Dose relationships of phase II, tachyphylaxis and train-of-four fade in suxamethonium-induced dual neuromuscular block in man.

In man under halothane-nitrous oxide-oxygen anaesthesia, suxamethonium-induced neuromuscular block can be divided into two phases using the train-of-four ratio: the first phase of minimal train-of-four fade and the second phase of marked train-of-four fade. The second phase is characterized by a low train-of-four ratio, 0.3 or less, and resembles tubocurarine-block) in many details. It corresponds to Phase II (dual block in the literature. Contrary to the prevailing belief, Phase II block has a rather abrupt onset. In this study of seven patients, Phase I covered up to a total of 1--3 mg/kg of suxamethonium, while 3--5 mg/kg established Phase II block, with a transitional zone of only 2 mg/kg. Establishment of Phase II also coincided with onset of unequivocal tachyphylaxis. The possibility of a causal relationship between tachyphylaxis and Phase II block is suggested.