High Altitude Pulmonary Edema (HAPE)
HAPE results from fluid build up in the lungs. This fluid prevents effective oxygen exchange. As the condition becomes more severe, the level of oxygen in the bloodstream decreases, which leads to cyanosis, impaired cerebral function, and death. Symptoms of HAPE include:

Shortness of breath at rest
Tightness in the chest
Persistent cough bringing up white, watery, or frothy fluid
Marked fatigue and weakness
A feeling of impending suffocation at night
Confusion, and irrational behavior

Confusion, and irrational behavior are signs that insufficient oxygen is reaching the brain. In cases of HAPE, immediate descent of around 2,000 feet (600 m) is a necessary life-saving measure. Anyone suffering from HAPE must be evacuated to a medical facility for proper follow-up treatment.