Vecuronium kinetics and dynamics in anesthetized infants and children

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Abstract
Vecuronium kinetics and dynamics were determined in 5 infants (3-11 mo. old) and 5 children (1-5 yr old) during anesthesia with 70% N2O and 0.9 MAC [minimum alveolar concentration] halothane. Vecuronium was infused i.v. at a rate of 2.5 .mu.g/kg per min while twitch tension of the adductor pollicis muscle was recorded and venous blood samples were drawn for determination of vecuronium concentrations by mass spectrometry. The elimination t1/2 [half life] was determined by linear regression of the log postdistribution concentration-time data; these values and noncompartmental techniques were used to calculate total plasma clearance (Cl), volume of distribution at steady state (Vdss), and a mean residence time. The steady-state plasma concentration resulting in 50% depression of twitch tension (CPss50) was determined by an effect compartment and a sigmoid concentration vs. paralysis model. Vdss was larger in infants (357 .+-. 70 ml/kg; .hivin.X [mean] .+-. SD) than in children (204 .+-. 116 ml/lkg), and Cl was of the same order for infant and children (5.6 .+-. 1.0 and 5.9 .+-. 2.4 ml/kg per min). Mean residence time was longer in infants (66.3 .+-. 22.9 min) than in children (34.3 .+-. 8.0 min). Cpss50 was lower in infants (57 .+-. 18 ng/ml) than in children (110 .+-. 28 ng/ml). The quantity of vecuronium in the body at steady state at 50% depression of twitch tension (Vdss .times. Cpss50) was similar in infants and children (21.2 .+-. 9.9 and 19.0 .+-. 3.3 .mu.g/kg). During comparable N2O-halothane anesthesia, age-related changes in Vdss, Cl and Cpss50 were much like those found for d-tubocurarine. There appear to be no significant age-related changes in the vecuronium dose-response relationship. The results explain why the duration of action of a single, body-weight-normalized dose of vecuronium is longer in infants than in children.