Ventilatory responses to carbon dioxide in children during nitrous oxide-halothane anaesthesia

The ventilatory response to carbon dioxide was studied in 12 unpremedicated children, aged 20-68 months, weighing between 10 and 20 kg, under nitrous oxide-halothane anaesthesia. Tidal volume (VT) and end-tidal carbon dioxide tension (PE'CO2) were continuously measured by pneumotachograph and capnograph. Minute ventilation (VE), respiratory rate (f), mean inspiratory flow (Vl) and effective inspiratory cycle (Tl/Ttot) were calculated during anaesthesia at three different inspired halothane concentrations (0.5, 1 and 1.5%). The ventilatory response to carbon dioxide was determined by relating the increase in ventilation during exposure to 2% carbon dioxide to the change in end-tidal carbon dioxide concentration. When the inspired concentration of halothane increased, there were significant decreases in VE, VT, Vl, and a significant increase in PE' CO2. The slope of the carbon dioxide response under light nitrous oxide-halothane anaesthesia (0.5% halothane) was relatively flat (18.64 ml min-1 kg-1 mm Hg-1) when compared with the mean values published for anaesthetized adults, children or neonates. When the inspired concentration of halothane was increased, the slope decreased significantly (39% of initial value at 1% inspired halothane, 26% at 1.5%). The addition of carbon dioxide produced significant increases in VE, VT and Vl but no change in respiratory rate. No statistical difference was observed in the slope of carbon dioxide response between the initial and "control" periods which were measured at the same inspired halothane concentration (0.5%).