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 (X1 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 T1/2 was therefore shorter in the young children (6.1 +/- 3.3 vs 11.1 +/- 3 sec) (P less than 0.025). T1/2 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). X1 90% was significantly larger in the young children (30 +/- 3% of the administered dose) than in the older children (26.7 +/- 2%) (P less than 0.05). Elimination T1/2 positively correlated with the amount of N2O which remained in the body at 90% steady-state (X1 90%) (r = 0.85, P less than 0.01), suggesting that the distribution of N2O and not its clearance is the main determinant of its elimination T1/2 in normal children.