The pharmacokinetics and pharmacodynamics of d-tubocurarine (dTc) were determined in neonates (0-2 months, n = 7), infants (2-12 months, n = 7), children (1-12 years, n = 9), and adults (12-30 years, n = 8) during 70% nitrous oxide, 0.58 MAC halothane anesthesia. dTc was administered by infusion, while blood for determination of plasma dTc concentrations was obtained, and the EMG of the adductor pollicis recorded. The plasma dTc concentration at which 50% depression of EMG twitch height occurs (Cpss(50)) was 0.18 +/- 0.09 micrograms/ml in neonates, and 0.27 +/- 0.06 micrograms/ml in infants, both significantly lower than the values of 0.42 +/- 0.14 and 0.53 +/- 0.14 micrograms/ml for children and adults, respectively. The steady-state distribution volume (Vdss) was 0.74 +/- 0.33 l/kg in neonates, significantly greater than the values of 0.52 +/- 0.22, 0.41 +/- 0.12, and 0.30 +/- 0.10 l/kg in infants, children, and adults, respectively. The elimination half-life (t beta 1/2) was 174 +/- 60 min in neonates, significantly longer than the values of 90 +/- 23 and 89 +/- 18 min in children and adults, respectively. Plasma clearance did not differ with age. We also determined D50, the product of Vdss and Cpss(50). D50, the quantity of drug present at steady-state to produce 50% paralysis, did not differ between groups. The authors conclude that during comparable nitrous oxide-halothane anesthesia, neonates and infants have an increased sensitivity to dTc, as determined by CPss(50). However, because of the larger Vdss in younger patients, dose size should not differ with age. In addition, because of the longer t beta 1/2 in neonates, second and subsequent doses should be required at less frequent intervals.
