Breathing patterns in infants and children under halothane anesthesia: effect of dose and CO2.

We studied the amplitude, timing, and shape of the airflow waveform at the mouth of spontaneously breathing children under two sets of conditions: 1) in 30 children aged 9 wk-4.5 yr at 2, 1, and 0% inspired halothane concentration and 2) in 22 children aged 5 mo-7 yr during hyperoxic CO2 rebreathing while recovering from anesthesia. Compared with control values, the relative changes in breath parameters at 1 and 2% halothane were, respectively, as follows: total cycle time -19 and -31%, tidal volume (VT) -30 and -44%, minute ventilation -11 and -17%, and VT/inspiratory time (TI) -16 and -20%. Parameters of timing and breath shape did not change except for the significant but small increase in TI/total cycle time (by 6 and 8%, respectively). With CO2 rebreathing, parameters reflecting inspiratory drive increased significantly in all patients as shown by the slopes of the regressions of these parameters against end-tidal PCO2. Mean slopes expressed in %control value per millimeter of mercury CO2 were 12.1 for minute ventilation, 8.3 for VT, and 10.67 for VT/TI. Parameters reflecting the timing and breath shape remained essentially unchanged. Our results suggest that, in children under halothane anesthesia, the amplitude, timing, and shape of the breathing pattern are controlled independently. In particular, the amplitude and timing of the breath may vary widely without any significant change in the shape.