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±4.8kg) during surgery under continuous extradural anaesthesia. Three different anaesthetic states were studied: (1) isoflurane 0.5 MAC in oxygen (27 study periods), (2) isoflurane 0.5 MAC with 50% nitrous oxide (32), (3) isoflurane 1 MAC in oxygen (25). End-tidal carbon dioxide (PE 1 CO2 ) and isoflurane, respiratory indices (tidal volume, VT; minute ventilation, VE; mean inspiratory flow, VI; respiratory frequency f, effective inspiratory timing T1/T tot 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 MA C fraction of isoflurane was to produce a smaller decrease in Vr responsible for a smaller decrease in VE without significant change in respiratory rate.