Low flow anaesthesia shall be defined as an inhalation anaesthetic technique via a rebreathing system in which the rebreathing fraction amounts to at least 50%. According to the literature, the term „Low Flow Anaesthesia“ should be used if the fresh gas flow is reduced to 1.0 l/min, and „Minimal Flow Anaesthesia“ if the flow is as low as 0.5 l/min. Both techniques are extreme variants of semiclosed use of rebreathing systems: a small to extremely small part of the exhaled air still is discharged as excess gas. In „Closed System Anaesthesia“, however, only that amount of gas is supplied to the breathing sytem which is really taken up by the patient. The excess gas valve remains closed, all the exhaled air is recirculated back to the patient after carbon dioxide elimination, and the gas volume within the rebreathing system remains constant. „Quantitative Closed System Anaesthesia“ only can be realized, if exactly that amounts of nitrous oxide, oxygen and volatile agent are delivered into the system being taken up at the respective time. Not only the volume but also the composition of the gas within the system remains constant. Actually there is a strong tendency to renounce the use of nitrous oxide completely. When N₂O is omitted in low flow anaesthesia the resulting increase of excess gas renders even possible the realization of Closed System Anaesthesia with conventional anaesthetic machines in clinical practice. Following advantages can be gained by low flow anaesthesia: A significant improvement of the anaesthetic gas climate, marked reduction in anaesthetic gas and vapour consumption with corresponding savings in costs, and a reduction in environmental and workplace pollution.

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