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

Since Bert (1878) and Barcroft (1925), studies on hypoxia are realized by lowering ambient O-2 partial pressure (PO2) either by barometric pressure reduction (hypobaric hypoxia HH) or by lowering the O-2 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-1,), minute ventilation WE), Q, and CO2 end-tidal fractions or pressures (FETO and FETCO2 or PETO2 and PETCO2 respectively), heart rate (HR) and O-2 arterial saturation by pulse oxymetry (SpO(2))] were measured throughout the tests. At the end of the tests, arterial blood samples were taken to measure arterial blood gases O-2 and CO2 arterial partial pressures (Pa-O2 and Pa-CO2), pH and O-2 arterial saturation (Sa(O2))]. Results show that during HH compared to NH, f is greater (P less than or equal to 0.001), V-1 and V-E under BTPS conditions are lower (P less than or equal to 0.05), and FETO2 and FFTCO2 are higher (P less than or equal to 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 less than or equal to 0.05) and SPO2 lower (P less than or equal to 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 Sa(O2) is lower (P ! 0.05). It is concluded that the physiological responses of humans submitted to an acute hypoxia at a P-O2 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 O-2 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.