Greater neuromuscular blocking potency of atracurium during hypothermic than during normothermic cardiopulmonary bypass.

Previous studies drew attention to the greater neuromuscular blocking potency of atracurium during, than before or after, hypothermic cardiopulmonary bypass (CPB) while disregarding the periods of normothermic perfusion. We recorded the evoked twitch tension in 15 patients during nitrous oxide/narcotic anesthesia who were undergoing open heart surgery. Atracurium was injected as an initial bolus dose of 460 micrograms/kg. Waning neuromuscular blockade was enhanced by repeat injections of 138 micrograms/kg whenever the twitch tension attained 25% of control. During hypothermic (< 32 degrees C) and normothermic (> 34 degrees C) CPB, the times of onset of the maintenance doses were 57% and 18% longer, respectively, than before CPB (P < 0.05). Maintenance doses of atracurium were required every 24 +/- 4 min (mean +/- SD) before CPB, every 45 +/- 8 min (P < 0.05) during hypothermia, every 22 +/- 3 min during normothermic perfusion, and every 23 +/- 3 min after CPB. In conclusion, the patients' changing demand of atracurium paralleled the changes of temperature rather than the institution and cessation of CPB.