Analysis of dynamic change in phrenic nerve activity following a sudden decrease in alveolar carbon dioxide

The effects of a sudden decrease in alveolar CO2 concentration (FCO2) on phrenic nerve activity (PNA) were studied in anesthetized and paralzyed cats. The vago-sympathetic and carotid sinus nerves were sectioned. The peak of integrated PNA was used as an index of the central inspiratory activity. 1) FCO2 decreased immediately and curvilinearly after hyperventilation. However, PNA did not change during the initial period of hyperventilation. After this time delay (Dt), the PNA began to decrease linearly over a certain period and finally disappeared. Toff, the time from the onset of hyperventilation to the disappearance of PNA, was in the range of 30-250 sec. This was related to both the level of FCO2 in a control period and the rate of decrease in FCO2 during hyperventilation, but Dt was mostly independent of these values. Mean (with SD) Dt was 10.68 +/- 7.01 sec (n = 68, from 6 cats). 2) VCO2, the quantity of CO2 eliminated through the lung during Toff, was measured in each experimental run. The VCO2 was directly proportional to the level of control FCO2 and, at a given level of control FCO2, was almost identical, irrespective of the different rates of decrease in FCO2. 3) We concluded that Dt is the time required for a change in the hydrogen ion concentration, [H+], in the brain interstitial fluids bathing the central chemosensitive structures, and that the central inspiratory activity, in the absence of the peripheral chemoreceptors, will be a single function of the [H+] in these fluids.