Variations in pancuronium requirement, plasma concentration, and urinary excretion induced by cardiopulmonary bypass with hypothermia.

To determine the effects of cardiopulmonary bypass (CPB) and hypothermia on the neuromuscular blockade produced by pancuronium, this relaxant was infused intravenously into 10 anesthetized patients to produce and maintain 90% depression of the twitch tension of the adductor pollicis muscle following supramaximal ulnar nerve stimulation. Infusion rates, plasma concentration of pancuronium, and adductor pollicis temperature were measured every 15 min. During the normothermic period preceding the start of CPB, the pancuronium requirement, the pancuronium plasma concentration, and muscle temperature were mean (mean +/- SEM): 238 +/- 12 micrograms . m-2 . 15 min-1, 0.31 +/- 0.01 microgram/ml, and 33.9 +/- 0.1 degrees C, respectively. At the beginning of CPB, the pancuronium infusion rate increased to 362 +/- 32 micrograms . m-2 . 15 min-1 (P less than 0.001) despite a decrease in the muscle temperature to 29.2 +/- 0.9 degrees C (P less than 0.001) and in pancuronium plasma concentration to 0.22 +/- 0.02 microgram/ml. During sustained muscle hypothermia to 28.3 +/- 0.4 degrees C the pancuronium plasma concentration remained constant at 0.22 +/- 0.01 micrograms/ml (P less than 0.001) while the requirement decreased to 94 +/- 15 micrograms . m-2 . 15 min-1 (P less than 0.001). After the muscle temperature was returned to 34 +/- 0.6 degrees C, the plasma pancuronium concentration and requirements increased to 0.35 +/- 0.05 microgram/ml and 392 +/- 32 micrograms . m-2 . 15 min-1 (P less than 0.001), respectively. After CPB, these values were 0.39 +/- 0.04 microgram/ml and 239 +/- 25 microgram . m-2 . 15 min-1. These results demonstrate that pancuronium requirements are increased at the beginning of CPB because of circulatory volume changes and again during rewarming of the patient once muscle temperature reaches about 34 degrees C.