The pharmacokinetics of inhaled N₂O in children

A new approach is described for the calculation of elimination half-life of inhaled N₂O. We generated the elimination rate constant (Ke) by measuring the cumulative fraction of exhaled N₂O versus time during induction of anaesthesia with oxygen and nitrous oxide. The amount of N₂O which remained in the body at 90% steady-state (Xl 90%) was estimated by subtracting the cumulative exhaled N₂O from the cumulative inhaled N₂O. In young children (age range 4-36 mo, mean 19 mo) the elimination rate constant was significantly higher than in adolescents and young adults (age range 14-20 yr, mean 16.8 yr) (0.14 ± 0.055/sec and 0.0646 ± 0.019/sec, respectively) (P = 0.025). Elimination Tl/2 was therefore shorter in the young children (6.1 ± 3.3 vs 11.1 ± 3 sec) (P < 0.025). Tl/2 was longer (14 ± 2.6 sec) in children with left to right shunt despite being younger than the adolescents and young adults (age range 3.25-15 yr, mean 9.9). Xl 90% was significantly larger in the young children (30 ± 3% of the administered dose) than in the older children (26.7 ± 2%) (P < 0.05). Elimination Tl/2 positively correlated with the amount of N₂O which remained in the body at 90% steate-state (Xl 90%) (r = 0.85, P < 0.01), suggesting that the distribution of N₂O and not its clearance is the main determinant of its elimination Tl/2 in normal children.