Fig. 7.

Diagrammatic representation of the relationship between alveolar ventilation (V_A) and alveolar Pco₂ (Pa_CO2) at a fixed resting CO₂ production (of 250 ml/min); Pa_CO2 = V̇co₂/ V̇_A × K. The schematic figure shows how changing plant gain (A, top) or controller gain (B, bottom) will influence the “CO₂ reserve” or ΔPa_CO2 between eupnea and apnea. A: changing the background drive to breathe without changing the slope of the ΔV_A vs. ΔPa_CO2 relationship above or below eupnea. For example, background hyperventilation raises V_A and lowers Pa_CO2 along the isometabolic hyperbola. This means that a greater transient increase in V_A and reduction in Pa_CO2 is required to reach the apneic threshold than it would be under control, normocapnic conditions. The reverse is true for conditions which reduce the background drive to breathe and cause hypoventilation. B: at any given level of background Pa_CO2, changing the slope (or responsiveness) of the ΔV_A-ΔPa_CO2 relationship below eupnea would alter the CO₂ reserve or the amount of reduction in Pa_CO2 required to cause apnea. Changing the slope of the ventilatory response to CO₂ above eupnea would alter the susceptibility for transient ventilatory overshoots. See text for a discussion of conditions which change controller and plant gain and therefore the susceptibility to transient ventilatory overshoots to apnea and ventilatory instability in sleep. [Adapted from Dempsey (127).]