Fresh gas flow is not the only determinant of volatile agent consumption: a multi-centre study of low-flow anaesthesia

Methods. Seven academic centres studied 302 patients, using desflurane, enflurane, halothane, or isoflurane using circle-systems and Drager Julian anaesthetic machines, with fresh gas flows ((V) over dot(F)) of 3, 1, and 0.5 litre min(-1). Volatile agent partial pressures in the breathing system were recorded and agent consumptions measured by weighing. Results. At these flows, desflurane consumption depended on (V) over dot(F). In contrast, halothane consumption was not influenced by (V) over dot(F) Isoflurane and enflurane showed differences in consumption between flows of 0.5 and 3 litre min(-1). Stepwise linear regression suggested that besides (V) over dot(F), other factors influenced consumption of the more soluble agents (sex, age, weight, height, altitude, and temperature). The partial pressure ratios were independent of (V) over dot(F) for desflurane (end-tidal to fresh gas=0.8), but the ratios of the more soluble agents varied with (V) over dot(F) (end-tidal to fresh gas=0.3-0.7). Conclusions. At (V) over dot(F) that involves significant re-breathing, consumption of soluble agents depends only partially on (V) over dot(F). These results can be explained using Mapleson's hydraulic analogue model.