THE USE OF NITROUS OXIDE IN MINIMAL FLOW ANAESTHESIA
G.G. Lockwood,
London.

Nitrous oxide can be used in low flow breathing systems with a moderate fresh gas flow with little difficulty. For instance, if a flow of 500 ml min"1 oxygen and 500 ml min'1 nitrous oxide is used then the inspired concentration of nitrous oxide rises towards 60% over the first hour and rarely exceeds 70%. On the other hand, the problems of its use in completely dosed systems is well known: after 30-40 min the patient uptake of nitrous oxide becomes less than that of oxygen, so nitrous oxide accumulates in the breathing system, generating a hypoxic mixture if the fraction in the fresh gas is not reduced with time [1]. Although the decline in oxygen concentration is gradual and can be prevented by an alert anaesthetist, attempts have been made to develop a different compromise between safety and simplicity. Virtue used 300 ml min '' oxygen and 200 ml min'1 in nitrous oxide in a minimal flow circle system but found that the inspired oxygen concentration sometimes fell below 30%, especially with his larger patients [2]. In fact he found little difference between this regime and the use of 500 ml min"1 each of oxygen and nitrous oxide.
Our interest in trunk ventilation has led us to an alternative approach. It is common practice to utilise a trunk of corrugated tubing to prevent mixing of the gas in a low flow breathing system and the driving gas of a ventilator. If no conventional fresh gas is supplied to such a system, titan as breathing system gas is taken up by the patient, so the ventilator driving gas moves along the trunk to replace it. If the driving gas is oxygen, such a system may be used safely and functions as a completely dosed system with oxygen supply exactly matched to consumption [3]. If a driving gas of 50% nitrous oxide in oxygen were to be used then a hypoxic mixture could develop in the breathing system in exactly the same way as in a conventional system. With the conventional system this problem can be overcome by increasing the fresh gas flow. The analogous manoeuvre when the trunk is supplying the fresh gas is to create a leak in the breathing system. The amount of fresh gas supplied by the trunk is now the sum of the total patient uptake and the leak. A convenient controlled leak is afforded by a gas analyser, if the sample is not returned to the breathing system. We have investigated such a system to see if ii overcomes the problem of patient variability when a fixed, conventional fresh gas flow is supplied [4]. We found that the leak provided by a gas analyser was not enough 10 prevent accumulation of nitrous oxide for the inspired oxygen concentration became less than 30% in most cases eventually. We therefore do not have cause to advocate a trunk rather than a conventional fresh gas supply.
References
1. Barton F, Nunn JF. Totally closed circuit nitrous oxide/oxygen anaesthesia. Br J Anaesth 1975; 47: 350-357.
2. Virtue RW, Minimal-flow nitrous oxide anesthesia. Anesthesiology 1974; 40: 196-198.
3. Jordan MJ, Bushman ]A- Closed-circuit halothane and enflurane using an in-circle Goldman vaporizer. Br J Anaesth 198); 53; 1285-1290,
4. Ncl MR, Watts JD, Lockwood GG, An alternative method of nitrous oxide delivery into a minimal flow circle breathing system. In press: Anaesthesia 1996.

 

 

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TRACE GAS ACCUMULATION IN CIRCLES
G. ROLLY

Ghent (Belgium)

It is already known for a long time that during closed circuit anaesthesia, the patients' inspired gas may progressively become contaminated by non-anaesthetic gases, such as nitrogen, volatile degradation products of halothane and carbon monoxide. Also methane, trace amounts of acetone, ethanol, hydrogen and argon, can appear in the patients' expired gases.
Nitrogen accumulation can appear during washout from body tissue stores, particularly if insufficient preoxygenation has been done. Alternatively, nitrogen can be suctioned into the closed circuit anaesthesia apparatus through leaks, if the system is not airtight. It was reported by Barton and Nunn in 1957, that nitrogen concentration during closed circuit anaesthesia gradually increased to 3.5 % after 80 minutes. Using a sophisticated experimental closed circuit rolling seal spirometer, we could evidence that the nitrogen concentration was 6.4 % after 60 minutes and 8.4 % after 120 minutes. Similar studies with the Engstrom Elsa apparatus during minimal flow conditions, showed that the N- accumulation was less and never exceeded 5 %. In an older study, done by Morita et al. (19.85), the influence of the time during which the denitrogenation is performed, was nicely demonstrated. After 6 to 8rfd.nu.tes denitrogenation, the average N., concentration was 6.4-16.2 %, whereas after 33 minutes denitrogenation it was only 1.0-5.1 %; the measurements were always made during closed circuit anaesthesia conditions.
Methane accumulation during closed circuit anaesthesia was already reported by Morita et al., in 1985. Methane is produced by microbial fermentation of carbohydrates under strict anaerobic conditions by Methanobacterium ruminatum. This primarily happens in the distal colon. The intestinal gas in humans contains up to 26 % methane , The human population can be divided into methane producers (1/3 of the adults) and none producers (2/3 of adults), exhaling less than 1 ppm methane. Many factors can influence the actual expired methane concentration. Morita evidenced methane concentrations ranging from 17 to 22 ppm, with a maximum of 229 ppm after 72 minutes. In our department we could find during rigid closed circuit conditions (PhysioFlex apparatus), an increase of methane of 703 ppm/hour, with a mean concentration of 861 ppm, but 1 patient had a concentration of 1187 ppm. As a side effect of the methane concentrations, present in the circuit, the infrared analyzer, working at a wavelength of 3.3 urn, evidenced "false" halothane values during TIVA conditions. The mean "false" halothane concentration was 1.0 %.
In a further study, it was possible to show that during TIVA some patients gave false halothane readings (2/3 of the patients), whereas others didn't (1/3 of the patients). After 105 minutes of closed circuit anaesthesia, a mean methane concentration of 941 ppm was present. In the low methane excreters, after 60 minutes, only 150 ppm methane was present, without gradual increase, in contrast to the previous group of patients.
Acetone is produced in the liver during metabolism of free fatty acids and is excreted partly by the lungs. During fasting there is an increased pulmonary excretion. The accumulation of acetone, reported by Morita et al., amounted to maximally 5.9 ppm. In our study, the acetone concentrations were on average double of those.
Ethanol in the same way as acetone is also exhaled by the lungs when using closed circuit anaesthesia. If a surgical intervention has to be performed urgently on an alcoholized patient, elimination of ethanol by exhalation would be made impossible by using a closed circuit system. It seems prudent in those circumstances not to reduce the fresh gas flow to low flow conditions.
Accumulation of carbon monoxide in closed circuit anaesthesia systems has already been described by Middleton et al. in 1965, with an average increase to 80 ppm. Carbon monoxide haemoglobin after 6 hours anaesthesia in closed circuit conditions, amounted to 0.5-1.5 % in non-smokers and to 3 % in smokers, as reported by Strauss et al., 1991. Moon et al., reported for the first time intra-operative carbon monoxide toxicity (1990). However, in a clinical trial of more than 1000 patients, no dangerous carbon monoxide haemoglobin concentrations could be found, after minimal flow anaesthesia and sodalime canisters used for several days (Baum et al., 1995). It is actually known that this "monday" disease occurred after long exposure• of the sodalime of unused anaesthetic machines to a high flow of oxygen, whereby extremely dry sodalime is generated. It has been shown that in those conditions sodalime and particularly baralyme can react with some inhalational anaesthetics (desflurane), to generate very high CO concentrations (Fang et al. 1995). Simply avoiding this situation and / or changing the sodalime before use, eliminates this problem. In our own studies, the CO concentration in the closed circuit was after 30 minutes 23.3 ppm with one highest value of 164 ppm. Besides the endogenously formed carbon monoxide as a by-product of haemoglobin catabolism and the reaction of the inhalation anaesthetics to sodalime, exogenous sources are cigarette smoke. Degradation products of volatile anaesthetics, resulting from chemical reaction with sodalime can also be formed during closed circuit or low flow conditions. With halothane, transformation to and accumulation of CF2CBrCl has been reported, without attaining toxic concentrations. Sevoflurane reacts with sodalime to form compound A. This degradation is considerably promoted by the heat which develops in the canister. Studies have shown that with low flow conditions or with closed circuit conditions, the compound A levels were below those supposed to be related with toxicity. In recent studies, we could prove that using' a PhysioFlex apparatus during clinical anaesthesia, the temperature of the sodalime canister was not exceeding 30 ° C.
Although trace amounts of foreign gases can accumulate during closed circuit anaesthesia, they are not attaining dangerous levels, particularly if the general recommendation is followed to flush the system every 30 minutes.

References
1. Barton F., Nunn J.F. Use of refractometry to determine nitrogen accumulation in closed circuits. Brit.J.Anaesth. 1975, 1Z, 346-349.
2. Morita S., Latta W., Hambro K., Snider M.T. Accumulation of Methane, Acetone, and Nitrogen in the inspired gas during closed circuit anesthesia. Anesth.Analg. 1985, 64, 343-347.
3. Roily G., Versichelen L.F., Mortier E. Methane Accumulation during Closed-Circuit Anesthesia. Anesth.Analg. 1994, 79 ,• 545-547.
4. Versichelen L., Roily G. and Vermeulen H. Accumulation of foreign gases during closed-system anaesthesia. Brit.J.Anaesth. 1996, ']€_, 668-672.
5. Moon R.E., Meyer A.F., Scott D.L., Fox E., Millington D.S.,
Norwood D.L. Intraoperative carbon monoxide toxicity. Anesthesiology 1990, 73, A 1049 (suppl. A3).
6. Baum J., Sachs G., v.d.Driesch Ch. and Stanke H.G. Carbon
monoxide generation in carbon dioxide absorbents. Anesth.Analg, 1995, .8J., 144-146.
7. Fang Z.X. and Eger II, E.I., Laster M.J., Chartkoff B.S.,
Kandel L. and lonescu P. Carbon monoxide production from degradation of desflurane, enflurane, isoflurane, halothane and sevoflurane by soda lime and baralyme . Anesth.Analg. 1995, 80, 1187-1193.

 

 

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THE PHARMACO-ECONOMICS OF LOW FLOW ANAESTHESIA Dr M Logan.
Edinburgh.

It has always proved difficult to assess the true economics and relative costs of low flow and high flow anaesthesia because of the wide differences in the techniques employed by anaesthetists. Even subtle changes in timing, flow rates, volatile agent concentrations and the use of concurrent drugs can each exert a huge effect on the final cost of a particular anaesthetic. The concept of 'savings' that can be made adds yet another dimension of error because the comparative costs depend not only on the uniformity of the low flow technique but also the many factors affecting the corresponding comparable high flow technique.
Computer predictions of the pharmaco-economics of volatile agent usage are governed by formulae that can include multiple factors that can be predicted to exert an influence in specific situations. Such predictions can not easily allow for the increased usage of volatile agent resulting from events such as the 'stormy induction'. Coughing and partial airway obstruction often cause a disproportionate increase in agent delivery for only a small increase in anaesthetic depth. The volatile agents available today vary significantly in their smoothness of handling the intravenous induction phase to volatile agent phase interface.
A low flow system using 11 min'1 maintenance fresh gas flows for the first hour of anaesthesia is likely to reduce the volatile agent cost by up to 80% when compared with a bain system. There are inherent expenses in purchasing and running circle systems. Thus any savings from reduced agent usage have to be balanced against these costs before true savings are possible. This threshold number of anaesthetics is dependent on both the volatile agent used and the use of nitrous oxide especially in the case of halothane anaesthesia. At present day prices in the UK, changing from using a Bain system to a low flow circle technique has a cost balancing threshold for the first hour of anaesthetic maintenance at 30 cases for Sevoflurane but 250-550 for Halothane depending on the concurrent use of nitrous oxide. The thresholds for Desflurane, Isoflurane and Enflurane lie between these two extremes at 40, 60 and 130 respectively. The cost of nitrous oxide per hour being 54% of that of halothane used at low flows is a significant factor when calculating economics. However If a vacuum insulated evaporator is used for hospital oxygen storage the use of low flow anaesthesia does not significantly alter the costs of oxygen usage.

The Theoretical Ideal Fresh-gas Flow Sequence
W W Mapleson
Cardiff

A computer-spreadsheet model has been- constructed representing a typical circle system, with the vaporizer outside the system, connected to a "standard" 70 kg patient. The patient part of the model is basically that specified by Davis and Mapleson (1981) but differs from some previous versions in that parallel and series deadspaces and "central" (right-to-left) shunt are incorporated explicitly, In accordance with figures for anaesthesia from Nunn's Applied Respiratory Physiology. In addition, an approximate allowance has been made for direct diffusion of anaesthetic in fat in the manner found necessary to match experimental data in the dog by Allott, Steward and Mapleson (1976).
In using the model, it will be assumed that. the patient is already preoxygenated, that anaesthesia has been induced intravenously, and perhaps some opioid has been given, but that nitrous oxide is not used. Fresh-gas flow and vaporizer concentration will start high and be reduced in steps with the aims of:
1. Raising the end-tidal concentration of volatile rapidly to 1 MAC and then keeping it constant.
2. Minimizing the wastage of volatile through the spill valve.
Sequences of fresh-gas flow and concentration will be sought which are simple and memorable rather than giving the absolute optimum solution. Results of simulations for desflurane, sevoflurane, isoflurane, enflurane and halothane will be presented at the meeting, with the usual disclaimer about consequential damage arising from the use of computer software!