Effect of change in P50 on exercise tolerance at high altitude: a theoretical study

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Abstract
Acclimatization to altitude often results in a rightward shift of the O2 dissociation curve (ODC). A left-shifted ODC is reported to increase exercise tolerance in humans at medium altitude and increase survival in rats breathing hypoxic gas mixtures. This paradox was examined using a computer model of pulmonary gas exchange. A Bohr integration procedure allowed for alveolar-capillary diffusion. When diffusion equilibration was complete, mixed venous (P.hivin.vO2) and arterial PO2 [partial pressure] fell as O2 consumption (.ovrhdot.VO2) was increased, but P.hivin.vO2 approached a plateau. Under these conditions a right-shifted ODC is advantageous (higher P.hivin.vO2) at all but very high altitudes. Diffusion limitation of O2 transfer may occur at any altitude if .ovrhdot.VO2 is increased sufficiently. If this occurs, a left-shifted ODC results in higher calculated .ovrhdot.VO2 max (compared with the standard ODC). Diffusion limitation always occurs at a lower .ovrhdot.VO2 with a right-shifted ODC than with a left-shifted ODC. Whether a leftward or rightward shift in the ODC is advantageous to gas exchange at an altitude depends on the presence or absence of diffusion limitation.

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