Normo- and hypobaric hypoxia: are there any physiological differences?

Since Bert (1878) and Barcroft (1925), studies on hypoxia are realized by lowering ambient O₂ partial pressure (PO₂) either by barometric pressure reduction (hypobaric hypoxia HH) or by lowering the O₂ fraction (normobaric hypoxia NH). Today, a question is still debated: "are there any physiological differences between HH and NH for the same ambient PO₂?" Since published studies are scarce and controversial, we submitted 18 subjects in a random order to a 40-min HH test and to a 40-min NH test at an ambient PO₂ equal to 120 hPa (4500 m). Cardioventilatory variables [breathing frequency (f), tidal volume (V _t), minute ventilation (V̇_E), O₂ and CO₂ end-tidal fractions or pressures (FET_O2 and FET_CO2 or PET_O2 and PET_CO2 respectively), heart rate (HR) and O₂ arterial saturation by pulse oxymetry (SpO₂)] were measured throughout the tests. At the end of the tests, arterial blood samples were taken to measure arterial blood gases [O₂ and CO₂ arterial partial pressures (Pa_O2 and Pa_CO2), pH and O₂ arterial saturation (SaO₂)]. Results show that during HH compared to NH, f is greater (P≤0.001), V _t and V̇_E under BTPS conditions are lower (P≤0.05), and FET_O2 and FET_CO2 are higher (P≤0.05). However, PET_O2 does not change during the last 25 min of the tests, and neither does PET_CO2 throughout the tests. HR is higher (P≤0.05) and SpO₂ lower (P≤0.05) in HH compared to NH. Arterial blood data reveal that hypoxemia, hypocapnia and blood alkalosis are greater in HH compared to NH and that SaO₂ is lower (P≤0.05). It is concluded that the physiological responses of humans submitted to an acute hypoxia at a PO₂ equal to 120 hPa differ according to the type of hypoxia. Compared to NH, HH leads to a greater hypoxemia, hypocapnia, blood alkalosis and a lower O₂ arterial saturation. These physiological differences could be the consequence of an increase in dead space ventilation, probably related to the barometric pressure reduction, and could be grouped together under the term "the specific response to hypobaric hypoxia". Knowledge of this specific response could improve the comprehension, prevention and treatment of altitude illnesses in the future.