Relationship between arterial oxygen saturation and heart rate variability at high altitudes

Autonomic nervous systems have important roles for survival of victims under hypobaric hypoxic condition. In the present study, we assessed the correlation between arterial oxygen saturation (SpO2) and heart rate variability (HRV) to identify the autonomic nervous responsiveness among trekkers at high altitude (n = 21). HRV was analyzed by the maximum entropy method. SpO2 among subjects at 3456 m (495 mm Hg) was 80% ± 5% (mean ± SD; range, 69%-93%). SpO2 and percentile entropy, and SpO2 and low-frequency variability, had positive correlation (r = 0.455 and 0.518, respectively). SpO2 value among subjects with mountain sickness symptoms was not different from that among subjects without the symptoms. In conclusion, autonomic responses among high-altitude trekkers may be blunted under hypobaric hypoxic conditions. Deterioration of autonomic function measured by HRV might be more sensitive to hypoxia than clinical symptoms at high altitudes.