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

Abstract

Since Bert (1878) and Barcroft (1925), studies on hypoxia are realized by lowering ambient O(2) partial pressure (PO(2)) 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 PO(2)?" 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 PO(2) equal to 120 hPa (4500 m). Cardioventilatory variables [breathing frequency (f), tidal volume (V(t)), minute ventilation (V(E)), O(2) and CO(2) end-tidal fractions or pressures (FET(O2) and FET(CO2) or PET(O2) and PET(CO2) 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 CO(2) arterial partial pressures ( Pa(O2) and Pa(CO2)), pH and O(2) arterial saturation (SaO(2))]. Results show that during HH compared to NH, f is greater (P</=0.001), V(t) and V(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 SpO(2) 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 SaO(2) is lower (P</=0.05). It is concluded that the physiological responses of humans submitted to an acute hypoxia at a PO(2) 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.

Acute Effects of Normobaric Hypoxia on Hand-Temperature Responses During and After Local Cold Stress

“…Loeppky et al, 1997;Savourey et al, 2003), it could be argued that the present results might not be representative of those expected in a hypobaric hypoxic setting. However, a previous study has shown that the hand CIVD responses are similar in normobaric and hypobaric hypoxia (Meeuwsen et al, 2009).…”

Effects of hypoxia and hypercapnia on human HRV and respiratory sinus arrhythmia

“…Normobaric hypoxia, in comparison to hypobaric hypoxia, represents a different challenge to ventilation and cardiac autonomic control (5,27,28). For example, increased breathing frequency with a lower tidal volume, higher end-tidal O 2 and CO 2 fractions, and a higher heart rate have been shown when hypobaric hypoxia is compared to normobaric hypoxia (27).…”

“…Normobaric hypoxia, in comparison to hypobaric hypoxia, represents a different challenge to ventilation and cardiac autonomic control (5,27,28). For example, increased breathing frequency with a lower tidal volume, higher end-tidal O 2 and CO 2 fractions, and a higher heart rate have been shown when hypobaric hypoxia is compared to normobaric hypoxia (27). To date there are no data on whether hypobaric hypoxia affects the distribution of heart beats throughout the ventilatory cycle, or whether any changes in this distribution affects ventilatory efficiency when exposed to low ambient PO 2 .A further technical limitation was that in the present study, isocapnia may not have been maintained during the hypoxia trial.…”

Can sildenafil improve physical performance at altitude? Current scientific evidence

“…These physiological differences could be the result of an increase in dead space ventilation, probably related to reduced barometric pressure in hypobaric hypoxia. 38 Another research showed that some variables of heart rate variability were affected in …”