The induction phase of neuromuscular block after suxamethonium and pancuronium: analysis of the time constant of single-twitch decay (tau T1).

The T1% (first unconditioned stimulus of train-of-four) decay was analysed in 55 patients paralysed with different doses of suxamethonium and pancuronium during minor elective ear and nose surgery. In patients given pancuronium, T1% decay is well described by a mono-exponential function: a semi-log plot of ln(T1%) versus time is in fact a straight line. Predicted values are quite comparable with mean and individual values recorded in patients. From these equations the time-constant of single-twitch decay, or tau T1, may be calculated easily: it is a good dynamic descriptor of the speed of onset of neuromuscular block. In patients paralysed with suxamethonium, the rapidly developing block makes it difficult to acquire a sufficient number of data for computation of regression lines: while T1% seems to decrease exponentially, as in the pancuronium groups, no definite conclusions are possible at present. The authors conclude that tau T1 may be a useful dynamic descriptor of neuromuscular block induced by non-depolarizing muscular blocking drugs. Two new concepts are introduced: (i) the time spent to overcome the 'margin of safety' of neuromuscular transmission, or T MOS; and (ii) the speed index, as a measure of the relative speed of the onset of block.