Methodological and physiological variability within the ventilatory response to hypoxia in humans

Measurement of the acute hypoxic ventilatory response (AHVR) requires caref ul choice of the hypoxic stimulus. if the stimulus is too brief, the respon se may be incomplete; if the stimulus is too long, hypoxic ventilatory depr ession may ensue. The purpose of this study was to compare three different techniques for assessing AHVR, using different hypoxic stimuli, and also to examine the between-day variability in AHVR. Ten subjects were studied, ea ch on six different occasions, which were greater than or equal to 1 wk apa rt. On each occasion, AHVR was assessed using three different protocols: I) protocol SW, which uses square waves of hypoxia; 2) protocol IS, which use s incremental steps of hypoxia; and 3) protocol RE, which simulates an isoc apnic rebreathing test. Mean values for hypoxic sensitivity were 1.02 +/- 0 .48, 1.15 +/- 0.55, and 0.93 +/- 0.60 (SD) l.min(-1).-%-1 for protocols SW, IS, and RE, respectively. These differed significantly (P < 0.01). The coe fficients of variation for measurement of AHVR were 20, 23, and 36% for the three protocols, respectively. These were not significantly different. The re was a significant physiological variation in AHVR (F-50,F-100 = 3.9, P < 0.001), with a coefficient of variation of 26%. We conclude that there was relatively little systematic variation between the three protocols but tha t AHVR varies physiologically over time.