High altitude pulmonary edema

Overview
High altitude pulmonary edema (HAPE) is a life-threatening form of non-cardiogenic pulmonary edema that occurs in otherwise healthy mountaineers at altitudes above. Some cases have however been reported also at lower altitudes (between 1500 and 2500 m in highly vulnerable subjects), although what makes some people susceptible to HAPE is not currently known. HAPE remains the major cause of death related to high altitude exposure with a high mortality in absence of emergency treatment.

Etiology
The initial insult that causes HAPE is a shortage of oxygen which is caused by the lower air pressure at high altitudes. The mechanisms by which this shortage of oxygen causes HAPE are poorly understood, but two processes are believed to be important:
 * 1) Increased pulmonary arterial and capillary pressures (pulmonary hypertension) secondary to hypoxic pulmonary vasoconstriction.
 * 2) An idiopathic non-inflammatory increase in the permeability of the vascular endothelium.

Although higher pulmonary arterial pressures are associated with the development of HAPE, the presence of pulmonary hypertension may not in itself be sufficient to explain the development of edema: severe pulmonary hypertension can exist in the absence of clinical HAPE in subjects at high altitude.

Incidence
The incidence of clinical HAPE in unacclimatized travelers exposed to high altitude (~ 4,000 m) appears to be less than 1%. In over 30 years of research experience, the U.S. Army Pike's Peak Research Laboratory, utilizing about 300 sea-level resident volunteers (and more than 100 staff members) rapidly and directly exposed to high altitude, only 3 were evacuated with suspected HAPE.

Predisposing factors
Individual susceptibility to HAPE is difficult to predict. The most reliable risk factor is previous susceptibility to HAPE, and there is likely to be a genetic basis to this condition, perhaps involving the gene for angiotensin converting enzyme (ACE).

Research
In order to help understand the factors that make some individuals susceptible to HAPE, the International HAPE Database was set up in 2004. Individuals who have previously suffered from HAPE can register with this confidential database in order to help researchers study the condition.