Chronic Mountain Sickness

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Chronic Mountain Sickness (CMS) is defined as elevated hematocrit (number of red blood cells) and Hypoxemia (lowered blood O2 saturation). Hematocrit > 65% and Arterial O2 saturation < 85%[1]. CMS may cause issues due to the increased blood viscosity. There is no evidence that Altitude Training causes CMS.

CMS is believed to be caused by the combination of high altitude and the insufficiency of other systems (heart/lungs), leading to an excessive compensation of the hematocrit[2]. Many patients with hematocrit between 55% and 70% function normally, only seeking medical help when other factors lead to a decreased O2 saturation[3].


  2. Chronic Mountain Sickness: The Reaction of Physical Disorders to Chronic Hypoxia
  3. Triple Hypoxia Syndrome