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 (Vt), minute ventilation (V̇E), O2 and CO2 end-tidal fractions or pressures (FETO2 and FETCO2 or PETO2 and PETCO2 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 (PaO2 and PaCO2), pH and O2 arterial saturation (SaO2)]. Results show that during HH compared to NH, f is greater (P≤0.001), Vt and V̇E under BTPS conditions are lower (P≤0.05), and FETO2 and FETCO2 are higher (P≤0.05). However, PETO2 does not change during the last 25 min of the tests, and neither does PETCO2 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.
AltitudeAltitude illnessesHumanHypobaric and normoxic hypoxia