The additive contribution of nitrous oxide to isoflurane MAC in infants and children

The purpose of this study was to determine the contribution of nitrous oxide to isoflurane MAC in pediatric patients. MAC was determined in 47 infants and small children (mean ages 16.6 ± 6.7 months) during isoflurane and oxygen anesthesia (n = 11) and isoflurane and nitrous oxide anesthesia (25% nitrous oxide [n = 12], 50% nitrous oxide [n = 12], and 75% nitrous oxide [n = 12]). After assigning patients to one of four groups, anesthesia was induced with increasing inspired concentrations of isoflurane in oxygen. After anesthetic induction and tracheal intubation, ventilation was controlled (carbon dioxide partial pressure = 32 ± 5 mmHg), and nitrous oxide was added to the inspired gas mixture to achieve end-expired nitrous oxide concentrations of 0, 25, 50, or 75%. Inspired and expired gas samples were obtained from a distal sampling port in the tracheal tube. The response to skin incision in each patient was assessed at a previously selected end-tidal concentration of isoflurane. The MAC of isoflurane was determined in each group using the up-and-down method described for evaluating quantal responses. The mean duration of constant end-tidal concentrations prior to skin incision was 14 ± 7 min (range 6-46 min). The ratio of expired to inspired nitrous oxide and isoflurane concentrations during the period of constant end-tidal concentrations was 0.96 ± 0.01 and 0.93 ± 0.03 respectively. The MAC of isoflurane in oxygen was 1.69 ± 0.13 vol % (mean ± standard deviation). The MAC of isoflurane in the presence of 25, 50, and 75% nitrous oxide was 1.26 ± 0.10, 0.97 ± 0.10, and 0.58 ± 0.09 vol %, respectively. As the concentration of nitrous oxide increased, the MAC of isoflurane decreased linearly (r² = 0.93). The predicted MAC of nitrous oxide in children was 109 ± 5 vol %, a value similar to the predicted MAC of nitrous oxide in prior studies of adults.