[Interaction between vecuronium and atracurium revisited by adapting an alternative method]

Given that contradictory conclusions about the interaction between vecuronium and atracurium have been reached by previous investigators, the present study used an alternative method to analyze this interaction by way of the respective potencies of the two agents and the potency of their combination.

Effective doses for each relaxant were first calculated by administering small amounts of the agents to 3 similar groups of patients anesthetized with intravenous drugs and ventilated with nitrous oxide. Depression of the first response to train-of-four stimulation was obtained during electromyographic monitoring. That information was used to estimate the mean potency for each individual by an alternative method based on prior knowledge of the slope of the dose-response curve. In another group of patients, fractions equipotent to the median effective dose (ED50) were administered and the same procedure was used to establish the theoretical and calculated potencies of fractionated doses. The algebraic sum of the ratios between the calculated and effective doses was used to define the type of interaction.

The ED50 and the doses giving 90% and 95% efficacy were 24, 38 and 45 microg x Kg(-1) for vecuronium and 144, 234 and 277 microg x Kg(-1) for atracurium. After combination, the new figures were 9, 14 and 17 microg x Kg(-1) for vecuronium and 57, 93 and 110 microg x Kg(-1) for atracurium. The algebraic sums of the ratios between calculated and effective doses were less than 1 for each level of effective dose (0.77, 0.76 and 0.77, respectively), denoting synergism.

The alternative method presented for analyzing the potency of the individual components of the vecuronium-atracurium interaction demonstrated that their effect is greater after combination and definitively synergistic.