Pancuronium, gallamine, and d-tubocurarine compared: is speed of onset inversely related to drug potency?

The relative potency and speed of onset of action of pancuronium, gallamine, and d-tubocurarine was studied in 55 adult female patients receiving nitrous oxide/oxygen-narcotic anesthesia. The integrated electromyogram of the adductor pollicis muscle was monitored using a cumulative dose-response technique; train-of-four stimuli were administered at 0.05 Hz. The measured ED95 values for pancuronium and gallamine were 0.069 and 2.38 mg/kg, respectively. In three separate groups, pancuronium 0.07 mg/kg, gallamine 2.4 mg/kg, or d-tubocurarine 0.45 mg/kg were given as a single bolus and the speed of onset and time to maximum effect determined. Peak twitch depression was essentially identical in all groups (92.7 +/- 1.4 [SE] vs. 93.3 +/- 1.1 vs. 93.7 +/- 1.1%, respectively). The rate of onset of neuromuscular blockade in these three groups was, however, quite different. After administration of pancuronium (n = 10) the times to 5%, 20%, 50%, and 80% twitch depression were 68 +/- 5, 97 +/- 6, 141 +/- 8, and 222 +/- 18 s. The comparable times following gallamine (n = 10) were 29 +/- 2, 42 +/- 3, 66 +/- 5, and 136 +/- 14 s; d-tubocurarine (n = 10) was intermediate in speed with onset times of 40 +/- 4, 63 +/- 6, 99 +/- 11, and 178 +/- 25 s. It appears that the onset times of different nondepolarizing blocking agents (even when given in equipotent doses) may vary by clinically appreciable amounts. The results of this study support the hypothesis that nondepolarizing neuromuscular blocking agents of low potency may have a more rapid onset of action than that seen with agents of high potency.