High-altitude exposure of three weeks duration increases lung diffusing capacity in humans

Background: high-altitude adaptation leads to progressive increase in arterial Pa-O2. In addition to increased ventilation, better arterial oxygenation may reflect improvements in lung gas exchange. Previous investigations reveal alterations at the alveolar-capillary barrier indicative of decreased resistance to gas exchange with prolonged hypoxia adaptation, but how quickly this occurs is unknown. Carbon monoxide lung diffusing capacity and its major determinants, hemoglobin, alveolar volume, pulmonary capillary blood volume, and alveolar-capillary membrane diffusion, have never been examined with early high-altitude adaptation. Methods and Results: lung diffusion was measured in 33 healthy lowlanders at sea level (Milan, Italy) and at Mount Everest South Base Camp (5,400 m) after a 9-day trek and 2-wk residence at 5,400 m. Measurements were adjusted for hemoglobin and inspired oxygen. Subjects with mountain sickness were excluded. After 2 wk at 5,400 m, hemoglobin oxygen saturation increased from 77.2 +/- 6.0 to 85.3 +/- 3.6%. Compared with sea level, there were increases in hemoglobin, lung diffusing capacity, membrane diffusion, and alveolar volume from 14.2 +/- 1.2 to 17.2 +/- 1.8 g/dl (P < 0.01), from 23.6 +/- 4.4 to 25.1 +/- 5.3 ml.min(-1).mmHg(-1) (P < 0.0303), 63 +/- 34 to 102 +/- 65 ml.min(-1).mmHg(-1) (P < 0.01), and 5.6 +/- 1.0 to 6.3 +/- 1.1 liters (P < 0.01), respectively. Pulmonary capillary blood volume was unchanged. Membrane diffusion normalized for alveolar volume was 10.9 +/- 5.2 at sea level rising to 16.0 +/- 9.2 ml.min(-1).mmHg(-1).l(-1) (P < 0.01) at 5,400 m. Conclusions: at high altitude, lung diffusing capacity improves with acclimatization due to increases of hemoglobin, alveolar volume, and membrane diffusion. Reduction in alveolar-capillary barrier resistance is possibly mediated by an increase of sympathetic tone and can develop in 3 wk.