Alcuronium pharmacodynamics in dogs: effect-concentration relationships in the diaphragmatic and limb muscles

Previous studies suggest that the muscles of the diaphragm are less sensitive to neuromuscular blocking agents than the limb muscles. However, this difference has not been characterized directly in terms of relaxant drug plasma concentrations. The pharmacodynamics of the non-depolarizing muscle relaxant alcuronium were therefore investigated in nine dogs using a constant-rate infusion regimen with simultaneous measurement of muscle paralysis in the limb and diaphragm. Maximum paralysis between 95 and 100% was achieved in both muscle groups, within approximately the same time interval. However, during onset of and offset of effect, the pharmacodynamic parameters ECp50 and ECp95 for the limb muscle were lower than in the diaphragm. From a pharmacodynamic effect model it was also predicted that Css(50) and Css(95) for the limb muscles are half those values for the diaphragm. Thus, the diaphragm is less sensitive to the action of alcuronium than are limb muscles. The half-time for equilibration of alcuronium between plasma and the effect site was two-fold lower for the diaphragm, and the rate of recovery from paralysis in diaphragmatic muscles was twice that observed in limb muscles. Collectively, these data suggest that there is a greater margin of safety in the diaphragmatic muscles and that the response of the peripheral limb muscles to nerve stimulation provides only a conservative index of recovery from competitive neuromuscular block in the diaphragmatic muscles.