Changes in ventilatory responses at high altitude measured using rebreathing

Ventilatory responses to hypoxia and hypercapnia play a vital role in
maintaining gas exchange homeostasis, and in adaptation to high-altitude
environments. This study investigates the mechanisms underlying
sensitization of hypoxic and hypercapnic ventilatory responses (HVR and
HCVR, respectively) in individuals acclimatized to moderate high altitude
(3800 m). Thirty-one participants underwent chemoreflex testing using the
Duffin modified rebreathing technique. Measures were taken at sea level
and after 2 days of acclimatization to high altitude. Ventilatory
recruitment thresholds (VRT), HCVR-Hyperoxia, HCVR-Hypoxia, and HVR were
quantified. Acclimatization to high altitude resulted in increased HVR
(p<0.001) and HCVR-Hyperoxia (p<0.001), as expected. We also
observed that the decrease in VRT under hypoxic test conditions
significantly contributed to the elevated HVR at high altitude since the
change in VRT across hyperoxic and hypoxic test conditions was greater at
high altitude compared to baseline sea level tests (p=0.043). Pre-VRT, or
basal, ventilation also increased at high altitude (p<0.001), but
the change did not differ between oxygen conditions. Taken together, this
data suggests that the increase in HVR at high altitude is at least
partially driven by a larger decrease in the VRT in hypoxia versus
hyperoxia at high altitude compared to sea level. This study highlights
the intricacies of respiratory adaptations during acclimatization to
moderate high altitude, shedding light on the roles of the VRT, baseline
respiratory drive, and two-slope HCVR in this process. These findings
contribute to our understanding of how the human respiratory control
responds to hypoxic and hypercapnic challenges at high altitude.