SELF-ANTAGONISM - POSSIBLE MECHANISM OF TACHYPHYLAXIS IN SUXAMETHONIUM-INDUCED NEUROMUSCULAR BLOCK IN MAN

Adults (12) receiving halothane-N2O anesthesia for elective surgery were investigated during suxamethonium-induced neuromuscular block, using the train-of-four twitch for measurement of both the degree of block and the magnitude of fade (decrease in 4th-to-1st ratio). The residual neuromuscular block resulting from prolonged exposure to suxamethonium could be antagonized by bolus i.v. injections of suxamethonium itself, in doses which increased the block in Phase I. When self-antagonism was demonstrated, larger doses were necessary to increase the residual block, but only after overcoming the self-antagonizing effect. A biphasic response, first antagonizing then increasing the block, was observed following injection of a single bolus of suxamethonium. The self-antagonizing effect was more obvious in the 4th twitch of the train-of-four, resulting in a marked decrease in train-of-four fade (increase in train-of-four ratio). Self-antagonism may be an important cause of tachyphylaxis in suxamethonium-induced neuromuscular block in man.