Neuromuscular effects of mivacurium in 2-to 12-yr-old children with burn injury

Background: Burned patients are usually resistant to the neuromuscular effe cts of nondepolarizing relaxants, mostly because of receptor changes. The m agnitude of the resistance is related to burn size and time after burn. Miv acurium is a muscle relaxant, degraded by plasma cholinesterase, whose enzy me activity is decreased in burns. The present study tested the hypothesis that burn-induced depressed plasma cholinesterase activity counteracts the receptor-mediated resistance, resulting in a lack of resistance to mivacuri um. Methods: Burned patients (n = 23), aged 2-12 yr, subclassified into burns o f 10-30% or > 30% of body surface, were studied at less than or equal to 6 days and again at 1-12 weeks after burn if possible. Thirteen additional pa tients served as controls. Neuromuscular variables monitored included onset and recovery following bolus dose, continuous infusion rates required to m aintain 95 +/- 4% paralysis, and recovery rates following infusion. Results: The onset times of maximal twitch suppression were not different b etween burns and controls, but recovery to 25% of baseline twitch height wa s prolonged in patients with > 30% burn irrespective of time after injury. The continuous infusion rates to maintain twitch suppression at 95 +/- 4% w ere not different between groups. The recovery indices, including train-of- four to > 75%, 25-75%, or 5-95% in burned patients, were similar or prolong ed compared with controls. The prolonged recovery in burned patients was in versely related to plasma cholinesterase activity (R-2 = 0.86, r = -0.93, P < 0.001), and the decreased plasma cholinesterase activity was related to burn size and time after burn. Conclusions: A normal mivacurium dosage (0.2 mg/kg) effects good relaxation conditions in burned patients, with an onset time similar to that in contr ols. This finding contrasts with the response seen with other nondepolarizi ng drugs, higher doses of which are required to effect paralysis. The decre ased metabolism of mivacurium, resulting from depressed plasma cholinestera se activity, probably counteracts the receptor-mediated potential for resis tance. Because succinylcholine is contraindicated in burned patients, large r doses of nondepolarizing agents are advocated to effect rapid onset of pa ralysis. This generalization does not hold for mivacurium.