Prolonged Neuromuscular Block After Mivacurium

Mivacurium is a new short acting synthetic bis-benzylisoquinolinium diester nondepolarizing muscle relaxant. The reported ED95 in adults and children is 0.07–0.08 mg/kg (1,2) and 0.10--0.11 mg/kg (3,4), respectively. A dose two times the ED95 provides tracheal intubating conditions within 2.5 min (1,2,5). Usual duration of action is 15–30 min, one-third to one-half that of the intermediate acting nondepolarizing agents (6). Like succinylcholine, mivacurium depends primarily on plasma cholinesterase (pseudocholinesterase) for metabolism and termination of clinical effect (5,7). The in vitro rate of hydrolysis of mivacurium in pooled human plasma experiments ranges from 70% to 88% that of succinylcholine (5,7). Duration of drug action will vary in patients with reduced plasma cholinesterase activity. Ali et al. (6), for example, demonstrated a significant correlation between the infusion rate of mivacurium required to maintain 95% twitch depression and the plasma cholinesterase activity in individual subjects. Individuals known to have pseudocholinesterase deficiencies have been studied with mivacurium and demonstrate prolonged neuromuscular block (8). No reports exist documenting the unanticipated occurrence of prolonged neuromuscular block after mivacurium. We present a case of prolonged neuromuscular block in a patient with previously undiagnosed homozygous atypical pseudocholinesterase deficiency.