VENTILATORY DEPRESSION BY HALOTHANE IN INFANTS AND CHILDREN

The purpose of this study was to extend previous observations of a greater decrease in tidal volume in infants than in children during halothane anaesthesia. We analyzed the inspiratory flow waveform recorded during spontaneous ventilation in: infants, two to six months of age, and children, one to five years of age. In addition we analyzed the CO2 signal and the pressure waveform during an occluded inspiration. The pressure generated during the initial 100 msec of inspiratory occlusion, an index of respiratory drive was analyzed to give some insight into the aetiology of the age-related differences. In 15 infants and 15 children. Flow (V), pressure (Pao) and PCO2 were recorded at three concentrations of inspired halothane (FIH): 0% 1% and 2% which correspond to an endtidal halothane concentration of about 0.3%, 0.9% and 1.3% respectively. Data were analyzed for minute ventilation (Vi) and parameters of timing (Total time (Ttot), Inspiratory time (Ti)), the amplitude of the neural output (mean inspiratory flow (VT/Ti), tidal volume (VT)) and the shape of the inspiratory breath profile (the inspiratory centroid flow (Ci/Ti), the inspiratory duty cycle (Ti/Ttot)). In some, the airway was occluded at end expiration and the slope of the initial 100 msec of occlusion (dP/dt) together with the maximal negative pressure (PMAX) were measured. Estimates of respiratory mechanics E'rs (PMAX/VT) and (VT/Ti)/(dP/dt) were obtained. The VT and Ttot decreased with increasing FIH in both infants and children (P < 0.05). The PETCO(2) increased in both groups and the % increase was greater in infants. The parameters of breath shape were unchanged. Infants experienced a decrease in Vi as FIH increased (P < 0.01). In children the parameters of breath amplitude VT/Ti and dP/dt did not change with halothane administration but, in infants, VT/Ti and dP/dt showed opposite dependencies on FIH; VT/Ti decreased whereas the dP/dt increased with increasing FIH. The E'rs was higher in infants (P < 0.05). The (VT/Ti)/(dP/dt) decreased as FIH increased in infants (P < 0.05). We conclude that there are age-related changes in the ventilatory response to halothane.