Pakistani Army soldiers walk above their military outpost at the Siachen Glacier -- 18,655 feet (5,686.04 meters) above sea level. Soldiers generally spend 90 days at a high altitude base before descending.

Robert Nickelsberg/Getty Images

High Altitude Pulmonary Edema (HAPE)

In the last section, we discussed Acute Mountain Sickness, the first form of altitude sickness. Now we'll learn about the second, more serious form of altitude sickness: High Altitude Pulmonary Edema (HAPE).

HAPE is caused by excessive fluid in the lungs. For some people, the lack of oxygen at high altitudes can cause blood vessels to constrict. The body responds by forcing blood through unrestricted vessels, which results in high blood pressure and blood vessel leakage. This leaked fluid then builds up in the lungs, interfering with the normal exchange of oxygen. This results in further oxygen loss in the already-depleted bloodstream.

Because of some similar symptoms (such as fluid in the lungs and difficulty breathing), HAPE is sometimes misdiagnosed as pneumonia. This can be a fatal mistake. A person afflicted with HAPE may take a sudden turn for the worse, fall into a coma and die within a few hours. HAPE usually manifests at night, and many climbers have been found comatose in the morning by their companions.

HAPE symptoms include:

  • Trouble breathing when at rest
  • Persistent cough
  • Weakness
  • Feeling of tightness in the chest

If two or more symptoms are present, it's best to immediately descend and get medical attention [source: Shlim]. You should be aware of other signs of HAPE -- you may have a fever, make a "gurgling" sound while breathing or find that lying flat on the ground is extremely uncomfortable.

With advanced symptoms, you probably won't have the strength to descend on your own -- you'll need help. This is just as well, since HAPE can result in high blood pressure that only worsens with physical exertion. As with severe Acute Mountain Sickness, the most important thing is to descend as soon as possible. Even after medical treatment, there is still a 10 to 15 percent chance that HAPE will be fatal [source: Dietz].

Stop before you reach the summit! If you thought blood vessels leaking into your lungs were bad, wait until you read about blood vessels leaking into your brain.