Molar Potency is Not Predictive of the Speed of Onset of Atracurium

Abstract

In an effort to determine the extent to which atracurium may represent an exception to the rule that molar potency predicts onset time, we studied the onset profile of atracurium after a dose selected to produce approximately 95% twitch depression. We compared these results with data obtained in a previous study after the administration of vecuronium, rocuronium, and cisatracurium. Eighteen ASA physical status I and II patients were studied. After the induction of anesthesia, tracheal intubation was accomplished without relaxants. The evoked electromyographic response to 0.10-Hz single stimuli was continuously recorded. After baseline stabilization, a single bolus of atracurium, averaging 0.21 mg/kg, was administered. If peak twitch depression did not fall within the range of 90%–98%, the patient was excluded. The time to 50% and 90% of peak effect was recorded. The time to 90% of maximal effect (192 ± 23 s) was not different from that previously observed for vecuronium (201 ± 20 s). The time to 50% of peak effect (110 ± 15 s) was shorter (P< 0.05) after atracurium administration than after vecuronium (125 ± 9 s). The onset times recorded for atracurium were slower than previously observed after rocuronium and more rapid than that which was seen after cisatracurium (P< 0.001). The observed onset profile of atracurium was considerably slower than anticipated, based on the drug’s molar potency. The 95% effective dose (μM/kg) may not be a reliable predictor of a muscle relaxant’s onset time, when the drug administered is a mixture isomers of varying potency. Implications The speed of onset of atracurium is slower than predicted, based on its molar potency. Potency of a relaxant may not be a reliable predictor of its time to peak effect, when the drug administered is a mixture of isomers with widely different neuromuscular activities.