A protocol for determining the shape of the ventilatory response to hypoxia in humans

The ventilatory response to isocapnic hypoxia is biphasic, which makes any experimental assessment of the relationship between the acute (peak) ventilatory response and the level of hypoxia difficult. This study explored whether one particular protocol could be useful for determining this relationship. The protocol consisted of exposing subjects to seven different levels of isocapnic hypoxia, each of which lasted 50 sec. In order to test whether the order of the hypoxic exposure had any effect on the outcome, the steps were performed both in increasing and decreasing severity of hypoxia, and the ventilatory responses compared. Twelve subjects were studied, and each test was repeated four times in each subject. PETCO2 was held at 2 mmHg above resting throughout. The ventilations obtained at the lowest level of PETO2 employed were clearly different between the two protocols. However, provided that these ventilations were excluded, no significant differences were present between the results from the ascending and descending exposures (ANOVA). This finding suggests that the rate of change of PO2 in these protocols was sufficiently slow for a full ventilatory response to develop, but also sufficiently fast to prevent significant ventilatory depression from occurring.