MILD INTRAOPERATIVE HYPOTHERMIA DOES NOT CHANGE THE PHARMACODYNAMICS (CONCENTRATION-EFFECT RELATIONSHIP) OF VECURONIUM IN HUMANS

To investigate the effect of mild hypothermia on the neuromuscular jun ction sensitivity to vecuronium, we determined the pharmacodynamics (c oncentration-effect relationship) of vecuronium in 10 patients (ASA ph ysical class I or II; age range, 21-46 yr; weight range, 54-104 kg), d uring isoflurane-nitrous oxide-fentanyl anesthesia. Five were cooled t o a mean temperature of 34.4 degrees C and five were maintained normot hermic at a mean temperature of 36.8 degrees C. Neuromuscular function was monitored by measuring the evoked mechanical response of the addu ctor pollicis muscle after supramaximal train-of-four stimulation of t he ulnar nerve at the wrist. Vecuronium, 3 mu g.kg(-1) min(-1), was in fused for 10 min, venous blood sampled for 60 min, and twitch tension and plasma concentration data were used to determine pharmacodynamic v ariables in each patient. Results for the hypothermic and normothermic groups were compared by Mann-Whitney U-test. There were no difference s in any pharmacodynamic variable between the hypothermic and normothe rmic patients. For the hypothermic and normothermic patients, respecti vely, steady-state plasma concentrations of vecuronium producing 50% n euromuscular block (C-ss50) were 73 +/- 13 ng/mL (mean +/- SD) and 79 +/- 31 ng/mL; the rate constants for equilibration of vecuronium betwe en the plasma and the neuromuscular junction (K-eo) were 0.27 +/- 0.14 per min(-1) and 0.26 +/- 0.11 per min, and the power functions repres enting the slope of the concentration-effect relationship (gamma) were 5.7 +/- 1.9 and 4.4 +/- 1.8. We conclude that the pharmacodynamics (c oncentration-effect relationship) of vecuronium are similar at 34.4 an d 36.8 degrees C and that pharmacodynamic factors do not explain the p rolongation of action of vecuronium previously observed during mild hy pothermia.