Acclimatization  |   Acute Mountain Sickness  |   High Altitude Pulmonary Edema  |   High Altitude Cerebral Edema

There are several medical problems associated with high altitude. These generally stem from the lower oxygen concentration in the blood that is a result of the reduced atmospheric pressure at higher altitudes. Body tissues consume the same amount of oxygen for the same amount of work no matter what the altitude is. The oxygen-carrying portion of the blood, the hemoglobin, has to increase, therefore, in order to supply the tissues with the required amount of oxygen. There are 3 levels of altitude that should be considered - High, Very High and Extreme.

1. High Altitude is from 2,400 to 4,300 m (8,000 to 14,000 ft). Altitude illness rarely occurs under 2,400 m unless the individual is already fairly severely compromised by a debilitating medical illness.
    
   
2. Very High Altitude is from 4,300 to 5,500 m (14,000 to 18,000 ft). Rapid ascent to such elevations without prior acclimatization is dangerous and can cause all types of altitude illness. Only 30 - 40% of maximal sea level work capacity should be undertaken routinely at altitudes above 4,000 m. 
   
   
3. Extreme Altitude is from 5,500 to 8,800 m (18,000 to 29,000 ft). Very few people go to such elevations and problems here are usually more likely to be altitude deterioration rather than illness since prolonged stay at this level or higher results in gradual loss of physical conditioning.

Physical performance is lowest upon arrival at high altitudes but it is possible to progressively improve it. The volume of blood pumped per minute by the heart for any level of exercise is lower than during similar exercise at sea level. The maximum heart rate that can be attained is also lower at high altitudes. During exercise at high altitude the lower oxygen pressure in the air, and, therefore, in the lungs, results in a lower oxygen saturation of the red blood cells and so people get short of breath during only moderate exertion and have to rest more often in order for the oxygen saturation to rise again. The work required to simply breathe, to move the diaphragm and the chest wall muscles, is increased at high altitude. At the top of Everest, for example, the oxygen used to keep breathing consumes such a high proportion of the total oxygen available that there is little left for anything else! Rapid ascent to high altitude forces fluid out of the blood vessels into the surrounding tissues, resulting in a decrease in blood volume. This decrease can persist for several weeks and will be worsened if fluid intake is inadequate at the same time.

During sleep at sea level the normal decrease in the rate and depth of breathing results in a decrease in oxygen saturation of the blood. At high altitudes this decrease is more marked and can account for the inability of many individuals to sleep well there. It probably also accounts for the headaches suffered during Acute Mountain Sickness (see below) being more severe in the morning and why high altitude pulmonary edema (fluid collecting in the lungs) is usually more severe during the night. Sleeping pills and tranquilizers intensify these sleep changes and should be avoided at high altitudes.

Acclimatization

Acclimatization to high altitude consists of several changes in the body. The respiratory volume increases, the cardiac output increases, the number of red blood cells increases, the hemoglobin increases, the blood pressure in the lungs increases and the body tissues change to promote normal function at lower oxygen levels.

Acclimatization occurs on its own after a period of time at higher altitudes. This period of time varies from person to person according to several factors. It can be helped along in several ways, the best being graded ascents. Prior to going to high altitude (above 2,400 m) gradually increasing amounts of exercise, such as walking and climbing, should be carried out at an intermediate altitude (between 1,8900 and 2,400 m) for 3 to 4 days. If ascent to even higher altitudes (4,600 to 5,500 m) is planned a second stage of 2 or 3 days at the 4,000 m level is helpful. At altitudes above 4,500 m ascents should be limited to 150 - 300 m per day and every third day should be a rest day. If one-stage ascent to altitudes of about 3,500 m is planned then minimal activity for 3 to 4 days should be done before any heavy work such as climbing is undertaken. One-stage ascent to altitudes higher than this is foolhardy.

Nutrition at high altitude is important, especially since loss of appetite often occurs upon reaching higher elevations. Diets should contain large amounts of sweets and should be low in fat. Fluid intake must be encouraged (but coffee, tea or alcohol don't count since they are diuretics and cause you to lose more liquid than you take in with these drinks).

A medication called Acetazolamide (Diamox) is often helpful for those who have difficulty sleeping or experience other discomforts during the first few days at high elevations. The drug is taken in a dose of 250 mg twice daily and is continued for 3 to 5 days. It works best if it is started the day before arrival at the higher elevation. Those who expect to have problems; ie, those who are not regular exercisers, are smokers, who suffer some form of heart or lung disease or are anemic would be well advised to start it then. The medication is used only to assist in the acclimatization process until the kidneys can begin to compensate to the change in the blood pH. This medication can worsen dehydration if fluid intake is inadequate.

It is important to realize that acclimatization is lost at about the same rate as it is achieved. An acclimatized individual can spend a few days at sea level and return to high altitude without much loss of tolerance. If the sea level stay is longer than one week, though, all the initial steps of acclimatization must be taken.

Acute Mountain Sickness

AMS is a direct effect of low oxygen in the brain. Its occurrence depends upon the rate of ascent, the elevation attained and individual susceptibility. Symptoms usually start 12 - 24 hours after arrival and begin to decrease by about day 3. The most common symptoms are headache, dizziness, fatigue, shortness of breath, loss of appetite, nausea and vomiting, disturbed sleep and a general feeling of being unwell. Anxiety attacks and hyperventilation may occur. Rarely, impaired mental function occurs, manifested by a loss of memory, forgetfulness or the inability to solve simple problems. This, though, rarely occurs below 4,300 m.

Those with AMS should avoid heavy exertion but it is important to continue with light outdoor activity rather than to rest completely. Sedatives should be avoided. It is important to drink extra fluids and to eat a light, high carbohydrate diet. Aspirin can be taken for headache. Tobacco and alcohol should be avoided. If symptoms persist after 3 days then descent to a lower altitude, even by as little as 600 m, usually brings prompt relief. If individuals are unusually drowsy they should be encouraged to walk around in the open air and to take deep breaths rather than to sleep during the day. Acetazolamide could be used here.

High Altitude Pulmonary Edema

HAPE is a dangerous condition in which the lungs fill with fluid that has oozed through the walls of the blood vessels. It can end in death by suffocation. It is much more likely to occur in those who have ascended rapidly and then have engaged in heavy physical exertion. Very rapid ascents may result in this even in acclimatized individuals but this rarely occurs below 2,400 m. There is a higher likelihood of this occurring in those aged 10 to 21 years.

Symptoms consist of undue shortness of breath with moderate exertion, a feeling of tightness in the chest, weakness and marked fatigue. Headache, loss of appetite and nausea are frequently also present. Coughing is an important early sign. The cough is usually intermittent at first and then becomes persistent. Occasionally white frothy material is coughed up and there can be streaks of blood as well. Pulse is rapid, respirations are rapid and the skin may be pale and cold.

Impairment of mental functioning is a sign of severe disease. All these symptoms can be worse at night.

Treatment consists of immediate descent to a lower altitude and then resting for 2 to 3 days. Oxygen will be helpful if it is available. Diuretics such as Acetazolamide are usually not helpful here.

High Altitude Cerebral Edema

The onset of this is usually insidious, occurring after several days at altitudes greater than 3,700 m. The most common symptoms are headache which is usually severe and constant, abnormal mental function and unsteadiness, the latter being typical of this condition. HACE can occur in the absence of AMS or HAPE.

Treatment is rapid descent and administration of oxygen. Diuretics can be helpful here as can steroids.

The prime lesson to learn with regard to altitude, then, is to go slowly especially when first reaching a higher altitude. Acclimatization will come. Be patient. Listen to your body and rest when it tells you to do so. Do not rely on medications to assist in your acclimatization unless you have specific symptoms of altitude sickness. Then sit back and enjoy the view!

R.J. Birnbaum, MD, CCFP, FCFP
June 21, 1998