Theoretical predictions of maximal oxygen consumption in hypoxia: effects of transport limitations.

A Krogh-type model for oxygen transport is used to predict maximal oxygen consumption (V(.-) O(2max)) of human skeletal muscle under hypoxic conditions. Assumed values of capillary density, blood flow, and hemoglobin concentration are based on measurements under normoxic and hypoxic exercise conditions. Arterial partial pressure of oxygen is assumed to decrease with reductions in inspired partial pressure of oxygen (P(I)O(2)), as observed experimentally. As a result of limitations of convective and diffusive oxygen delivery, predicted V(.-) O(2max) values decline gradually as P(I)O(2) is reduced from 150 mmHg to about 80 mmHg, and more rapidly as P(I)O(2) is further reduced. At very low levels of P(I)O(2), V(.-) O(2max) is limited primarily by convective oxygen supply. Experimentally observed values of V(.-) O(2max) in hypoxia show significant dispersion, with some values close to predicted levels and others substantially lower. These results suggest that maximal oxygen consumption rates in hypoxia are not necessarily determined by oxygen transport limitations and may instead reflect reduced muscle oxygen demand.