The role of brief hypocapnia in the ventilatory response to CO2 with hypoxia.

In conscious cats the ventilatory response curve to physiological range of CO2 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 CO2 ventilatory response curve will shift to the left, rise steeply, particularly in the early phase, 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.