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HAPE - High Altitude Pulmonary Edema
Another form of severe altitude illness is High Altitude Pulmonary Edema, a potentially deadly condition that develops because the lung arteries develop excessive pressure in response to low oxygen, resulting in overflow of fluid in the lungs. Though it often occurs with AMS, it is not felt to be related and the classic signs of AMS may be absent. Signs and symptoms of HAPE include any of the following:

Extreme fatigue
Breathlessness at rest
Fast, shallow breathing
Cough, possibly productive of frothy or pink sputum
Gurgling or rattling breaths
Chest tightness, fullness, or congestion
Blue or gray lips or fingernails
Drowsiness
HAPE usually occurs on the second night after an ascent, and is more frequent in young, fit climbers or trekkers.

Descend immediately
Immediate descent is the best treatment for HACE. This is of the utmost urgency, and cannot wait until morning (unfortunately, HACE often strikes at night). Delay may be fatal. The moment HACE is recognized is the moment to start organizing flashlights, helpers, porters, whatever is necessary to get this person down. Descent should be to the last elevation at which they woke up feeling well. Bearing in mind that the vast majority of cases of HACE occur in persons who ascend with symptoms of AMS, this is likely to be the elevation at which the person slept two nights previously. If you are uncertain, a 500-1000 meter descent is a good starting point. Other treatments include oxygen, hyperbaric bag, and dexamethasone. These are usually used as temporizing measures until descent can be effected (see physician section for more details).

In some persons, the hypoxia of high altitude causes constriction of some of the blood vessels in the lungs, shunting all of the blood through a limited number of vessels that are not constricted. This dramatically elevates the blood pressure in these vessels and results in a high-pressure leak of fluid from the blood vessels into the lungs. Exertion and cold exposure can also raise the pulmonary blood pressure and may contribute to either the onset or worsening of HAPE.

Immediate descent is the treatment of choice for HAPE; unless oxygen is available delay may be fatal. Descend to the last elevation where the victim felt well upon awakening. Descent may be complicated by extreme fatigue and possibly also by confusion (due to inability to get enough oxygen to the brain); HAPE frequently occurs at night, and may worsen with exertion. These victims often need to be carried.

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).

HAPE usually resolves rapidly with descent, and one or two days of rest at a lower elevation may be adequate for complete recovery. Once the symptoms have fully resolved, cautious re-ascent is acceptable...