A comparison of acceleromyography and mechanomyography for determination of the dose-response curve of rocuronium in children

In order to compare an acceleromyograph (TOF-Guard(TM)) with a mechanomyograph (Grass FT03), the dose-response relationship of rocuronium was simultaneously determined in both arms of 15 children aged 3-11 years during anaesthesia with thiopentone, alfentanil and nitrous oxide. Three subgroups of five children received rocuronium 120, 180 or 240 mu g.kg(-1) randomly. The effective doses to produce 50% and 95% depression of the first twitch of the train-of-four determined by acceleromyography were 206 and 337 mu g.kg(-1), respectively, while these values determined by mechanomyography were 151 and 331 mu g.kg(-1), respectively. The dose-response curve obtained by acceleromyography was steeper and shifted to the right compared with that obtained by mechanomyography (p < 0.0001). The difference between the effective dose producing 50% twitch depression determined by the two devices was highly significant (p < 0.0001). In 13 out of 15 children, the acceleromyograph control train-of-four ratio was significantly greater than unity. Although there was a good correlation (r = 0.85) between simultaneous pairs of measurements of neuromuscular block, the acceleromyograph exhibited a bias of -25% relative to the mechanomyograph with wide limits of agreement (-62 to +12%). We conclude that acceleromyographic and mechanomyographic measurements should not be used interchangeably when determining the potency of muscle relaxants.