High altitude hypoxia is a condition where the body does not receive sufficient oxygen due to exposure to high elevations, typically above 8,000 feet (2,500 meters). Understanding this phenomenon is necessary for safely enjoying high-altitude environments, as the body’s response dictates the difference between successful adaptation and serious illness.
Why Altitude Causes Low Oxygen
The root cause of hypoxia at high altitudes is a change in atmospheric physics, not a change in air composition. The air we breathe always contains about 21% oxygen, regardless of how high up we are. The problem arises because atmospheric pressure decreases significantly as altitude increases.
This drop in barometric pressure causes the partial pressure of oxygen to fall proportionally. Partial pressure is the force driving oxygen molecules from the air into the lungs and bloodstream. Since the air pressure is lower, this driving force is greatly reduced, making it harder to saturate the blood with oxygen.
How the Body Adapts to High Altitudes
The body attempts to counteract the reduced oxygen availability through a process called acclimatization, involving immediate and long-term changes. The most immediate response is hyperventilation, where the body increases its breathing rate and depth to take in more air. This reflex is triggered by sensors in the carotid arteries that detect the lower oxygen levels in the blood.
Simultaneously, the heart rate increases to pump available oxygenated blood more rapidly. Over a period of days, the kidneys excrete bicarbonate, which slightly acidifies the blood. This adjustment helps sustain the higher rate of breathing.
For longer-term adaptation, the body begins producing more red blood cells, stimulated by the hormone erythropoietin. This change increases the blood’s capacity to carry oxygen. However, this adaptation takes weeks and is not a factor in the initial days of a trip.
Identifying Acute Altitude Illnesses
When the body’s compensatory mechanisms fail to keep pace with the ascent, acute altitude illnesses can develop. Acute Mountain Sickness (AMS) is the most common and mildest form, often resembling a severe hangover. Symptoms typically include headache, nausea, fatigue, and difficulty sleeping, appearing within the first day of reaching a high elevation.
Two more severe and potentially fatal conditions are High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE). HACE is a swelling of the brain, usually progressing from untreated AMS. Warning signs include ataxia (loss of coordination noticeable during a heel-to-toe walking test), confusion, and irrational behavior.
HAPE involves fluid accumulation in the lungs and can occur without prior AMS symptoms, making it insidious. Key indicators are unusual shortness of breath at rest and a persistent, dry cough that may progress to produce pink or frothy sputum. Both HACE and HAPE require immediate intervention.
Strategies for Prevention and Treatment
The most effective strategy for preventing altitude illness is a slow, staged ascent, which allows time for natural acclimatization to occur. Above 10,000 feet (3,000 meters), experts recommend limiting the daily sleeping elevation gain to about 1,000 to 1,600 feet (300 to 500 meters). A common practice is to “climb high, sleep low,” meaning you ascend to a higher point during the day but return to a lower elevation to sleep.
Staying well-hydrated and avoiding excessive exertion upon arrival at a new altitude also helps minimize risk. For individuals with a history of altitude sickness or those planning a rapid ascent, prophylactic medication is often recommended. Acetazolamide, commonly known as Diamox, is the preferred medication because it speeds up the acclimatization process by altering blood chemistry to increase the respiratory drive.
For mild AMS, rest and simple pain relievers may be sufficient, but worsening symptoms demand immediate action. Descent to a lower altitude is the definitive treatment for severe cases like HACE and HAPE. Supplemental oxygen or a portable hyperbaric chamber can temporarily stabilize a patient until a safe descent is possible.