Following 10 min of quiet sitting for baseline measurements (Table 1), the HVR test was carried out (see Pfoh et al., 2016, 2017 for detailed methodology and background). The transient HVR test consisted of three consecutive inspired tidal breaths of 100% N2 from a 50 L Douglas bag. Expired gases were directed to room air once passing by the gas analyzer port proximal the mouthpiece of the participant. The transient HVR test was repeated five times, each separated by 1–2 min to ensure respiratory gases (P ETCO2 and P ETO2) and SpO2 levels returned to baseline. To administer the transient test, an investigator manually switched the three‐way valve between room air and the 100% N2 gas mixture while monitoring the participants’ respiratory flow signal. Switching between the inspired gas mixtures was performed during expiration, largely without the participants’ awareness.

Breath‐hold duration and baseline variables in room air and in steady‐state normobaric hypoxia

Baseline data obtained while breathing room air and after reaching steady state while breathing 13.5%–14% FIO2 (~30‐min).

Abbreviations: and HVR, hypoxic ventilatory response (tested via 5× transient N2 test). EI‐BHD, end‐inspiratory breath‐hold duration; HR, heart rate; MAP, mean arterial pressure; P ETCO2, partial pressure end‐tidal carbon dioxide; P ETO2, partial pressure end‐tidal oxygen; R R, respiratory rate; ScO2, calculated oxygen saturation; SpO2, peripheral oxygen saturation by pulse oximetry; SS‐CD, steady‐state chemoreflex drive; I, ventilation; V TI, inspired tidal volume.

EI‐BHD in hyperoxia (third column) is following five consecutive tidal breaths of 100% FIO2. Values are mean ± SD.

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