Cardiopulmonary bypass reduces the minimum alveolar concentration for isoflurane

Objective: The purpose of this investigation was to determine the influence of cardiopulmonary bypass (CPB) on the minimum alveolar concentration (MAC) of isoflurane in a rat model of CPB. Design: Prospective. Setting: University research laboratory. Participants: Sprague-Dawley rats. Interventions: Using tail-clamp methodology, the pre- and post-CPB MAC for isoflurane was studied. Methods and main results: Rats were anesthetized with isoflurane, intubated, ventilated, and surgically prepared for CPB, after which they were randomized to either Sham-operated or CPB groups. The CPB group (n = 10) underwent 90 minutes of normothermic nonpulsatile CPB. The Sham group (n = 13) were cannulated but did not undergo CPB. Pre- and post-CPB MAC determinations were compared within groups using a paired Student t test. The CPB group had a pre-CPB baseline isoflurane MAC of 1.09% ± 0.11% versus 1.09% ± 0.08% in the Sham group ( p = 0.90). Twenty minutes after CPB, the CPB group exhibited a decrease in MAC to 0.98% ± 0.14% ( p = 0.0026, compared with baseline). The MAC in the Sham group was unchanged ( p = 0.5852, compared with baseline). Two hours after CPB, the MAC in the CPB group remained lower compared with baseline at 0.99% ± 0.14% ( p = 0.0032). Conclusions: CPB resulted in a small (10%) but significant reduction in the MAC for isoflurane. The mechanism behind this reduction in MAC is not clear but may be related to CPB-induced cerebral injury.