Pulmonary capillary reserve and exercise capacity at high altitude in healthy humans

Purpose: We determined whether well-acclimatized humans have a reserve to recruit pulmonary capillaries in response to exercise at high altitude. Methods: At sea level, lung diffusing capacity for carbon monoxide (DLCO), alveolar-capillary membrane conductance (Dm_CO), and pulmonary capillary blood volume (V_c) were measured at rest before maximal oxygen consumption ($$\dot{V}{\text{O}}_{2\hbox{max} }$$V˙O2max) was determined in seven adults. Then, DLCO, Dm_CO and V_c were measured pre- and post-exhaustive incremental exercise at 5150 m after ~40 days of acclimatization. Results: Immediately after exercise at high altitude, there was an increase in group mean Dm_CO (14 ± 10 %, P = 0.040) with no pre- to post-exercise change in group mean DLCO (46.9 ± 5.8 vs. 50.6 ± 9.6 ml/min/mmHg, P = 0.213) or V_c (151 ± 28 vs. 158 ± 37 ml, P = 0.693). There was, however, a ~20 % increase in DLCO from pre- to post-exercise at high altitude (51.2 ± 0.2 vs. 61.1 ± 0.2 ml/min/mmHg) with a concomitant increase in Dm_CO (123 ± 2 vs. 156 ± 4 ml/min/mmHg) and V_c (157 ± 3 vs. 180 ± 8 ml) in 2 of the 7 participants. There was a significant positive relationship between the decrease in $$\dot{V}{\text{O}}_{2\hbox{max} }$$V˙O2max from sea level to high altitude and the change in DLCO and lung diffusing capacity for nitric oxide (DLNO) from rest to end-exercise at high altitude. Conclusion: These data suggest that recruitment of the pulmonary capillaries in response to exercise at high altitude is limited in most well-acclimatized humans but that any such a reserve may be associated with better exercise capacity.