Total intravenous anesthesia with propofol augments the potency of mivacurium

Background: Little is known about the potentiating effect of propofol on neuromuscular blocking drugs. However, some animal studies indicate a dose-dependent increase of the potency of neuromuscular blocking drugs by propofol. This study compared mivacurium potency after five minutes and after 20 min of total intravenous anesthesia with propofol (TIVA propofol). Methods: Twenty-eight patients were randomized into two groups, after approval of the Ethics Committee and written consent. Anesthesia was induced, in all patients, using remifentanil 0.5 mu g.kg(-1).min(-1) for two minutes, after which: 3 mg.kg(-1) of propofol was injected; a laryngeal mask airway was inserted; and intermittent, positive pressure ventilation was initiated. Anesthesia was maintained using TIVA propofol (titrated using bispectral index monitoring to 40-45). Neuromuscular monitoring consisted of phonomyography at the adductor pollicis muscle. In Groups 5 min and 20 min, a tetanic stimulation of the ulnar nerve commenced after four minutes and after 19 min of TIVA, respectively, followed by controlled, single twitch stimulation at 1Hz for one minute. Boli of 60, 30, 30, and 30 mu g.kg(-1) mivacurium, respectively, were administered (each drug increment was administered after the effect of the previous dose had caused a stable response), and single twitch stimulation continued at 0.1 Hz. The dose-response curve was determined for both groups; potency was calculated using log-probit analysis. Data were presented as mean (SD) and were compared using two-sided analysis of variance, P < 0.05. Results: Patient characteristics were similar in the two groups. The corresponding ED(50) and ED(95) values were greater, at 76.7 +/- 12.4 mu g.kg(-1) and 146.6 +/- 27.6 mu g.kg(-1) for Group 5 min, vs 46.7 +/- 12.2 mu g.kg(-1) and 101.1 +/- 20.2 mu g.kg(-1) for Group 20 min, respectively. Conclusions: After 20 min of TIVA propofol, the potency of mivacurium is approximately 50% greater than after five minutes of TIVA propofol. For clinical purposes, it is important, therefore, to consider the duration of TIVA propofol before determining the dose of neuromuscular blocking drug.