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European Journal of Anaesthesiology:

Low-flow anaesthesia

Baum, J. A.

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Damme, Germany

As early as 1850 John Snow (1813-53) recognized that inhalation anaesthetics were exhaled mostly unchanged in the expired air of anaesthetized patients. He concluded that the narcotic effect of the volatile anaesthetics could be markedly prolonged by reinhaling these unused vapours [1]. About 75 years later, in 1924, the rebreathing technique with carbon dioxide absorption was introduced into routine clinical practice by Ralph Waters (1883-1979), who inaugurated the to-and-fro absorption system. At the same time Carl Gauss (1875-1957) published his clinical experiences of the successful use of a circle absorption system for use with narcylene, i.e. acetylene, as an inhalation anaesthetic. Since 1933, with an increasing clinical use of cyclopropane, it became nearly essential to close the overflow valve of the breathing system and to reuse the exhaled air completely. As a result of the serious danger of explosions, any discharge of excess gas out of the breathing system had to be minimized. In 1954 halothane was introduced, a volatile anaesthetic characterized by high anaesthetic potency but a narrow therapeutic index. To ensure patient safety, the use of this highly potent anaesthetic necessitated knowledge of the inhaled vapour concentration. Its estimation was easier the higher the fresh gas flow and the lower the proportion of rebreathing. This was particularly important, as both the draw over and the copper-kettle vaporizers were insufficiently reliable and accurate in the low-flow range. Thus, although most anaesthetic machines were equipped with circle systems, it became clinical routine to use high fresh gas flows so that rebreathing became negligible [2]. This technique was called the semi-closed use of rebreathing systems. Knowledge about the performance of low-flow and closed system anaesthesia was gradually forgotten.

In addition to other American anaesthetists [3], Harry J. Lowe and Edward A. Ernst were the first to actively resurrect the ideas of low-flow and closed system anaesthesia [4]. Their mathematical approach to the clinical practice of inhalation anaesthesia did not make many friends. Nevertheless, their efforts did attract interest in this subject again and for the past 10 years a marked resurgence of low-flow techniques has been observed.

There can be little doubt that a reduction in the fresh gas flow from the commonly used 4 to 6 L min−1 to 1 or even 0.5 L min−1, according to the schemes of low flow or minimal flow anaesthesia, will result in a marked reduction in the consumption of anaesthetic gases and vapours [5]. The corresponding significant savings of costs for inhalation anaesthetics and the decrease of work place and environmental pollution are obvious and indisputable advantages of low-flow anaesthetic techniques. These advantages meet perfectly well the demands resulting from the present decrease in economic resources and increasing ecological awareness.

The latter supports the return towards low-flow anaesthesia as the volatile anaesthetics belong to the partially substituted halogenated hydrocarbons, and together with nitrous oxide contribute to both the greenhouse effect and the depletion of the stratospheric ozone layer [6]. The proportion of environmental destruction by anaesthetic gases and vapours is comparatively small, compared with the huge amount of nitrous oxide generated by bacterial decomposition of nitrate in fertilized soil and the amount of fully halogenated CFCs released by industry. Nevertheless, all measures should be taken to avoid unnecessary pollution with anaesthetic gases by the improper use and handling of the available anaesthetic equipment, especially designed for rebreathing techniques [7].

The introduction of the new generation of exclusively fluoro-substituted inhalation anaesthetics, sevoflurane and desflurane, will further enhance the trend towards low-flow anaesthetic techniques [8]. Because of their low solubility, the amounts of anaesthetic vapour taken up by patients are very small. Surprisingly, because of their low anaesthetic potency, the partial pressures which need to be established within the breathing system have to be comparatively high. If high fresh gas flows are used and, correspondingly, most of the exhaled air is wasted, large amounts of anaesthetic vapours will have to be delivered into the breathing system only to re-establish the required high partial pressure. The use of expensive inhalation anaesthetics, featuring low solubility and low anaesthetic potency, will only be economically justifiable if used with low-flow anaesthetic techniques. This would be particularly significant if Xenon were to be used as an anaesthetic gas in clinical practice [9].

Apart from all the economic and ecologic advantages, low-flow anaesthesia significantly increases the warmth and humidity of the anaesthetic gases [10]. A raised temperature and water vapour content of the inhaled anaesthetic gases preserves the anatomical and functional integrity of the ciliated epithelium of the respiratory tract. Although the preservation of a physiological mucociliary clearance will undoubtedly be beneficial for the patients, no clinical study has up to now succeeded in demonstrating a significant decrease in post-operative pulmonary or respiratory disturbances by flow reduction.

Compared with the early 1950s, the technical preconditions for safe low-flow anaesthesia have changed completely [11]. The flow compensation achieved in modern plenum vaporizers and the calibration and graduation of the gas controls are designed not only for the use with low fresh gas flows but perform with sufficient accuracy throughout this flow range. Modern compact rebreathing systems and modern anaesthetic ventilators are extremely gas tight. Several ventilators of the new generation of anaesthetic machines feature flow decoupling, thus, the delivered tidal volume can be set independent of the fresh gas flow. In increasing frequency, multi-gas monitors are available making possible a comprehensive analysis of the anaesthetic gases. In many of the European countries continuous monitoring of the inspired oxygen concentration is already mandatory. Capnometry and continuous monitoring of the anaesthetic agent concentration will be obligatory with the forthcoming common European technical standard for anaesthetic workstations, EN 740 [12]. Thus, although the settings of the gas controls and the vaporizer in low-flow anaesthesia do not reflect the anaesthetic gas composition within the breathing system, the anaesthetist can obtain accurate data about the gas composition supplied to the patient. The safety features of anaesthetic machines and the availability of comprehensive gas monitoring today overcome all the technical shortcomings and counteract former resistance to the routine performance of low-flow anaesthetic techniques.

Last but not least, the clinical application of low flow anaesthesia is further facilitated by the availability of reliable guidelines for the safe performance of these techniques in routine clinical practice, without the need to resort to difficult mathematical calculations [11,13].

However, if low flow anaesthesia is performed with older types of conventional anaesthetic machines, the specific features of this anaesthetic technique will have to be considered. The lower the flow, the greater the difference between the fresh gas and the gas composition within the breathing system. If the fresh gas flow is lower than 1 L min−1 even experienced anaesthetists may fail to estimate precisely the gas composition within the breathing system from the settings of the fresh gas controls. Furthermore, in a flow range lower than 1 L min−1 the performance of the fine needle valves and the flow meter tubes approaches the limits of accuracy. The implementation of advanced computer technology, electronically controlling the fresh gas supply by closed loop feedback according to preset values, will overcome these technical problems. These highly sophisticated machines will become available in the near future. The Dräger company already offers commercially such a machine, featuring electronic control of the anaesthetic gas composition and volume circulating within the rebreathing system [14].

A matter of concern remains the accumulation of trace gases resulting from the diminution of the wash out effects. Foreign gases may decrease the concentration of nitrous oxide and oxygen. That may, for instance, be the case if nitrogen accumulates because of insufficient denitrogenation, or the argon concentration may rise as a result of the use of an oxygen concentrator. Methane, exhaled physiologically by the patient, in high concentrations may compromise anaesthetic gas monitoring, e.g. measurement of halothane concentration. Accumulation of acetone may prolong the emergence from anaesthesia and provoke nausea or vomiting. However, only in the very rare cases of severely ketoacidotic patients does this become clinically relevant. Recently it was revealed, that the volatile anaesthetic agents desflurane, enflurane, isoflurane, and presumably also halothane, are liable to react with absolutely dry carbon dioxide absorbents to generate carbon monoxide [15]. Subsequent recommendations have been made not to use fresh gas flows lower than 5 L min−1 to safely avoid accidental carbon monoxide intoxication resulting from trace gas accumulation [16]. However, this conclusion must strongly be rejected, as high fresh gas flows are liable to dry out the absorbents. On the contrary, low-flow anaesthesia, preserves the moisture content of the absorbents, and can be regarded as a measure to prevent carbon monoxide generation [17]. Halothane and sevoflurane, in their part, may react with carbon dioxide absorbents by generating haloalkenes, e.g. 1, bromo-1,chloro-2,2,difluoro-ethylene, (BCDFE), or fluoromethyl-2,2,difluoro-1,trifluoromethyl-vinyl-ether, (Compound A) [18]. Both agents have been used extensively in several million cases over many years, even in low-flow anaesthetic techniques. However, not a single clinical report has shown any evidence that these haloalkenes did accumulate at toxic levels or that any patient really suffered harm from accidental accumulation of these degradation products. Thus, in contrast with the USA, and Switzerland where anaesthetists are obliged to use sevoflurane with fresh gas flows of at least 2 L min−1, sevoflurane has been approved for clinical use in most European countries (Austria, Belgium, Denmark, Finland, France, Greece, Italy, The Netherlands, Norway, Portugal, Spain, Sweden, United Kingdom and Germany) without any restriction concerning fresh gas flow. It is still controversial whether or not Compound A is nephrotoxic to humans. Thus, whenever the possibility of accumulation of potentially harmful trace gases might arise in clinical practice, for safety reasons, a low-flow technique with a flow of at least 1 L min−1 should be used, guaranteeing a sufficient continuous wash out effect.

All the economical, ecological and clinical advantages therefore are in favour of returning towards rebreathing techniques. More so, as all these advantages can be gained easily by the judicious use of the already available anaesthetic machines. The main obstacle against the revival of low-flow anaesthesia remains the disinterest and the reluctance of anaesthesists to change their usual practice.

J. A. Baum

Damme, Germany

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© 1996 European Academy of Anaesthesiology