Cardiorespiratory Responses to Exercise in Hypobaric versus Normobaric Hypoxia: A Randomized, Single-Blind, Crossover Study

PURPOSE: There is controversy whether there are meaningful physiological differences between hypobaric (HH) and normobaric hypoxia (NH). This study aimed to compare the cardiorespiratory responses to acute HH and NH under strictly controlled conditions. We hypothesized no differences at rest and during submaximal exercise, whereas during maximal exercise, a higher maximal ventilation (V̇ Emax ), peripheral oxygen saturation (SpO 2 ), and maximal oxygen consumption (V̇O 2max ) in HH than in NH.

METHODS: In a randomized, single-blind, crossover design, eight young healthy subjects (three females) were studied in an environmental chamber in which either the barometric pressure (HH) or the inspired oxygen fraction (NH) was reduced to the equivalent of ~4000 m altitude. Measurements were taken at rest, and during submaximal (moderate and high intensity) and maximal cycling exercise.

RESULTS: All resting parameters were similar between HH and NH, except for a lower root mean square of the successive R-R interval differences in HH ( P < 0.05). SpO 2 was 2% higher in HH at all exercise intensities ( P < 0.05). During submaximal exercise, minute ventilation was similar between HH and NH. However, HH yielded a 7% lower tidal volume during moderate-intensity exercise ( P < 0.05) and a lower respiratory exchange ratio during high-intensity exercise ( P < 0.01). V̇ Emax and V̇O 2max were 11% and 6% higher in HH, respectively ( P < 0.01 for both). SpO 2 at maximal exercise was positively correlated with V̇ Emax , V̇ Emax /V̇O 2max , and V̇O 2max .

CONCLUSIONS: The higher V̇O 2max found in HH than in NH can be attributed to the higher V̇ Emax counteracting desaturation at maximal exercise. Conversely, submaximal SpO 2 improved in HH through mechanisms other than increased ventilation. These findings are likely due to respiratory muscle unloading in HH, which operated through different mechanisms depending on exercise intensity.