We would like to make a minor but needed correction to a study we recently published in Anesthesiology. 1Historically, labels of vials or ampules containing neuromuscular blocking drugs indicate the total weight of the salt of the drug per milliliter. For example, vecuronium labeling indicates that when the drug is reconstituted, each milliliter contains 1 mg of vecuronium bromide. The GRAM molecular weight of this salt is 637.7. The GRAM molecular weight of bromine is 79.9. Thus each milliliter of vecuronium only contains 0.87 mg of the active moiety or base. The same labeling convention also applies to atracurium, rocuronium, mivacurium, and succinylcholine.

Dr. Francois Donati (Departement d’Anesthesie, Center Hospitalier de l’Universite de Montreal, Quebec, Canada) recently pointed out to us that cisatracurium is labeled differently. Each milliliter of cisatracurium contains 2.0 mg of the active moiety not the salt. Thus the molar potency of the drug is considerably lower (0.066 μm/kg vs.  0.050 μm/kg) than we indicated in our article. We have consequently recalculated all our published data regarding the relationship between molar potency and onset time. We stand by our conclusion that there is a linear relationship between the log of onset time and the log of molar potency. However, the coefficient of determination (R2) of this relationship is somewhat less impressive than before. The R2 for potency versus  time to 50% effect (see fig. 3 in our article) 1is now 0.958 not 0.984. Similarly, the R2 for potency versus  time to 90% effect is now 0.951 not 0.977.

It appears that the pharmaceutical industry has recently changed its labeling convention without notifying the consumers of its products. Early studies with rapacuronium used the old labeling convention. 2However, the current label expresses the drug concentration in milligrams of the active moiety or base per milliliter. To the best of our knowledge cisatracurium and rapacuronium are the only two drugs so labeled.

Kopman AF, Klewicka MM, Kopman DJ, Neuman GG: Molar potency is predictive of the speed of onset of neuromuscular block for agents of intermediate-, short-, and ultra-short duration. A nesthesiology 1999; 90:425–31
Wierda JMKH, van den Broek L, Proost JH, Verbaan BW, Hennis PJ: Time course of action of endotracheal intubating conditions of Org 9487, a new short-acting steroidal muscle relaxant; A comparison with succinylcholine. Anesth Analg 1993; 77:579–84