HALOTHANE AFFECTS VENTILATORY AFTERDISCHARGE IN HUMANS

In awake humans, when ventilatory stimulation is suddenly removed, the subsequent change in minute ventilation (which remains at higher levels for longer times than expected from the dynamics of the chemoreceptors) is termed ventilatory after discharge. In this study we investigated the effects of subanaesthetic concentrations of halothane on afterdischarge. The ventilatory pattern after sudden termination of brief periods (90-180 s) of isocapnic hypoxia (PE'(CO2) approximately 0.1 kPa above initial resting values; PE'(O2) 6.5 kPa) by normoxia (PE'(O2) 14 kPa) was determined in healthy volunteers. Six subjects underwent 13 studies without halothane (control) and six others 10 studies during inhalation of 0.22% halothane. Isocapnic hypoxia caused a mean increase in ventilation of 10.8 (SD 2.4) litre min(-1) in the control and 4.2 (2.4) litre min(-1) in the halothane studies (P < 0.01). The transition to normoxia caused a slow ventilatory decay in the control and a fast decay in the halothane groups: the interval that occurred between the ''last hypoxic'' breath and the time required for ventilation to return to 110% of baseline was 60.7 (23) s for the control and 12.3 (6.0) s for the halothane studies (P < 0.05). Taking into consideration the different factors that determine the pattern of breathing immediately after termination of a brief period of hypoxia by normoxia (PE'(O2) waveform, transport delay time between lungs and carotid bodies, time constant of the peripheral chemoreflex loop and afterdischarge), the faster ventilatory decay observed with halothane is probably related to suppression of afterdischarge. We conclude that afterdischarge was activated by brief periods of isocapnic hypoxia, but not when combined with inhalation of a subanaesthetic concentration of halothane.