Respiratory depression in children at different end tidal halothane concentrations.

Respiratory motor function and timing were investigated at end tidal halothane concentrations of 1.5%, 1.0% and 0.5% before and during 4% carbon dioxide stimulation in 10 spontaneously breathing children who weighed between 10.2 and 25.2 kg, during hypospadias repair under halothane anaesthesia. Their tracheas were intubated and all received a caudal block to eliminate surgical stimulation. Pneumotachography and capnography were used and in three cases movements of ribcage and abdomen were also studied by magnetometers. Respiratory drive was evaluated by occlusion tests. Ventilation was depressed at an end tidal halothane concentration of 1.5%, with smaller tidal volumes, higher respiratory rates, higher end tidal carbon dioxide tensions and a weaker respiratory drive compared with 1.0% and 0.5% halothane. Paradoxical breathing was noted at 1.5% as well as at 1.0% but not at 0.5% halothane anaesthesia; the ribcage moved inwards during inspiration. Respiratory compensation during periods of 4% carbon dioxide stimulation was inadequate at 1.5% halothane, as indicated by higher end tidal carbon dioxide tensions, less negative occlusion pressures and movements of ribcage and abdomen that were unresponsive to 4% carbon dioxide, when compared with 1.0% and 0.5% halothane. Respiratory rates were higher and duration of inspiration longer at 1.5% than at 1.0% and 0.5% halothane. Respiratory timing was unaltered by carbon dioxide stimulation. It is concluded that the ventilatory motor response to carbon dioxide is dose dependent and improves at more superficial anaesthetic levels, while respiratory timing is unresponsive to carbon dioxide stimulation irrespective of the halothane concentration used. Paradoxical breathing existed at end tidal halothane concentrations higher than 1%.