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

Abstract

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.

FIGURE 1

SpO₂, ${\dot{\mathrm{V}}}_{\mathrm{E}}$, and $\dot{\mathrm{V}}{\mathrm{O}}_2$ as a function of absolute mechanical power output. Arrow indicates the increase in maximal power output in HH compared with NH. Open symbols, HH; closed symbols, NH. *P < 0.05, ** P < 0.01 versus NH.

FIGURE 2

RER and the ventilatory equivalent for oxygen ( ${\dot{\mathrm{V}}}_{\mathrm{E}}/\dot{\mathrm{V}}{\mathrm{O}}_2$) as a function of percentage utilization of the condition-specific $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$. The arrows indicate the shift of the % $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ in HH compared with NH. The RER points lie on the same relationship, while the ${\dot{\mathrm{V}}}_{\mathrm{E}}/\dot{\mathrm{V}}{\mathrm{O}}_2$ versus % $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ curve is shifted leftward and upward in HH compared with NH. Open symbols, HH; closed symbols, NH. *P < 0.05, ** P < 0.01, and *** P < 0.001 versus NH.

FIGURE 3

A, Relationship between V_T and ${\dot{\mathrm{V}}}_{\mathrm{E}}$. Dashed lines represent isopleths for the indicated respiratory frequency (f_R), and arrows indicate the increase in ${\dot{\mathrm{V}}}_{\mathrm{E}}$ in HH compared with NH. B, Relationship between exercise HR and $\dot{\mathrm{V}}{\mathrm{O}}_2$. Dashed lines represent isopleths for the indicated $\dot{\mathrm{V}}{\mathrm{O}}_2/\mathrm{HR}$ ratio, and arrows indicate the increase in maximal $\dot{\mathrm{V}}{\mathrm{O}}_2$ in HH compared with NH. For all panels: open symbols, HH; closed symbols, NH. *P < 0.05, ** P < 0.01, and *** P < 0.001 versus NH.

FIGURE 4

A and B, At maximal exercise, SpO₂ increases as a function of maximal minute ventilation, both absolute ( ${\dot{\mathrm{V}}}_{\text{Emax}}$) and normalized for maximal oxygen uptake ( ${\dot{\mathrm{V}}}_{\text{Emax}}/\dot{\mathrm{V}}{\mathrm{O}}_{2max}$). C, $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ increases as a function of SpO₂. D, Relationship between measured and predicted (21) $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$, with the dashed line representing the identity line. For all panels: open symbols, HH; closed symbols, NH; gray symbols, group means.