Does hypothermia or hyperventilation affect enflurane MAC reduction following partial cardiopulmonary bypass in dogs?

This study in dogs determined the effect of systemic cooling and arterial hypocarbia during cardiopulmonary bypass (CPB) on the requirements for enflurane anaesthesia (MAC) before and after CPB. Twelve mongrel dogs were each anaesthetized with enflurane in oxygen on two separate occasions. End-tidal enflurane concentration was measured with a Puritan-Bennett Anaesthesia Agent Monitor. Using the tail-clamp method, MAC was determined twice with a one-hour interval between measurements (MAC 1 and MAC 2). Partial CPB was then initiated using femoral arterio-venous cannulation and maintained for one hour. Following separation from CPB, MAC was again determined twice with a one hour interval between measurements (MAC 3 and MAC 4). Dogs were randomly assigned according to PaCO₂ management during CPB (low, 17.6 ±8.6 mmHg vs high, 38.9 ± 11.5 mmHg), and then subjected to two experimental conditions. The first experiment on each dog was undertaken using normothermia during CPB (warm, 35-37° C) while the second experiment (at least two weeks later) was conducted using hypothermia during CPB (cold, 30° C). Analysis of the data, using ANOVA for repeated measures, revealed MAC 3 (1.95 ± 0.33%, post-CPB) to be reduced when compared with MAC 1 (2.18 ± 0.28%, P < 0.01) or MAC 2 (2.10 ± 0.22%, P < 0.01), determined before CPB. Multivariate repeated measures analysis revealed no independent effects of hypothermia or arterial hypocarbia during CPB, on MAC reduction. By the time of the second experiment in each dog (two weeks later), MAC had returned to baseline levels. The authors conclude that, in the dog, partial CPB causes a short-term small reduction of enflurane MAC, but that neither hypothermia nor hypocarbia during CPB affected MAC.