The nature of suxamethonium neuromuscular block in the dog assessed by train-of-four stimulation.

Evoked muscle responses from continuous train-of-four (frequency 2Hz) electrical nerve stimuli, with an interval of 10 sec between each train, were recorded in the intact anaesthetised dog receiving multiple doses of suxamethonium at 0.035 mg/kg. The train-of-four twitches showed a slight progressive fade from the first to the fourth and this fade increased as additional doses of the muscle relaxant were given. The train-of-four ratio was used to divide the suxamethonium neuromuscular block into phase 1 and phase 2 components. Dogs receiving one dose of suxamethonium at 0.3 mg/kg were developing the phase 2 block during the return of neuromuscular activity and after the third to fourth dose this block was fully established. Evidence of tachyphylaxis was present in the phase 1 block which persisted into the phase 2 component.