2.2. Participant recruitment and inclusion criteria

A total of 16 participants volunteered for the study (29.9 ± 8.0 years; BMI 23.9 ± 3.5 kg/m²; six males), with only a subset (n = 12) completing the transient HVR test (see Section 2.4, below). Participants were all non‐hypertensive with no reported history of neurological, cardiovascular, or respiratory illness and were not taking any prescription medications aside from hormonal birth control. Because the monthly fluctuation of cycling ovarian hormones (Macnutt et al., 2012) and gender (Pfoh et al., 2016) have been previously shown to be not related to HVR magnitude, no regard was given to the position in the ovarian cycle in the recruitment of women in this study. All participants were non‐smokers and abstained from caffeine, alcohol, and exercise for at least 12 hr prior to participation. Following pre‐screening and written informed consent, participants were familiarized with the protocol prior to instrumentation (Figure 1). All data were collected in a quiet and darkly lit laboratory during mid‐day. A subset of these participants were also recruited for a previously published study (Pfoh et al., 2017), and some HVR and SS‐CD values are overlapping in both studies. However, the question of this study was determined a priori in advance and is independent from Pfoh et al. (2017), which compared various chemoreflex tests.

Protocol schematic. Following instrumentation, participants breathed room air for 10 min. We then carried out five consecutive transient hypoxic ventilatory response tests (TT‐HVR), comprising three consecutive normal tidal breaths of 100% N₂. Following a return to baseline values, participants carried out two voluntary end‐inspiratory breath holds (EI‐BH), the first under room air conditions, and the second following five tidal breaths of 100% O₂. Participants then breathed F_IO₂ 0.13.5–014 (13.5%–14%) for ~30 min to each steady state, after which they performed a third EI‐BH in hypoxic conditions