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Hypoxic syncope

R G Westendorp1, G J Blauw, M Frölich

  • 1Department of Clinical Epidemiology, Leiden University Hospital, The Netherlands.

Aviation, Space, and Environmental Medicine
|May 1, 1997
PubMed
Summary
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Hypoxia can cause syncope in healthy individuals, involving the sympathetic-adrenergic system. Prompt recovery occurs with oxygen and Trendelenburg positioning, indicating no further clinical examination is needed.

Area of Science:

  • Cardiovascular Physiology
  • Altitude Medicine
  • Neuroautonomic Function

Background:

  • Syncope in healthy individuals at altitude is a known phenomenon.
  • This study investigates cardiovascular effects during hypoxia exposure to clarify the sympathetic-adrenergic system's role in hypoxic syncope.
  • Unexpected episodes of syncope were observed during simulated altitude trials.

Purpose of the Study:

  • To examine cardiovascular responses to hypoxia in healthy volunteers.
  • To elucidate the role of the sympathetic-adrenergic system in altitude-induced syncope.
  • To assess the impact of atrial natriuretic peptide on cardiovascular function during hypoxia.

Main Methods:

  • Two crossover trials were conducted using a low-pressure chamber to simulate altitude.

Related Experiment Videos

  • Study A involved acute severe hypoxia (20,000 ft) in 30 males for psychological performance assessment.
  • Study B involved prolonged moderate hypoxia (13,500 ft) in 5 volunteers, with and without atrial natriuretic peptide infusion.

    • Main Results:

    • In study A, 5% of exposures to severe hypoxia caused lightheadedness, pallor, sweating, and bradycardia; 2% resulted in syncope with cardiac asystole.
    • In study B, moderate hypoxia alone caused no adverse reactions.
    • Atrial natriuretic peptide infusion during moderate hypoxia led to near syncope, significant drops in mean arterial pressure and heart rate, and an eightfold increase in epinephrine levels, while norepinephrine remained unchanged.

    Conclusions:

    • Acute severe hypoxia is sufficient to cause syncope in healthy individuals.
    • Enhanced epinephrine-induced vasodilation may trigger sympathetic withdrawal and parasympathetic enhancement, leading to vascular collapse (Bezold-Jarisch reflex).
    • Recovery is rapid with Trendelenburg positioning and oxygen; further clinical examinations are typically unnecessary.