Respiratory, cerebrovascular and cardiovascular responses to isocapnic hypoxia.

We simultaneously measured respiratory, cerebrovascular and cardiovascular responses to 10-min of isoxic hypoxia at three constant CO(2) tensions in 15 subjects. We observed four response patterns, some novel, for ventilation, middle cerebral artery blood flow velocity, heart rate and mean arterial blood pressure. The occurrence of the response patterns was correlated between some measures. Isoxic hyperoxic and hypoxic ventilatory sensitivities to CO(2) derived from these responses were equivalent to those measured with modified (Duffin) rebreathing tests, but cerebrovascular sensitivities were not. We suggest the different ventilatory response patterns reflect the time course of carotid body afferent activity; in some individuals, carotid body function changes during hypoxia in more complex ways than previously thought. We concluded that isoxic hyperoxic and hypoxic ventilatory sensitivities to CO(2) can be measured using multiple hypoxic ventilatory response tests only if care is taken choosing the isocapnic CO(2) levels used, but a similar approach to measuring the cerebrovascular response to isocapnic hyperoxia and hypoxia is unfeasible.