Twitch augmentation and train-of-four fade during onset of neuromuscular block after subclinical doses of suxamethonium.

We have studied the train-of-four (TOF) response mechanomyographically during onset of neuromuscular block produced by subclinical doses of suxamethonium in order to follow the augmentation of the first twitch of the TOF (T1) and TOF fade compared with control TOF responses before the drug was given. In the groups given suxamethonium 0.05, 0.1, 0.2 and 0.3 mg kg-1, the increments in T1 after administration of the drug were observed before twitch depression occurred; these were mean 22.3 (SEM 8.1)%, 19.2 (3.3)%, 10.8 (2.0)% and 4.2 (2.2)%, respectively. This effect was more marked with the lower doses (P < 0.05). The degree of TOF fade was moderate during onset of neuromuscular block and depended on the dose of drug. The results of this study suggest that low doses of suxamethonium produced transient increase in muscle tension and twitch depression with significant TOF fade. We conclude that suxamethonium was associated with presynaptic effects as a consequence of brief stimulation of acetylcholine release followed by progressive diminution at the neuromuscular junction.