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 Dräger Julian anaesthetic machines, with fresh gas flows (V(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(F). In contrast, halothane consumption was not influenced by V(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(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(F) for desflurane (end-tidal to fresh gas=0.8), but the ratios of the more soluble agents varied with V(F) (end-tidal to fresh gas=0.3-0.7).


CONCLUSIONS
At V(F) that involves significant re-breathing, consumption of soluble agents depends only partially on V(F). These results can be explained using Mapleson's hydraulic analogue model.
Coetzee
J F
JF
Department of Anesthesiology, Faculty of Medicine, University of Stellenbosch, Tygerberg, South Africa.
Stewart
L J
LJ
D000293:Adolescent; D000328:Adult; D000368:Aged; D000764:Anesthesia, Closed-Circuit; D018685:Anesthetics, Inhalation / Q000008:administration & dosage* / Q000493:pharmacokinetics; D000886:Anthropometry; D000077335:Desflurane; D004334:Drug Administration Schedule; D004737:Enflurane / Q000008:administration & dosage / Q000493:pharmacokinetics; D005260:Female; D006221:Halothane / Q000008:administration & dosage / Q000493:pharmacokinetics; D006801:Humans; D007530:Isoflurane / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000493:pharmacokinetics; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D010313:Partial Pressure
D018685:Anesthetics, Inhalation; D004737:Enflurane; D000077335:Desflurane; D007530:Isoflurane; D006221:Halothane