If a participant on an Everest trek suffers from a mild headache and nausea at Namche Bazaar (12,300ft), he might take an aspirin and wait for these symptoms to go away; however if the symptoms progress to vomiting and a splitting headache, he must assume that he is suffering from AMS and make plans to descend. It is amazing how many people in this situation ignore the dangers and continue to ascend with their friends, trying to blame their symptoms on poor fitness or flu. For some people, it's the high investment of time, effort and money, for others perhaps it's peer pressure or reluctance to accept defeat. A further problem is that many in the burgeoning adventure travel industry are clueless about mountain sickness. 

AMS may set in within hours to days of arrival at high altitude: the onset of symptoms is usually gradual, which is why it is so vital to watch out for early warnings: does a person feel excessively tired; is she the last one to drag herself in to camp? 

 

AMS is caused by a lack of oxygen. Although the proportion of oxygen in the atmosphere always remains the same (21%), as we go higher the "driving pressure" decreases. The driving pressure depends directly on the barometric pressure, and forces
oxygen from the atmosphere into the capillaries of the lungs. Reduced driving pressure results in decreased saturation of oxygen in the blood and throughout the tissues. 

Just what causes some people to suffer from AMS but not others is largely unknown, but there are clear-cut and important preventive factors that are now well established (see below). The exact mechanism (pathophysiology) of AMS has similarities to that of HACE. 

 

Our trekker in the above example would probably go on to suffer from HACE if he continued to ascend despite the headache and vomiting; the symptoms of HACE are an extension of those of AMS. 

From fatigue, there is progression to lethargy and then to coma. Or there may be confusion and disorientation. A useful test is to see if the person can walk a straight line. If he walks like a drunk or is unsteady, it has to be assumed that he has life-threatening HACE and needs to descend promptly with assistance. This situation is serious enough to justify immediate helicopter evacuation.

HACE is probably caused by shifts of fluid into the tissues of the brain. Reduced oxygen levels cause swelling within the confines of the bony skull. The resulting rise in pressure may lead to lethargy and eventually coma. 

 

This disease may follow AMS, but often it may appear independently. The typical scenario would be a trekker who has no headache or nausea, but finds he has a harder time walking uphill, that he is out of breath on slight exertion compared with the initial days of the trek. There may be a nagging cough and he too may have ascribed these symptoms to a cold. He may be suffering from subclinical or early HAPE, a well-recognized entity. With further ascent this may progress to shortness of breath
even at rest - descent is now obligatory, or the outcome may be fatal. Low oxygen causes the pulmonary artery to narrow and this results in exudation of blood near the smaller branches of the lungs (the alveoli). If the exudation continues, blood may
escape into the alveoli leading to a cough with watery, blood-tinged phlegm. Such exudation, or "water logging" of the lung tissue interferes further with oxygenation. A popular, compact device called a pulse oximeter can measure the oxygen level in
the blood simply and rapidly, using a sensor attached to the index finger. It can be very helpful in confirming if HAPE is present.

 

 

Acclimatization is a state of physiological "truce" between the body of a visitor and the hostile low-oxygen environment of high altitude. This truce permits the trekker to ascend gradually. (This is distinct from "adaptation" - permanent change to the organism, perhaps over thousands of years, perhaps even at a genetic or evolutionary level, to facilitate survival at altitude.
Scientists are trying to decipher if the Sherpas or Tibetans have made such an adaptation.) For acclimatization to take place the single most important step is hyperventilation - the trekker unconsciously breathes faster and more deeply than normal, even at rest, to make up for the lack of oxygen. However, hyperventilation also leads to loss of carbon dioxide from the blood, making the blood more alkaline, and in turn depressing ventilation. However, 48 to 72 hours after exposure to high altitude, the kidney comes to the rescue and begins to excrete alkali from the blood to restore a more balanced environment in which hyperventilation can continue unabated.