Ventilatory response during halothane and enflurane anaesthesia

In six children with body weights between 11.4-18.7 kg, minute ventilation, tidal volume, respiratory rate, end-tidal CO2 concentration and CO2 elimination were measured during both CO2 free breathing and CO2 breathing due to low fresh gas flows (maximal inspired CO2 about 2%) or the addition of CO2 from Rotameters (mean inspired CO2 about 1.5%) during both halothane and enflurane anaesthesia. All patients were undergoing hypospadias repair, received caudal analgesia prior to surgery and were intubated and allowed to breathe halothane/enflurane in O2/N2O (FIO2 0.5) spontaneously through a modified T-piece system (Mapleson F). End-tidal CO2 concentrations were similar with both agents during CO2-free breathing and did not increase during CO2 breathing because of increased minute ventilation, of the same magnitude with both agents, which was achieved by larger tidal volumes. Respiratory rates were unchanged. No differences were found between halothane and enflurane at the light levels of general anaesthesia made possible by combination with caudal block.