Does age or pseudocholinesterase activity predict mivacurium infusion rate in children?

Previous studies have suggested that the mivacurium infusion rate to maintain target twitch depression is greater in children than in adults, and that there is only a limited relationship between pseudocholinesterase activity and mivacurium infusion rate in children. We therefore examined whether mivacurium infusion rates are larger in children than in adults, and whether pseudocholinesterase activity influences mivacurium infusion rate in children. In 20 children aged 1-9 yr, mechanical twitch response to ulnar nerve train-of-four stimulation was measured; concurrent data were obtained in 14 adults aged 18-58 yr. All patients were anesthetized with N2O and isoflurane, 0.75 minimum alveolar anesthetic concentration (MAC) (age-adjusted). Mivacurium was infused at constant rates for > 15 min targeting 50% and 90% twitch depression. The Hill equation was fit to the resulting values for twitch depression versus mivacurium infusion rate to predict infusion rates producing 50% and 90% twitch depression (IR50 and IR90, respectively). The relationship of IR50 and IR90 to pseudocholinesterase activity was determined by linear regression; values for children and adults were compared by analysis of covariance. For children, IR50 (r2 = 0.22, P = 0.038) but not IR90 (r2 = 0.11 P = 0.21) was related to pseudocholinesterase activity. Infusion rates were approximately twice as large in children as in adults. We confirm that mivacurium infusion rates are larger in children than in adults and demonstrate a relationship between pseudocholinesterase activity and infusion rates.