Factors influencing MAC reduction after cardiopulmonary bypass in dogs

Background: Anaesthetic requirements may be reduced following surgery employing cardiopulmonary bypass (CPB). This study, in dogs, determined the role of a) volatile agents (enflurane [E] vs isoflurane [I]), b) oxygenator (bubble [B] vs membrane [M]), and c) presence [FL] vs absence [NoFL] of an in-line arterial filter in the bypass circuit in altering anaesthetic requirements following CPB. Methods: Male mongrel dogs were anaesthetized with either enflurane (n = 24) or isoflurane (n = 24). They were randomly assigned to one of eight groups (n = 6 per group); Group 1 (E/B/FL), Group 2 (E/M/FL), Group 3 (E/M/NoFL), Group 4 (E/B/NoFL), Group 5 (I/M/FL), Group 6 (I/B/FL), Group 7 (I/M/NoFL) or Group 8 (I/B/NoFL). MAC was determined using the tail-clamp method at hourly intervals, twice before and three times after a one hour normothermic perfusion using aortoatrial cannulation and CPB. Results: Prior to CPB, MAC was reproducible (enflurane: MAC1 2.17 ± 0.29 vs MAC2 2.14 ± 0.28%; isoflurane: MAC1 1.42 ± 0.31 vs MAC2 1.41 ± 0.33%) and differed among groups only for the volatile agent employed. Following CPB, MAC was reduced in all groups (P < 0.05 vs pre-CPB measurements) except Group 1 (E/B/FL). The degree of MAC reduction in other groups ranged from 39-64% and was not different based on type of agent employed, use of a membrane or bubble oxygenator, or presence or absence of an in-line arterial filter. Conclusion: In dogs, MAC reduction following CPB was variable, not related to type of volatile agent employed, use of a membrane or bubble oxygenator, or presence or absence of an in-line arterial filter. The explanation for reductions in anaesthetic requirements following CPB in this model remains speculative.