Analysis of Dynamic Change in Phrenic Nerve Activity Following a Sudden Decrease in Alveolar Carbon Dioxide

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The effects of a sudden decrease in alveolar CO₂ concentration (F_CO2) on phrenic nerve activity (PNA) were studied in anesthetized and paralyzed 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) F_CO2 decreased immediately and curvilinearly after hyperventilation. However, PNA did not change during the initial period of hyperventilation. After this time delay (D_t), the PNA began to decrease linearly over a certain period and finally disappeared.T_off, 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 F_CO2 in a control period and the rate of decrease in F_CO2 during hyperventilation, but Dt was mostly independent of these values.Mean (with SD) D_t was 10.68±7.01 sec (n=68, from 6 cats).
2) V_CO2, the quantity of CO₂ eliminated through the lung during T_off, was measured in each experimental run.The V_CO2 was directly proportional to the level of control F_CO2 and, at a given level of control F_CO2, was almost identical, irrespective of the different rates of decrease in F_CO2.
3) We concluded that D_t 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.