So it is very difficult to get O2 to the place in the body that needs it under high altitude and low-pressure conditions. This leads to swelling of the brain. The pressure from this can cause certain vital parts, like the brain stem to get damaged. It can be fatal if swelling happens for a long time. The best solution is to bring the person suffering from HACE to a lower altitude.
Low oxygen causes vasoconstriction in the alveolar capillaries.
This is because the body senses the low O2 and does not want to waste energy sending blood to places that are not oxygenating it. This results in the blood going elsewhere, as in blood vessels of the lungs. This results in increased pressure in the pulmonary circulatory system. Further, there is swelling due to fluid loss in non-alveolar capillaries. Normally, pressure drops from 150 mm Hg to 100 mm Hg.