Operation Everest II and the 1978 Habeler/Messner ascent of Everest without bottled O2: what might they have in common?

In 1978, Peter Habeler and Reinhold Messner climbed Everest without supplemental O 2 . Subsequently, Oelz et al. (Oelz O, Howald H, Di Prampero PE, Hoppeler H, Claassen H, Jenni R, Bühlmann A, Ferretti G, Brückner JC, Veicsteinas A, Gussoni M, Cerretelli P. J Appl Physiol (1985) 60: 1734–1742, 1986) assessed their cardiopulmonary function, finding no advantageous physiological attributes to explain their success, and leading West (West JB. High Life: A History of High-Altitude Physiology and Medicine. New York: Oxford University, 1998) to suggest that grit and determination were more important. In 1985, Charlie Houston, John Sutton, and Al Cymerman hosted a scientific project assessing a simulated ascent of Everest (OE II) at the U.S. Army Research Institute of Environmental Medicine. Included were measurements of O 2 transport. In particular, mixed venous Po 2 was measured at/near maximal exercise, for calculating pulmonary O 2 -diffusing capacity. A serendipitous observation was made: while both V̇o 2max and mixed venous Po 2 fell with altitude (as expected), it was how they fell—in direct proportion—that was remarkable. It later became clear that this reflected diffusion limitation of O 2 transport from muscle microvessels to the mitochondria, and that this last step in O 2 transport plays a major role in limiting V̇o 2max . Thus, how Habeler and Messner made it up Everest without bottled O 2 and no special cardiopulmonary attributes might be explained if their muscle O 2 -diffusing capacity, which depends largely on muscle capillarity, was unusually high. Oelz et al. mention that muscle capillary density was substantially—40%—above normal, but did not suggest that this accounted for the climbersʼ success. Therefore, high muscle capillarity, enhancing diffusive unloading of O 2 , may have been a major enabling physiological attribute for Habeler and Messner and that OE II, by chance, played a key role in bringing this to light.