Figure 4. Ventilatory response to CO₂ below the eupnoeic P_CO2.

The continuous lines were drawn between the point of eupnoeic ventilation at eupnoeic P_CO2 and the point of P_CO2 at the apnoeic threshold P_CO2 using data from the present study during normoxia and hypoxia. The slope of the ventilatory response below eupnoeic P_CO2 was greater during hypoxia compared to normoxia. The dashed line represents a hypothetical non-hypoxic ventilatory stimulant that lowered the eupnoeic P_CO2 to the same extent as we observed during hypoxia, but did not change the slope of the ventilatory response below eupnoea compared to room air breathing. Note that a much smaller reduction in P_CO2 is required to produce apnoea during hypoxia compared to the non-hypoxic ventilatory stimulant.