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 (pe ak) ventilatory response and the level of hypoxia difficult. This stud y explored whether one particular protocol could be useful for determi ning 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 an y effect on the outcome, the steps were performed both in increasing a nd decreasing severity of hypoxia, and the ventilatory responses compa red. Twelve subjects were studied, and each test was repeated four tim es in each subject. PET(CO2) was held at 2 mmHg above resting througho ut. The ventilations obtained at the lowest level of PET(O2) employed were clearly different between the two protocols. However, provided th at these ventilations were excluded, no significant differences were p resent between the results from the ascending and descending exposures (ANOVA). This finding suggests that the rate of change of P-O2 in the se protocols was sufficiently slow for a full ventilatory response to develop, but also sufficiently fast to prevent significant ventilatory depression from occurring.