Does a subparalysing dose of vecuronium enhance diaphragm fatigue?

We have examined, in six healthy volunteers, the effect of a subparalysing dose of vecuronium on the development of diaphragm fatigue. Vecuronium was given as a 0.5-mg bolus i.v. followed by 0.5 mg infused over 30 min; as a control, saline was given in random order. Diaphragm strength was assessed by measuring transdiaphragm pressure and by electromyography. Diaphragm fatigue was induced by breathing against an inspiratory resistance. The plasma concentration of vecuronium varied between 15 and 30 ng ml-1 15 min after administration of vecuronium was started. Peripheral neuromuscular block was not detected in any subject. Diaphragm fatigue developed within the same period in both groups: mean 334 (SD 166) s after saline and 345 (190) s after vecuronium. The electromyographic pattern of diaphragm fatigue and the time constant of relaxation of transdiaphragm pressure after fatigue were similar in both groups. We conclude that, at low plasma concentrations of vecuronium, similar to those present in the postoperative period, there was no predisposition to diaphragm fatigue.