Recovery of mivacurium and doxacurium versus vecuronium in the isolated forearm

To assess rate of biophase recovery, the recovery from neuromuscular block with mivacurium in the isolated forearm was compared with that from vecuronium simultaneously administered into the other isolated forearm of six volunteers. In a second series of similar experiments, recovery from doxacurium was compared with that from vecuronium. Neuromuscular block was monitored using the adductor pollicis mechanomyographic response to ulnar nerve stimulation at 0.2 Hz. Comparable degrees of maximum twitch tension depression were obtained in each series. In the first series, mean (SD) 25-75% recovery index for mivacurium was 8.4 (1.5) min and 10.5 (1.9) min for vecuronium. In the second series, mean (SD) recovery index for doxacurium was 18.3 (4.2) min and 12.2 (5.0) min for vecuronium. The recovery index of doxacurium in the isolated forearm was significantly greater, and the recovery index of mivacurium significantly less, than the recovery index of simultaneously administered vecuronium. Mivacurium block in the isolated forearm recovers rapidly, although not faster than after systemic injection; this is consistent with a drug that is retained in the biophase despite rapid plasma metabolism. Doxacurium block in the isolated forearm is slow to recover, compared with vecuronium; this suggests that high affinity for the biophase may contribute to its long duration of action.