Background: High altitude pulmonary oedema (HAPE) that is severe enough to require urgent medical care is infrequent. We hypothesised that subclinical HAPE is far more frequent than suspected during even modest climbs of average effort. Methods: We assessed 262 consecutive climbers of Monte Rosa (4559 m), before ascent and about 24 h later on the summit 1 h after arriving, by clinical examination, electrocardiography, oximetry, spirometry, carbon monoxide transfer, and closing volume. A chest radiograph was taken at altitude. Findings: Only one climber was evacuated for HAPE, but 40 (15%) of 262 climbers had chest rales or interstitial oedema on radiograph after ascent. Of 37 of these climbers, 34 (92%) showed increased closing volume. Of the 197 climbers without oedema, 146 (74%) had an increase in closing volume at altitude. With no change in vital capacity, forced expiratory volume in 1 s and forced expiratory flow at 25-75% of forced vital capacity increased slightly at altitude, without evidence of oedema. If we assume that an increased closing volume at altitude indicates increased pulmonary extravascular fluid, our data suggest that three of every four healthy, recreational climbers have mild subclinical HAPE shortly after a modest climb. Interpretation: The risk of HAPE might not be confined to a small group of genetically susceptible people, but likely exists for most climbers if the rate of ascent and degree of physical effort are great enough, especially if lung size is normal or low.
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