Key result
Acute isocapnic intermittent hypoxia elicited long-term facilitation in ventilation following both 5-minute and 90-second cycles (P<0.001 for both), but did not induce progressive augmentation.
Why the study?
It was unknown whether cardiorespiratory experience-dependent effects differ between different stimulus durations of acute intermittent hypoxia, and whether animal model findings translate to humans.
Do cardiorespiratory experience-dependent effects differ between 5-min and 90-s cycles of acute isocapnic intermittent hypoxia in healthy humans?
Do cardiorespiratory experience-dependent effects differ between 5-min and 90-s cycles of acute isocapnic intermittent hypoxia in healthy humans?
p-value: p=<0.001
Acute isocapnic intermittent hypoxia elicits long-term facilitation in ventilation and blood pressure regardless of cycle length, but does not produce all experience-dependent effects seen in animal models.
Acute isocapnic intermittent hypoxia elicits ventilatory long-term facilitation independent of cycle length in humans; leaves open translation of other experience-dependent effects from animal models.
New Findings What is the central question of this study? Do cardiorespiratory experience‐dependent effects (EDEs) differ between two different stimulus durations of acute isocapnic intermittent hypoxia (IHx; 5‐min vs. 90‐s cycles between hypoxia and normoxia)? What is the main finding and its importance? There was long‐term facilitation in ventilation and blood pressure in both IHx protocols, but there was no evidence of progressive augmentation or post‐hypoxia frequency decline. Not all EDEs described in animal models translate to acute isocapnic IHx responses in humans, and cardiorespiratory responses to 5‐min versus 90‐s on/off IHx protocols are largely similar. Abstract Peripheral respiratory chemoreceptors monitor breath‐by‐breath changes in arterial CO 2 and O 2 , and mediate ventilatory changes to maintain homeostasis. Intermittent hypoxia (IHx) elicits hypoxic ventilatory responses, with well‐described experience‐dependent effects (EDEs), derived mostly from animal work involving intermittent 5‐min cycles of hypoxia and normoxia. These EDEs include post‐hypoxia frequency decline (PHxFD), progressive augmentation (PA) and long‐term facilitation (LTF). Comparisons of these EDEs between animal models and humans using similar IHx protocols are lacking. In addition, it is unknown whether shorter bouts of hypoxia, which may be more relevant to clinical conditions, elicit EDEs of similar magnitudes in humans. Respiratory (frequency, tidal volume and minute ventilation ( ) and cardiovascular (heart rate and mean arterial pressure (MAP)) variables were measured during and following two patterns of acute isocapnic IHx in 14 healthy human participants (four female): (1) 5 × 5 min and (2) 5 × 90 s on/off hypoxia. Participants’ end‐tidal was clamped at 45 Torr during hypoxia and 100 Torr during normoxia. We found that (1) PHxFD and PA were not present in either IHx pattern ( P > 0.14), (2) LTF was present in following both 5‐min ( P < 0.001) and 90‐s isocapnic IHx trials ( P < 0.001), and (3) LTF was present in MAP following 5‐min isocapnic IHx ( P < 0.001), and trended towards significance following 90‐s IHx ( P = 0.058). We demonstrate that acute isocapnic IHx alone may not elicit all of the EDEs that have been described in animal models. Additionally, ventilatory LTF occurred regardless of the length of hypoxia–normoxia cycles.
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Keough et al. (2021) studied Healthy (n=14). Acute isocapnic intermittent hypoxia vs. Between protocols was evaluated on Long-term facilitation (LTF) in ventilation (p=<0.001). Acute isocapnic intermittent hypoxia elicited long-term facilitation in ventilation following both 5-minute and 90-second cycles (P<0.001 for both), but did not induce progressive augmentation.
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