Neuromuscular blocking characteristics of vecuronium after tubocurarine-induced "fade". An experimental double-blind clinical study.

Objective: The fade in train-of-four (TOF) monitoring is considered to be due to blocking of the prejunctional nicotinic acetylcholine receptors (AchRs). During onset of the neuromuscular block (NMB) tubocurarine (TC) causes more fade in the TOF responses than vecuronium (VEC). Therefore we wanted to investigate whether onset or duration of action of VEC or TC would be improved with a priming dose of an agent with different prejunctional activity.

Methods: The rates of NMB were measured following priming doses of 0.15 mg x kg(-1) of TC and 0.015 mg x kg(-1) of VEC with 6 min priming time. The individual time course of action of 0.6 mg x kg(-1) of TC (1.13 x ED 95) and 0.1-0.2 mg x kg(-1) of VEC (1.75-3.5 x ED95) were examined with a priming dose of the same agent or the other agent, by measurement of changes in the evoked compound EMG from the hypothenar muscle.

Results: Priming doses of TC decreased mean TOF ratio to 67% [95% confidence interval (CI) = 56-78] during priming time, which was significantly lower than after priming with VEC 87% (76-97; P < 0.001). Despite the higher TOF ratio, the priming dose of VEC accelerated the onset time of intubation dose of TC more than the priming dose of TC (P = 0.0018). Priming with TC prolonged the duration of VEC-induced NMB by 35-70 min compared with priming with VEC, which means that a small priming dose of TC changes VEC from a muscle relaxant with intermediate action to a long-acting agent.

Conclusion: Priming with TC caused a lower TOF ratio; however, priming with TC did not accelerate the onset time of either agent as much as priming with VEC. It appears that potentiation of NMB after combination of VEC and TC is not dependent on "fade" receptors.