What would happen to you if you were taken immediately to the summit of Mt. Everest? The answer is you would pass out, and likely die within minutes.

The problem is not a lack of oxygen at altitude, as the oxygen content of the atmosphere is stable up to 10,000m+ (32,800 ft). The issue is that the pressure of oxygen decreases logarithmically as altitude increases, which causes hypobaric hypoxia (low oxygen pressure). This is the reverse effect of diving, where the pressure of oxygen increases dramatically. Altitude illness is extremely rare at elevations below 2,000m (6,500 ft).

In simple terms, we need the pressure to push oxygen into our bloodstream. If there is not enough pressure when a person is at altitude, then the body will reduce the pressure inside the vessels by lowering the levels of carbon dioxide, by hyperventilating. Unfortunately, hyperventilating removes the CO2 that drives us to breathe and eventually lowers are breathing. Rate. To allow us to continue breathing fast, our kidneys secrete bicarbonate, causing people to urinate more at altitudes.

Swelling

The most severe symptoms of altitude sickness arise from oedema, or fluid accumulation, in the body. This can occur anywhere, including the tissues under the skin. The most severe consequences of this swelling happen in the brain and the lungs.

At very high altitudes, swelling in the brain is called acute mountain illness (AMS), which progresses as the oedema increases to be called high altitude cerebral oedema (HACE). As oedema forms in the lungs, it is called high altitude pulmonary oedema (HAPE).

The physiological cause of altitude-induced oedema is not conclusively established. It is currently believed, however, that HACE is caused by the opening of cerebral blood vessels, resulting in higher blood flow and, consequently, higher pressures in the vessels in the brain. On the other hand, HAPE may be due to general vessel constriction in the lung circulation which, with constant or increased blood flow out of the heart. This also leads to increases in lung vessel pressures.

Acute Mountain Sickness (AMS)

Medical history is the key to diagnosing Acute Mountain Sickness (AMS) because there are no specific physical exam findings. It is vital to assess the rate of ascent and the total elevation gain. AMS is common. It is diagnosed as a headache, and at least one of the following symptoms:

• Dizziness or lightheadedness

• Fatigue or weakness

• Nausea/vomiting/anorexia

• Insomnia

The most significant risk factors for AMS are a prior history of AMS, fast or high ascents, and obesity. Men and women and children are equally susceptible.

High Altitude Cerebral Edema (HACE)

As oedema in the brain increases, symptoms become more profound, and AMS progresses to HACE, a life-threatening disease.

HACE is defined as severe AMS symptoms with additional apparent neurologic dysfunction:

• Poor muscle control: this is the most common sign of HACE

• Altered level of consciousness

• Severe lack of energy: While the boundary between AMS and HACE can be blurry, HACE almost never occurs without AMS symptoms first. The progression of AMS to coma typically occurs over 1 – 3 days. HACE and HAPE are often present simultaneously.

High Altitude Pulmonary Edema (HAPE)

HAPE usually evolves over two to four days after ascent to altitude. The criteria for HAPE diagnosis are symptoms of at least two of the following.

• Shortness of breath at rest

• Cough

• Weakness or decreased exercise performance

• Chest tightness or congestion

The primary symptoms are shortness of breath at rest, cough, and exercise intolerance. The initial sign will often be a marked decrease in exercise tolerance in an individual as compared to previous days. Occasionally, frothy pink sputum is produced, but this is usually later in the illness. Mild cases may resolve within hours after a descent. In contrast, severe cases may progress to death within 24 hours, particularly if descent is delayed.

Prevention

Slow ascent is the safest method to facilitate acclimatization and to prevent any altitude illness. Current recommendations for climbers without experience at high altitude are to spend two to three nights at 2500 – 3000 meters before a further ascent. Increases of greater than 600 meters in sleeping altitude should be avoided. One should consider an extra night of acclimatization for every 300 – 900 meters of altitude gain. Medicines can help but are no substitute for a gradual ascent.

AMS/HACE

There is a medicine called acetazolamide that can help. But going up slowly is a better way to prevent altitude illness. This medicine works by increasing the breathing rate. Since, during a hike, people are already breathing fast, its effects are felt mostly at night during sleep.

HAPE

Prevention for HAPE is to limit the ascent rate to no more than 350 meters (1155 ft) a day.

Treatment

AMS

The treatment for acute mountain sickness (AMS) is to discontinue ascent and rest. Descent is the best treatment.

HACE

The treatment for HACE is IMMEDIATE descent (almost always with assistance). This is imperative and should not be delayed. Even modest elevation losses can be helpful. In addition to descent, administering a medicine called dexamethasone can be used as you go down. Recovery with prolonged problems can last for weeks. Most who survive eventually fully recover neurologically.

HAPE

The treatment for HAPE is IMMEDIATE descent. All that may be required is 500 to 1000 meters of descent before improvement is observed. The patient should rest after a descent. No intervention should delay an opportunity to descend.