Journal of the New Zealand Medical Association, 16-May-2003, Vol 116 No 1174
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.
Data 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.
Data 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.
We 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: firstname.lastname@example.org
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