The physiological basis of reduced VO2max in Operation Everest II

While it is well-known that maximal VO2 falls progressively with increasing altitude, discussion continues on how each step of the O2 transport pathway contributes to this loss in exercise capacity. By analyzing the O2 transport pathway as an in-series system of four transport processes (ventilation, alveolar-capillary diffusion, circulation, and muscle capillary–mitochondrial diffusion) and applying this analysis to published data from Operation Everest II, it is concluded that in OE II the passive diffusive steps in the lungs and muscles explain most of the decrement in O2 availability, while changes in the mostly convective processes of ventilation and blood flow contribute little or no impediment to O2 transport. Potentially deleterious effects of decreased maximal cardiac output and mitigating effects of increased [Hb] on circulatory O2 transport are likely offset by corresponding changes in diffusion equilibration in the lungs and muscles. These conclusions are consistent with the laws of diffusion that depend on partial pressure, which falls progressively with increasing hypoxia.