Enzymatic Versus Pharmacologic Antagonism of Profound Mivacurium-induced Neuromuscular Blockade

Background: Mivacurium, a nondepolarizing muscle relaxant, is hydrolyzed by butyrylcholinesterase. The use of butyrylcholinesterase for antagonism of profound mivacurium-induced blockade has not been studied in humans. In part 1 of this two-part study, the authors examined the relationship between the posttetanic count (PTC) and recovery from profound mivacurium-induced blockade. In part 2, an attempt was made to antagonize a quantified level of profound mivacurium-induced blockade using either butyrylcholinesterase, edrophonium, or neostigmine.

Methods: Eighty-seven ASA physical status 1 or 2 adult patients were given 0.15 mg *symbol* kg sup -1 mivacurium during fentanyl-thiopental-nitrous oxide-isoflurane anesthesia. They were randomly assigned to eight groups. Neuromuscular function was monitored by recording the mechanomyographic response of the adductor pollicis to PTC and train-of-four (TOF) stimulation in all patients except those in group 1 where the TOF was the only pattern used. In part 1, neuromuscular function was allowed to recover spontaneously in ten patients (group 1; control-TOF) until TOF ratio (the amplitude of the fourth evoked response as a fraction of the first evoked response: T4/T1) had reached 0.75. The temporal relationship between PTC and the first reaction to TOF stimulation was determined in another 21 patients, and neuromuscular function in 10 of these patients was allowed to recover spontaneously until TOF ratio had reached 0.75 (group 2; control-PTC). In part 2, the antagonism of mivacurium-induced profound (PTC greater or equal to 1; groups 3-6) and 90% block (groups 7-8) of twitch height were investigated in another 56 patients. Groups 3 and 7 received neostigmine 0.06 mg *symbol* kg sup -1 whereas groups 4 and 8 received edrophonium 1 mg *symbol* kg sup -1, respectively. Groups 5 and 6 received exogenous human butyrylcholinesterase equivalent to activity present in 25 or 70 ml *symbol* kg sup -1 of human plasma, respectively.

Results: Neither butyrylcholinesterase nor edrophonium shortened the times from first PTC response to TOF = 0.75 compared to group 2. Neostigmine resulted in prolongation of recovery time. There was a linear relationship (r = -0.80; P = 0.00001) between PTC and time of onset of TOF response.