Accentuated hypoxemia at high altitude in subjects susceptible to high-altitude pulmonary edema

To investigate the hypotheses that activated coagulation, catecholamine release, or arginine vasopressin release are involved in the pathogenesis of high-altitude pulmonary edema (HAPE), we measured these variables in seven subjects susceptible to HAPE and in nine control subjects at an altitude of 1,600 m, and after 6 and 12 h at a simulated altitude of 4,150 m. Each subject was studied twice, once after 3 days of placebo medication and once after 3 days of premedication with aspirin and dipyridamole. At high altitude, HAPE-susceptible subjects showed significantly exaggerated hypoxemia and a slightly higher end-tidal carbon dioxide partial pressure that did not account fully for the hypoxemia. Fibrinolytic activity was significantly accelerated in both groups at high altitude, whereas other coagulation measurements, catecholamines and arginine vasopressin levels, and pulmonary function tests were not significantly changed. Similar findings were obtained after both placebo and platelet-inhibitor premedication. The results indicate that none of the three hypothesized mechanisms, i.e., activated coagulation, excessive catecholamine release, or antidiuresis, would account for HAPE susceptibility. Instead, HAPE-susceptible subjects exhibited exaggerated hypoxemia associated with relative hypoventilation and a widened alveolar-arterial gas pressure difference.