Molar potency is predictive of the speed of onset of neuromuscular block for agents of intermediate, short, and ultrashort duration.

BACKGROUND: The times to peak effect of rocuronium, vecuronium, cisatracurium, mivacurium, and succinylcholine were evaluated to confirm that the correlation between potency and onset time observed for long-acting relaxants also held for drugs of intermediate and short duration.

METHODS: The authors recruited 99 patients classified as American Society of Anesthesiologists physical status score 1 or 2 for the study. After anesthesia was induced, tracheal intubation was accomplished without relaxants. Anesthesia was maintained with nitrous oxide and 3% or 4% end-tidal desflurane plus intravenous narcotic supplementation. The evoked electromyographic response to single stimuli administered at 0.10 Hz was recorded continuously. Drug doses were selected to produce approximately 95% twitch depression. If peak twitch depression did not fall in the range of 90% to 98%, the patient was excluded from the study. The time to 50% to 90% of peak effect was plotted as a function of the administered dose.

RESULTS: There was no difference in the onset profiles of mivacurium and vecuronium, or in the time to 50% of peak effect between succinylcholine and rocuronium. For all other parameters, onset times ranked as follows: succinylcholine < rocuronium < vecuronium-mivacurium < cisatracurium (P < 0.05). When the log of the ED95 in micromoles per kilogram for all five drugs was plotted against the log of onset time to 50% peak effect, the R2 value for the best fit line was more than 0.98.

CONCLUSIONS: The inverse correlation between the molar potency and speed of onset previously described for agents of long duration also applies to nondepolarizing agents of intermediate and short duration. The onset time of succinylcholine also appears to be compatible with this relation.