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

Gustave Savourey1, Jean-Claude Launay, Yves Besnard

  • 1Département des Facteurs humains, Centre de recherches du service de santé des armées, 38702 La Tronche cedex France. gsavourey@crssa.net

European Journal of Applied Physiology
|April 1, 2003
PubMed
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Hypobaric hypoxia (HH) causes greater physiological differences than normobaric hypoxia (NH) at 4500m. HH results in more severe hypoxemia, hypocapnia, and lower oxygen saturation compared to NH.

Area of Science:

  • Physiology
  • Environmental Medicine
  • Altitude Research

Background:

  • Studies on hypoxia historically involve reducing ambient oxygen partial pressure (PO2) via hypobaric hypoxia (HH) or normobaric hypoxia (NH).
  • A long-standing debate questions whether physiological responses differ between HH and NH at equivalent PO2 levels.
  • Existing research on this topic is limited and yields conflicting results.

Purpose of the Study:

  • To investigate and compare the cardioventilatory and arterial blood gas responses to acute hypobaric hypoxia (HH) versus normobaric hypoxia (NH) at a PO2 of 120 hPa (approximately 4500 m).
  • To determine if distinct physiological differences exist between HH and NH under controlled conditions.

Main Methods:

  • Eighteen healthy subjects underwent both a 40-minute HH test and a 40-minute NH test in a randomized order.

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  • Tests were conducted at a standardized ambient PO2 of 120 hPa.
  • Cardioventilatory variables (breathing frequency, tidal volume, minute ventilation, end-tidal O2 and CO2, heart rate, SpO2) were measured continuously.
  • Arterial blood gases (PaO2, PaCO2, pH, SaO2) were analyzed at the conclusion of each test.

    • Main Results:

    • Hypobaric hypoxia (HH) led to significantly higher breathing frequency, lower tidal volume and minute ventilation (BTPS), and higher end-tidal O2 and CO2 compared to normobaric hypoxia (NH).
    • Heart rate was elevated, and arterial oxygen saturation (SpO2) was lower during HH compared to NH.
    • Arterial blood analysis revealed greater hypoxemia, hypocapnia, and blood alkalosis in HH, with a significantly lower SaO2.

    Conclusions:

    • The physiological responses to acute hypoxia at 120 hPa differ significantly depending on whether the hypoxia is hypobaric or normobaric.
    • Hypobaric hypoxia elicits a more pronounced hypoxemia, hypocapnia, and blood alkalosis, alongside reduced arterial oxygen saturation, compared to normobaric hypoxia.
    • These observed differences, termed 'the specific response to hypobaric hypoxia,' may stem from increased dead space ventilation due to barometric pressure reduction, potentially improving future understanding and management of altitude-related illnesses.