Transient and steady state responses of pulmonary ventilation to the medullary extracellular pH after approximately rectangular changes in alveolar $$P_{CO_2 } $$

The extracellular pH (pH_e) either on the ventral surface of the medulla oblongata or the parietal cortex, the tidal volume, the expiratory\(P_{CO_2 } \) and the arterial blood pressure were continuously recorded in anaesthetized or unanaesthetized decerebrate cats. The concentration of the inspired CO₂ was manipulated in order to obtain a nearly rectangular increase in the end-tidal\(P_{CO_2 } \). The responses of\(V_{{\text{T}}^ \cdot } f,\dot V_{\text{E}}\) and pH to such a change in\(P_{CO_2 } \) were observed. The observations from such a preparation were: 1 pH_e responded with a delay of 5–7 s to a rectangular variation in end-tidal\(P_{CO_2 } \). 2. The time constant of the change in the medullary extracellular pH was in the range of 50 s and a similar value was found forV _T and\(\dot V_{\text{E}}\). 3. The response ofV _T or\(\dot V_{\text{E}}\) to a change in the medullary pH_e was approximately linear in anaesthetized as well as in unanaesthetized decerebrate cats. The slope of the respiratory response ofV _T to pH_e in decerebrate cats was about 3 times greater than that in anaesthetized cats. There were only slight differences in the relation ofV _T to pH_e between the transient and steady state responses. This means that the “upstroke” of the on-transient ofV _T was approximately the same as the ‘downstroke’ of the off-transient. On the other hand, a slight delay was observed when\(\dot V_{\text{E}}\) was plotted against pH_e. Pronounced delay occurred when respiratory frequency was plotted against pH_e for on-and off-CO₂ transients. 4. A marked hyteresis was observed whenV _T or\(\dot V_{\text{E}}\) was plotted against the cortical pH_e for on-and off-CO₂ inhalation. 5. Such a precise time correlation of the medullary surface pH andV _T changes could only be possible if the pH on the ventral medullary surface is representative for the pH at the sensor.