Backcountry health & safety

High altitude illnesses

by Susan Bradford, Ph.D., Richard Fantozzi, M.D., and Bill Fantozzi, Wilderness Remote and ER Instructor

Canine search and rescue personnel may not be called out to Mt. Everest but may still encounter a victim of Acute Mountain Sickness or even fall victim themselves.

AMS is brought on by the combination of reduced air pressure and lower oxygen concentration that occurs at high altitudes. Symptoms can range from mild to life threatening, affecting the nervous system, lungs, muscles or heart.

AMS can be divided into three main groups:

1) Acute mountain sickness

2) High altitude pulmonary edema (HAPE)

3) High altitude cerebral edema (HACE).

The faster one climbs to a high altitude, the greater one’s chance of becoming sick. Typically AMS is encountered in victims at altitudes above 8,000 ft. Mild symptoms may be experienced in up to 20% of people at altitudes ranging from 6,300 to 9,700 ft (2).

The reduced air pressure at high altitudes, combined with lower oxygen concentrations, result in AMS and may target the heart, lungs and central nervous system. Generalized swelling of lung and brain tissues may result from fluid leaking from capillaries (2).

Anyone who goes to altitude can get AMS. It is primarily related to individual physiology (genetics) and the rate of ascent; there is no significant effect of age, gender, physical fitness, or previous altitude experience. Some people acclimatize quickly, and can ascend rapidly; others acclimatize slowly and have trouble staying well even on a slow ascent. The same person may get AMS on one trip and not another, despite an identical ascents. Unfortunately, no way has been found to predict who is likely to get sick at altitude.

It is remarkable how many people mistakenly believe that a headache at altitude is "normal"; it is not. Denial is common. Being willing to admit that you have altitude illness is the first step to staying out of trouble.

Understanding the physiology

The atmosphere puts pressure on our bodies at all times. The weight of the air above us compresses the air around us, making it denser. As you ascend up a mountain, the air becomes less and less compressed, or thinner, such that in a given volume of air, there are fewer molecules present. So, while the percentage of molecules that are oxygen remains constant at 21%, fewer total molecules in the high, thin air means there are fewer oxygen molecules to breathe.

Barometric pressure measures the force exerted against a surface by the weight of air. At sea level, the barometric pressure averages 760 mmHg, representing the pressure exerted at the base of a 1-milimeter high column of mercury.

At 12,000 feet (3,658 meters), where the barometric pressure is only 483 mmHg, there are roughly 40% fewer oxygen molecules per breath. At this altitude increases, you breathe in fewer oxygen molecules per breath. Even with increased respiration, the blood’s oxygen content does not reach sea level concentrations. Lower air pressure also causes fluid to leak from the capillaries (small blood vessels), which can cause fluid build-up in both the lungs and the brain. Normal physiologic changes that occur in every person who goes to altitude include:

Hyperventilation (breathing faster and/or deeper)

Shortness of breath during exertion

Changed breathing pattern at nights

Awakening frequently during the night

Increased urination.

Acclimatizing to altitude

Over time at higher altitudes, the body adjusts (or acclimates) to having less oxygen. This process of adjusting to decreased oxygen generally takes several days to weeks. Specific physiological changes occur in body chemistry, including an increase in rate of respiration and the need to urinate more often. These changes are part of the normal response to high altitude. If one is not careful to sufficiently replace lost fluids, dehydration may occur. Drink plenty of liquids to prevent dehydration, and get plenty of rest. A mild analgesic (such as acetaminophen, aspirin or ibuprofen) can help with headaches. Do not take sleeping pills or drink alcohol (which can slow your breathing).


Without proper acclimatization, high altitude can lead to serious, even life-threatening illness. You need to recognize the symptoms of altitude sickness, so you can avoid worsening illness.

AMS can cause mild to life-threatening illness. AMS may cause headache, weakness, light-headedness, trouble sleeping, upset stomach and loss of appetite (2). If one experiences any of these symptoms, it is important to stop and acclimatize.

HAPE is a form of severe altitude illness that often occurs with AMS, although the classic signs of AMS may be absent. HAPE is characterized by fluid in the lungs and typically occurs 48 hours after an ascent. HAPE frequently occurs at night, and may worsen with exertion. HAPE is more frequent in the young and physically fit. Oxygen exchange within the lungs is impaired and cyanosis results. The victim may develop a cough with frothy fluid (2). Other symptoms include any of the following: extreme fatigue; breathlessness at rest; fast, shallow breathing; gurgling or rattling breaths; chest tightness, fullness or congestion; blue or gray lips or fingernails; or drowsiness. HAPE usually resolves itself with descent to a lower elevation.

It is common for persons with severe HAPE to also develop HACE, presumably due to the extremely low levels of oxygen in their blood (equivalent to a continued rapid ascent).

The HACE form of AMS is a dangerous life-threatening condition that typically occurs after one week at higher elevations. The vast majority of HACE cases occur in those who ascend with symptoms of AMS and can progress rapidly and can be fatal within a few hours to one or two days. A leakage of fluid creates brain swelling, and the hallmark of HACE is a decreased ability to think. There may be confusion, changes in behavior, lethargy, or lack of coordination (ataxia), the inability to walk a straight line (similar to the way a person walks when intoxicated on alcohol). The victim may be confused and not recognize that they are ill or even lose consciousness. People with HACE usually survive and recover completely if they descend soon enough and far enough, although the staggering gait may persist for days after descent.


The International Society for Mountain Medicine’s Golden Rules:

1) If you are not feeling well at high altitudes, assume it is AMS unless diagnosed as another medical condition

2) Never ascend with AMS symptoms

3) If you begin to feel worse or have HACE or HAPE, descend immediately.

If you ascend with AMS you will get worse, and you might die. Even a day hike to a higher elevation is a great risk. Treatment of AMS includes descending to a lower altitude and letting your body adjust (1). Do not ignore the symptoms.

For HAPE, immediate descent is the treatment of choice; delay may be fatal. Descent may be complicated by extreme fatigue and possibly also by confusion (due to inability to get enough oxygen to the brain). Victims often need to be carried.

For HACE, immediate descent is the best treatment. This is of the utmost urgency and cannot wait until morning; unfortunately, HACE often strikes at night.


Remember, it's how high you sleep each night that really counts. The following recommendations from the International Society for Mountain Medicine are said to keep most people from getting AMS:

Spend at least one night at an elevation below 3000 meters

At altitudes above 3000 meters (10,000 feet), your sleeping elevation should not increase more than 300-500 meters (1000-1500 feet) per night

For very 1000 meters (3000 feet) of ascent, spend a second night at the same elevation

Drink plenty of fluids, at least 3-4 quarts per day.

Climbers have a maxim: "climb high, sleep low." Day hikes to higher elevations that you take on "rest days" (when you spend a second night at the same altitude) help your acclimate. This second night also ensures that you are fully acclimatized and ready for further ascent.

Drugs that may be taken to prevent or treat AMS include acetazolamide (Diamox) and nifedipine (Procardia) (3), but are more for people ascending to very high elevations. Portable oxygen and baric chambers are carried for very high altitude climbing.

Recognizing and treating AMS in dogs

Dogs are prone to the same type of effects from high altitude as are their handlers. Dogs may stop eating, show discomfort and have no energy. Treatment is the same for dogs as it is people: don't take the dog higher if it's showing symptoms, make sure it has plenty of water and if symptoms doesn't improve, get the dog to a lower elevation.


Copyright 2010 California Karelians
San Diego, California