The role of brief hypocapnia in the ventilatory response to CO 2 with hypoxia

Abstract

In conscious cats the ventilatory response curve to physiological range of CO 2 is displaced upward by hypoxia (about 45 torr), but it rises, either parallel with, or convergent on, the normoxic curve. Thus, a positive interaction of hypoxia and hypercapnic stimuli is not observed under these circumstances. However, if during the hypoxic exposure, hypocapnia is allowed to develop, the subsequently determined CO 2 ventilatory response curve will shift to the left, rise steeply, particularly in the early pliase, and demonstrate a positive hypoxic-hypercapnic interaction. A demonstrable interactive effect was dependent on a conditioning period of hypocapnia, and this was shown to be associated with an elevated level of lactic acid to a greater degree in cerebral venous blood than in CSF or arterial blood. The interpretation is discussed without reaching a firm conclusion of mechanism, but the results emphasize how a minor change of experimental protocol affects a basic phenomenon in the chemical control of breathing.