Not an expert but am finishing up my RRT education and that was a topic this last semester. As you ascend although the partial pressures decreases the % of gas concentration does not.(at 9000 ft barometric pressure =570mmHg while 75%less that sea level it does not have as much effect on fluid shift as the normal gradient obtained by hydrostatic vs osmotic ) With HAPE it is not so as much as a reduction in alveolar pressure (Palv), but pulmonary vascular constriction brought on by hypoxia. PADP(Pulmonary Artery Diastolic Pressues as hight as 90mmHg have been reported in patients suffering from HAPE. This high pressure causes both hemorrhage and fluid shifts that leads to this
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http://physiologyonline.physiology.o.../2/55.full.pdf