Variability of onset times within and among relaxant regimens

To evaluate the consistency of times to 95% twitch height depression (T95%) in groups of patients receiving identical induction and relaxant regimens.

Prospective, noncontrolled, blinded study.

Ambulatory surgical unit at a university medical center.

Seventy-five ASA physical status I and II patients undergoing general endotracheal anesthesia.

Patients received succinylcholine 1.5 mg/kg or a nondepolarizing regimen with doses ranging from approximately 1.5 to 6 times the ED95, with or without a priming dose.

For each of the eight relaxant regimens used in five or more patients, the intraregimen variability of T95% (at the adductor pollicis muscle upon ulnar stimulation at 0.1 Hz) was expressed as SD and range, and the individual data points were displayed. There was wide intraregimen variability. For each regimen, the slowest T95% was at least 73% longer than the fastest T95%. For the 16 patients receiving a priming dose plus an intubating dose 5 or more times the ED95, the median T95% was 95 seconds; however, T95% was beyond 120 seconds in 5 of the 16 cases.

The wide variability in onset times among subjects receiving the same regimen indicates that monitoring of neuromuscular response, preferably to a relatively slow rate of neurostimulation, is essential if one elects to use moderate to high doses of atracurium and/or vecuronium for rapid-sequence induction in a patient in whom movement or coughing is unacceptable. Since onset times were not symmetrical about the mean, the magnitude and frequency of unacceptable onset times would not be fully appreciated unless the individual data points were displayed. Such information may be critical when reporting the suitability of a neuromuscular blocking drug for rapid intubation.