AltitudeOmics: Breathing variability at rest and during exercise across 16 days of acclimatization to hypobaric...

Ventilatory acclimatization to high altitude is an important adaption to prolonged time at altitude. Changes in breathing variability over a 16 day acclimatization period to altitude, when awake, were quantified. We tested the hypothesis that the increase in ventilatory loop gain with acclimatization would exacerbate breathing variability at rest and during exercise and that these changes would be correlated with the changes in cerebrovascular CO₂, chemoreceptor sensitivity and successful ventilatory acclimatization. Breath-by-breath ventilation, partial pressure of end-tidal O₂ and CO₂ in 21 healthy lowlanders were recorded at sea level (SL), at day 1 at high altitude (ALT1) and after 16 days of acclimatization (ALT16) to 5260 m at rest and during submaximal and maximal exercise. Power spectrum density (PSD) and the frequency of ventilatory oscillations (f_max) were compared across conditions. At rest, PSD of oscillations were significantly greater at ALT1 and ALT16 compared to SL and f_max of oscillations was significantly lower at ALT16 compared to SL and ALT1. During submaximal exercise PSD of these oscillations was also significantly greater at ALT1 and ALT16 compared to SL. Their f_max were not different between SL, ALT1 and ALT16. The area under the curve (AUC) of ventilation at SL rest and the AUC of ventilation increase from SL to ALT1 was positively associated with the increase in ventilation from SL to ALT16. These data highlight the intrinsic oscillatory nature of the closed loop ventilatory system during and after altitude acclimatization. The AUC metric at SL and acute altitude may be important for predicting successful ventilatory acclimatization with chronic altitude. KEY POINTS: The amplitude of the ventilatory oscillations in acute and chronic hypoxia were greater than at sea level (SL). The frequency of oscillations was slower after acclimatization. These changes may be a result of the increased chemosensitivity and to the closed loop system of the control of ventilation. These changes may also contribute to the adaptation of this closed loop to chronic altitude by limiting the decrease in arterial $P_{O_2}$ . The area under the curve (AUC) of ventilation at SL rest and the change in AUC from SL to day 1 at high altitude (ALT 1) were both positively associated with the greatest increases in VE from SL to ALT 16, highlighting the potential importance of this metric in predicting successful ventilatory acclimatization to high altitude.