Respiratory effects of nitrous oxide during isoflurane anaesthesia in children

Respiratory effects of nitrous oxide and isoflurane were studied in 13 children (mean age 45.6 +/- 19.3 months, mean weight 14.9 +/- 4.8 kg) during surgery under continuous extradural anaesthesia. Three different anaesthetic states were studied: isoflurane 0.5 MAC in oxygen (27 study periods), isoflurane 0.5 MAC with 50% nitrous oxide (32), isoflurane 1 MAC in oxygen (25). End-tidal carbon dioxide (PE' CO2) and isoflurane, respiratory indices (tidal volume, VT; minute ventilation, VE; mean inspiratory flow, VI; respiratory frequency f, effective inspiratory timing TI/Ttot were measured. The addition of nitrous oxide (comparison of respiratory variables obtained in 25 successive periods at (1) and (2)) produced a significant increase in PE' CO2' significant decreases in VT, VE and VI, a significant increase in f. The increase in alveolar concentration of isoflurane ((1) compared with (3) in 25 successive periods) was associated with a significant increase in PE' CO2' significant decreases in VT, VE, VI and a significant increase in f. The equipotent anaesthetic states (2) and (3) were compared in 21 successive periods. In children, the net result of substituting nitrous oxide for an equal MAC fraction of isoflurane was to produce a smaller decrease in VT responsible for a smaller decrease in VE without significant change in respiratory rate.