Onset properties of mivacurium measured by mechanomyography depend on the twitch height of the adductor pollicis muscle

Introduction: The influence of the twitch height of the adductor pollicis muscle during baseline measurements on the pharmacodynamic parameters of mivacurium was prospectively evaluated. Patients and methods: Fifty adult patients were anaesthetized with propofol and alfentanil. Neuromuscular function was monitored mechanomyographically by measuring the force of the adductor pollicis muscle following stimulation of the ulnar nerve. Following a stabilization period of 20 min, the individual twitch height of the adductor pollicis muscle was determined before a single bolus of mivacurium (75 μg kg^-1) was administered. Patients were divided into two groups. The data of patients whose thumb adduction force was below the median value of all patients were the 'low force' group (9.1 ± 1.4 N) and the data of all other patients were the 'high force' group (13.7 ± 1.8 N). Results: In the 'high force' group, maximum neuromuscular blockade of mivacurium was deeper (0.97 ± 0.05 vs. 0.93 ± 0.06; P < 0.05) and onset faster (2.9 ± 1.1 min vs. 4.0 ± 1.2 min; P < 0.05). Neuromuscular recovery did not differ between the groups. Conclusion: The different onset speeds reflect either different sensitivity to neuromuscular blocking agents with respect to patients' muscle power or a problem of the mechanomyographic measuring technique.