Repeated hypoxic exposures change respiratory chemoreflex control in humans

Abstract

A group of seven volunteers (5 male, 2 female) were exposed to 20 min isocapnic (eucapnic) hypoxia once daily for 14 consecutive days. Their chemoreflexes were measured before and after each exposure. The same volunteers repeated the exposures with air substituted for the hypoxic gas mixture in a pseudorandom crossover design. On day 1 an initial ventilatory response to hypoxia and subsequent decline was discernible in two volunteers, but the mean response for all volunteers at this stage was not significant. However, the response gradually increased, and by day 14 was discernible in six volunteers making the mean response for all volunteers significant. No change was observed over the 14 days of air exposure. Only the chemoreflex threshold measured in iso‐oxic (hypoxic) modified rebreathing tests changed significantly, and only for the series of exposures to hypoxia. Over 14 days, the mean ±s.e.m. threshold for all volunteers fell proportionately, 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 exposures. We conclude that the enhancement of the initial ventilatory response to hypoxia induced by repeated hypoxic exposure is produced by a decrease in chemoreflex threshold. However, the decline in the ventilatory response during a single exposure is produced by an increase in the chemoreflex threshold. Since threshold changes were only found for hypoxic (iso‐oxic) modified rebreathing tests, we conclude that only the peripheral chemoreflex changed.

Hypoxic ventilatory response is correlated with increased submaximal exercise ventilation after live high, train low

“…In that study, P ET CO 2 was further reduced after two more nights of hypoxia, suggesting that chronic hypocapnia was progressively developed, which is typical during the early phase of ventilatory acclimatisation. Additionally, it has been reported that central chemosensitivity is enhanced following chronic hypoxia exposure lasting 48 h Tansley et al 1998), but not after 2 weeks of daily intermittent hypoxic exposures lasting 20 min (Mahamed and Duffin 2001). A limitation to our study, though, is that we did not conduct a measure of central chemosensitivity.…”

Impact of intermittent hypoxia on long-term facilitation of minute ventilation and heart rate variability in men and women: do sex differences exist?

“…The decrease in ⌬V E/⌬PET O 2 we observed might also have occurred because the peripheral chemoreflex threshold gradually decreased from the first to the last hypoxic episode. Although increases in the peripheral chemoreflex threshold have not been documented during exposure to intermittent hypoxia, increases have been observed during exposure to short periods of sustained hypoxia (31). Nonetheless, we believe that it is unlikely that decreases in peripheral chemoreflex sensitivity and/or increases in the chemoreflex threshold are responsible for the decreases in ⌬V E/⌬PET O 2 observed.…”

Cerebral hypoperfusion during hypoxic exercise following two different hypoxic exposures: independence from changes in dynamic autoregulation and reactivity

“…Thus IH or CH induces some adaptation at the muscle level and lowers MCAv and cerebral oxygenation during hypoxic exercise, potentially mediated by the greater hypocapnia, rather than a compromise in CA or MCAv reactivity. hypoxia; exercise; intermittent and continuous hypoxia; cerebral blood flow ELEVATIONS IN VENTILATORY sensitivity to hypoxia following exposure to either continuous hypoxia [CH, e.g., high altitude (7, 35)] or intermittent hypoxia [IH (11,20,28,42)] results in subsequent hypocapnia (i.e., reduction in end-tidal CO 2 ). Because hypocapnia results in cerebral vasoconstriction (23,32), elevations in ventilatory chemosensitivity may result in cerebral hypoperfusion.…”