Plasma concentrations of pancuronium during predetermined intensities of neuromuscular blockade

The plasma concentrations of pancuronium were monitored during i.v. infusions of the relaxant in dogs. Pancuronium was administered at rates which maintained the degree of neuromuscular blockade at three predetermined levels. The concentrations of the drug in the blood were consistent for any one animal but showed considerable overlap for the three levels of paralysis between animals. Concentrations obtained during infusion and which maintained the twitch response at 20% and 80% of control were compared, in the same dogs, with concentrations obtained during recovery from a bolus injection of pancuronium. When the infusion maintained the twitch response at 20% of the control value, the mean plasma concentration of pancuronium was 0.152 microgram ml-1. That measured after the bolus injection was 0.156 microgram ml-1. The concentrations at 80% of control were 0.094 microgram ml-1 and 0.083 microgram ml-1 respectively. The agreement between these results suggests a relationship between the plasma concentration of the relaxant and its effect during the termination of the action after a large bolus injection of the drug. As this occurs chiefly during the postdistribution equilibrium, the relatively slow decrease in plasma concentration would appear to become the rate-limiting factor in recovery from paralysis.