Electrophysiological effects of atracurium and vecuronium on normal and denervated hearts

Atracurium and vecuronium offer some advantages with respect to other nondepolarizing muscle relaxants. However, their administration has been associated with episodes of bradycardia and asystole, the mechanism of which has not yet been clarified. In an effort to analyze the effects of these drugs on automaticity, refractoriness, and conduction, 30 dogs underwent heterotopic heart transplantation to establish a model in which a normal heart (recipient) and a denervated heart (donor) could coexist. The electrophysiological studies included the determination of cycle length (CL), sinoatrial conduction time (SACT), antegrade and retrograde atrio-ventricular (A-V) node and ventricular block points (ABP and RBP), as well as effective antegrade and retrograde A-V node (ARPAVN and RRPAVN) and ventricular refractory periods (VRPs) in both hearts. These parameters were measured in a basal state and after injection of pancuronium (0.5 mg/kg) in 10 animals, atracurium (0.5 mg/kg) in another 10 dogs, and vecuronium (0.12 mg/kg) in the remaining animals. The results showed that pancuronium produced a tachycardia, significantly facilitated A-V conduction in the donor and the recipient hearts, and lowered the A-V node refractoriness in the donor organ, while atracurium and vecuronium induced no changes from the baseline values, either in normal or denervated hearts. Based on the results of this study, the episodes of sustained bradycardia reported in association with atracurium and vecuronium are attributed to the absence of antagonism of vagal stimuli provoked by pancuronium, and/or by surgical or anesthetic manipulations.