Problems with determining the hypoxic response in humans using stepwise changes in end-tidal PO2

This study examined whether the form of the acute ventilatory response to different levels of end-tidal PO2 (PETO2) could be determined from a progressive series of steps in PETO2 at constant end-tidal PCO2. Seven levels of PETO2 were employed each lasting 2 min. The steps were performed in both ascending and descending order to test whether the ventilatory responses were affected by the order of the hypoxic exposures. These exposures were carried out both with and without a prior 20 min period of isocapnic hypoxia (PETO2 = 50 mmHg). Each protocol was undertaken 6 times in each of 6 subjects. With prior exposure to hypoxia, the order in which the steps were performed affected the ventilatory response (P < 0.005, ANOVA). Without prior exposure to hypoxia, this finding did not quite reach significance (P < 0.056), unless one particular abnormal subject was excluded (P < 0.001). It is concluded that the order of the hypoxic exposures in this particular test affects the form of the hypoxic response.