Due to the worldwide restriction of economical resources and an increasing environmental awareness a recollection towards low flow techniques can be observed. The more, as modern anaesthetic apparatus and new inhalational anaesthetics became available, perfectly suited to be used with low fresh gas flows. In low flow anaesthesia the rebreathing fraction shall be at least 50%, i.e. 50% of the exhaled gas is led back to the patient during the next inspiratory stroke. Low Flow- (1.0 l/min), Minimal Flow- (0.5 l/min), and Closed System Anaesthesia  are the different low flow techniques given in the literature. The generally acknowledged advantages of low flow anaesthetic techniques  are: 1. A significant improvement of the anaesthetic gas climate, 2. a considerable decrease of anaesthetic gas and vapour consumption, 3. a corresponding marked reduction of costs, and 4. the reduction of atmospheric and workplace contamination.  All these advantageous features of the rebreathing technique only can be gained if the fresh gas flow rate is reduced appropriately. Low flow anaesthetic techniques, on the other hand, are specifically characterized by a considerable difference between the gas composition of the fresh gas and the composition of the anaesthetic gas within the breathing system. The time constants of the system, i.e. the time it takes that alterations of the fresh gas composition result in corresponding alterations of the anaesthetic gas composition, are the longer the lower is the fresh gas flow. Last, but not least, fresh gas flow reduction results in a marked decrease of the excess gas volume, i.e. less excess gas is available to compensate for gas loss via leakages. Nevertheless, modern anaesthetic apparatus, equipped with highly gas tight breathing systems, vaporizers and gas flow controls precisely working in the low flow range, ventilators featuring fresh gas flow compensation, and breathing systems with gas reservoirs, perfectly meet all technical needs to safely perform low flow techniques. The essential monitoring, comprising the continuous measurement of the inspiratory oxygen concentration, the concentration of the anaesthetic agents, the ventilatory parameters airway pressure and tidal or minute volume, and the carbon dioxide concentration, is mandatorily stipulated by the common European technical norm for anaesthetic workstations EN 740. Desflurane, an anaesthetic agent featuring very low solubility, is perfectly suitable to be used with even extremely low fresh gas flows: the wash-in and wash-out is rapid, thus, induction and emergence phase can be kept very short, and, due to the high output of the agent specific vaporizer and, simultaneously, small individual uptake, the time constants are specifically short even at very low fresh gas flow rates. Thus, modern anaesthetic equipment together with new anaesthetic agents considerably facilitate the routine performance of low flow anaesthetic techniques. When, furthermore, under these preconditions the use of nitrous oxide is omitted, even Closed System Anaesthesia can become a routinely performed anaesthetic method. On the other hand, however, anaesthetic agents, characterized by low solubility and low anaesthetic potency, from economical point of view must be applied with low flow techniques to gain an acceptable efficiency.

Further reading:

H. J. Lowe, E. A. Ernst: The Quantitative Practice of Anaesthesia, Use of Closed Circuit. Williams & Wilkins, Baltimore, 1981
Hargasser S. et al.: A lower solubility recommends the use of desflurane more than isoflurane, halothane, and enflurane under low flow conditions. J Clin Anesth 1995: 7: 1-5
J. A. Baum: Low Flow Anaesthesia, The theory and practice of low flow, minimal flow and closed system anaesthesia, 2nd ed. Butterworth-Heinemann, Oxford, 2001
 

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