Repeated hypoxic exposures change respiratory chemoreflex control in humans

1. A group of seven volunteers (5 male, 2 female) were exposed to 20 min is ocapnic (eucapnic) hypoxia once daily for 14 consecutive days. Their chemor eflexes were measured before and after each exposure. The same volunteers r epeated the exposures with air substituted for the hypoxic gas mixture in a pseudorandom crossover design. 2. On day 1 an initial ventilatory response to hypoxia and subsequent decli ne was discernible in two volunteers, but the mean response for all volunte ers at this stage was not significant. However, the response gradually incr eased, and by day 14 was discernible in six volunteers making the mean resp onse for all volunteers significant. No change was observed over the 14 day s of air exposure. 3. Only the chemoreflex threshold measured in iso-oxic (hypoxic) modified r ebreathing tests changed significantly, and only for the series of exposure s to hypoxia. 4. Over 14 days, the mean +/- S.E.M. threshold for all volunteers fell prop ortionately, from 42 +/- 1.1 mmHg on day 1 to 39 +/- 1.0 mmHg on day 14. By contrast, the mean +/- S.E.M. threshold, for all volunteers and all days, rose from 40 +/- 0.4 mmHg before to 42 +/- 0.5 mmHg after the hypoxic expos ures. 5. We conclude that the enhancement of the initial ventilatory response to hypoxia induced by repeated hypoxic exposure is produced by a decrease in c hemoreflex threshold. However, the decline in the ventilatory response duri ng a single exposure is produced by an increase in the, chemoreflex thresho ld. Since threshold changes were only found for hypoxic (iso-oxic) modified rebreathing tests, we conclude that only the peripheral chemoreflex change d.