Cardiac Output and Onset Time of Different Muscle Relaxants

To the Editor: We read with interest the recent article by Munoz et al. [1]. In this study, they found that ephedrine reduced the onset time of rocuronium, probably secondary to increased cardiac output and muscle blood flow. In a similar study [2], we found that atropine [3] did not reduce the onset time of atracurium. These results seem to be contradictory. However, Audibert and Donati [4,5] found that decreasing blood flow by the inflation of a tourniquet modified (delayed) the onset of drugs with a fast onset times, such as rocuronium and suxamethonium, and had little effect on the onset time of drugs with an intermittent onset, such as vecuronium and mivacurium. This implies that the circulation time to the muscle is important in determining the onset time of only some blockers. Atracurium, which is also a blocker with intermediate onset, is therefore unlikely to have its onset greatly altered by changes in blood flow. The association of assumed increases in cardiac output with reduced onset time of rocuronium [1], but not of atracurium [2], is consistent with the hypothesis that blood flow and noncirculatory factors have different effects in determining the onset time of different relaxants. Eliahu Simhi, MD Department of Anesthesiology; Schneider Children's Medical Center of Israel; Petah-Tikva 49202, Israel Barbara W. Brandom, MD Department of Anesthesiology; Children's Hospital of Pittsburgh; Pittsburgh, PA 15213-2583