Potency estimation of mivacurium: comparison of two different modes of nerve stimulation

We have assessed the potency of mivacurium, a new non-depolarizing neuromuscular blocker, using two different modes of nerve stimulation in patients anaesthetized with thiopentone, fentanyl and nitrous oxide in oxygen. The force of contraction of adductor pollicis was measured after single twitch stimulation at 0.1 Hz or train-of-four stimulation (TOF) at 2 Hz every 10 s. Dose-response curves were constructed using a single-dose method for each mode of stimulation. The ED50 and ED95 were 43 micrograms kg-1 and 83 micrograms kg-1, respectively, for the single twitch responses and 34 micrograms kg-1 and 66 micrograms kg-1, respectively, for the first response of the TOF stimulation. The difference between the ED95 doses was not significant (P = 0.051), but the difference between the ED50 doses was significant (P = 0.03), suggesting greater sensitivity of the neuromuscular junction using TOF stimulation. The results show that the information obtained using single twitch stimulation at 0.1 Hz is not the same as that obtained from the first response of the TOF stimulation.