Some Medical Precautions for Working at High Altitude

Prof. John B. West, Division of Physiology, School of Medicine, UCSD

The Problem. Because the barometric pressure falls at high altitude, the partial pressure of oxygen (Po₂) is reduced. For example, at the MMA site (altitude about 5010 m) in north Chile the barometric pressure is about 419 mmHg (sea level value 760 mmHg). Physiologists calculate the inspired Po₂ as 0.2093 (P_B - 47) where 0.2093 is the fractional concentration of oxygen in air, and 47 mmHg is the water vapor partial pressure at the body temperature of 37C. This means that while the inspired Po₂ is 149 mmHg at sea level, it is only 78 at the site, that is about half the normal value. The lower inspired Po₂ means that the Po₂ of the arterial blood and the tissues is reduced, a condition known as hypoxia.

Hypoxia impairs mental performance, productivity and general well being. Because the radioastronomy team sleep at about 2400 m (P_B 572 mmHg) and commute to the site, they will develop some high-altitude acclimatization which will alleviate some of the potential problems at high altitude. For example, there will be an increase in rate and depth of breathing (ventilation) which raises the Po₂ in the alveolar gas in the depths of the lung. Nevertheless, the team will not be able to work as efficiently at the site as at sea level, and cognitive function, for example in problem solving, will be impaired. If repairs are to be made to the radiotelescope, it is important to draw up a checklist in San Pedro de Atacama. If the component to be repaired can be brought down to San Pedro de Atacama the problem will be solved more easily. Arithmetical calculations at the site are likely to have errors.

Most people will not have a problem at the site if they have already slept at San Pedro de Atacama, and they are only up at the site for a few hours. However they will notice some increased shortness of breath while walking uphill, and they will fatigue more easily. A few people may develop altitude sickness.

Acute Mountain Sickness (AMS). This is characterized by headache, fatigue, lightheadedness, palpitations, loss of appetite and perhaps nausea. These symptoms should rapidly disappear when the person returns to San Pedro de Atacama. In general, no one should try to sleep at the site because high-altitude problems are typically much worse during the night.

High-Altitude Pulmonary Edema (HAPE). The subject becomes increasingly breathless, may have some chest discomfort, and develop a cough. Often, though not always, this is accompanied by symptoms of acute mountain sickness. When the disease is severe, the subject may cough up frothy sputum, often pink. High-altitude pulmonary edema is a medical emergency and the subject should descend to San Pedro de Atacama as soon as possible. Typically recovery is rapid upon descent. Untreated HAPE can be fatal.

High-Altitude Cerebral Edema (HACE). This is relatively rare but potentially very serious. The symptoms begin like acute mountain sickness but progress to difficulty with walking (ataxia), mental confusion, dislike of intense light (photophobia), changes of mood, hallucinations and irrational behavior. Again this is a medical emergency and the person should be taken down to San Pedro de Atacama immediately. Most cases recover rapidly on descent, but the condition is life threatening if the subject remains at high altitude.

Medications. Mild headaches caused by AMS usually respond to aspirin or Tylenol. People who are commuting from San Pedro de Atacama should not need any other medications though rescue oxygen could be useful if there is an accident at the site and someone has to be brought down. Newcomers who go straight to the site without acclimatization, or spend only one night at San Pedro, might benefit from one 250 mg tablet of Diamox (acetazolamide) taken in the morning before going up. This prescription drug works by causing a metabolic acidosis which stimulates ventilation. However it is a powerful drug with a number of side effects and should not be used on a routine basis.

Additional Reading. Mountain Sickness by P.H. Hackett, New York, American Alpine Club, 1978 has a lot of useful information in a non-technical form. A standard reference text is High Altitude Medicine and Physiology by M.P. Ward, J.S. Milledge and J.B. West, 2nd edition, London, Chapman and Hall, 1995.