Low and minimal flow inhalational anaesthesia.

Abstract

To describe the pharmacokinetic behaviour and practical aspects of low (0.5-1 l.min-1) and minimal (0.25-0.5 l.min-1) flow anaesthesia. A Medline search located articles on low flow anaesthesia, and computer simulated anaesthetic uptake models are used. Most, 85-90%, of anaesthetists use high fresh gas flow rates during inhalational anaesthesia. Low/minimal flow anaesthesia with a circle circuit may avoid the need for in-circuit humidifiers, raise the temperature of inspired gases by up to 6 degrees C, reduce cost by about 25% by reduction of fresh gas flows to 1.5 l.min-1, and reduce environmental pollution with scavenged gas. Knowledge of volatile anaesthetic pharmacokinetic behaviour facilitates the use of minimal/low flow rates. Small amounts of nitrogen or minute amounts of methane, acetone, carbon monoxide, and inert gases in the circuit are of no concern, but the degradation of desflurane (to carbon monoxide by dry absorbent) and sevoflurane (to compound A by using a fresh gas flow of > 2 l.min-1) must be avoided. With modern gas monitoring technology, safety should be no more of a concern than with high flow techniques. The use of fresh gas flow rates of < 1 l.min-1 for maintenance of anaesthesia has many advantages, and should be encouraged for inhalational anaesthesia with most modern volatile anaesthetics.