information:
Ross Kennedy, Specialist Anaesthetist, Department of Anaesthesia, Christchurch
Hospital, and Clinical Senior Lecturer, Department of Anaesthesia, Christchurch
School of Medicine and Health Sciences, University of Otago; Richard French,
Specialist Anaesthetist, Department of Anaesthesia, Christchurch Hospital,
Christchurch
Acknowledgements: We
thank Elisabeth Wells, Biostatistician, for her advice during the preparation of
this manuscript.
Correspondence: Dr
Ross Kennedy, Department of Anaesthesia, Christchurch Hospital, Private Bag
4710, Christchurch. Fax: (03) 364 0289; email: ross.kennedy@chmeds.ac.nz
References:
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