CARDIOPULMONARY BYPASS DOES NOT ALTER CANINE ENFLURANE REQUIREMENTS

This study determined the effect of cardiopulmonary bypass (CPB) on canine enflurane minimum alveolar concentration (MAC). Fourteen dogs were anesthetized with enflurane in N2O and O2, and after tracheal intubation, the N2O was discontinued. Femoral arterial and pulmonary arterial catheters were placed, and MAC was determined with the tail-clamp method. CPB was initiated via the femoral artery-vein route, with additional venous return obtained from an external jugular vein. Partial CPB was used in the first 10 dogs. In 4 dogs, a membrane oxygenator (group 1) was used, and in the next 6 dogs a bubble oxygenator (group 2) was used. In 4 additional dogs (group 3), using bubble oxygenators, total CPB was achieved by occlusion of the pulmonary artery via a left thoracotomy. The CPB circuit was primed with Ringer's lactate, and circuit blood flows were 70-125 ml.kg-1.min-1, with mean arterial pressures maintained at 50-110 mmHg. MAC was determined again after termination of CPB. In 10 dogs, MAC was also measured during CPB. In 5 dogs MAC was measured after administration of protamine. MAC in all 14 dogs did not change (2.2 +/- 0.3 vs. 2.3 +/- 0.3). MAC remained constant in group 1 (2.4 +/- 0.3 vs. 2.3 +/- 0.4), group 2 (2.2 +/- 0.2 vs. 2.3 +/- 0.3), and group 3 (2.2 +/- 0.1 vs. 2.3 +/- 0. 1). Similarly, MAC was unchanged during CPB (2.2 +/- 0.2 vs. 2.2 +/- 0.2) and after protamine (2.3 +/- 0.2 vs. 2.2 +/- 0.3). Temperature was 38.3 +/- 1.2 prebypass and 37.9 +/- 0.9 postbypass. Hematocrit decreased from 38% to 28% post-CPB (P < 0.05), and pulmonary artery pressure increased slightly (P < 0.05). We conclude that CPB does not alter canine enflurane MAC.