Vecuronium kinetics and dynamics in anesthetized infants and children

Vecuronium kinetics and dynamics were determined in five infants (3 to 11 months old) and five children (1 to 5 years old) during anesthesia with 70% nitrous oxide and 0.9 MAC halothane. Vecuronium was infused intravenously at a rate of 2.5 μg/kg/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 t½ was determined by linear regression of the log postdistribution concentration—time data; these values and noncompartmental techniques were used to calculate total plasma clearance (C1), volume of distribution at steady state (Vd ss ), and mean residence time. The steady‐state plasma concentration resulting in 50% depression of twitch tension (Cp ss50 ) was determined by an effect compartment and a sigmoid concentration vs. paralysis model. Vd ss was larger in infants (357 ± 70 ml/kg; X̄ ± SD) than in children (204 ± 116 ml/kg), and Cl was of the same order for infants and children (5.6 ± 1.0 and 5.9 ± 2.4 ml/kg/min). Mean residence time was longer in infants (66.3 ± 22.9 minutes) than in children (34.3 ± 8.0 minutes). Cp ss50 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 (Vd ss × Cp ss50 ) was similar in infants and children (21.2 ± 9.9 and 19.0 ± 3.3 μg/kg). During comparable nitrous oxide–halothane anesthesia, age‐related changes in Vd ss , Cl, and Cp ss50 were much like those found for d ‐tubocurarine. These findings are consistent with our observation that there appear to be no significant age‐related changes in the vecuronium dose‐response relationship. Our results explain why the duration of action of a single, body‐weight‐normalized dose of vecuronium is longer in infants than in children. Clinical Pharmacology and Therapeutics (1985) 37, 402–406; doi: 10.1038/clpt.1985.62