Muscle blood flow and distribution determine maximal VO2 of contracting muscle

During repetitive contractions, the VO2 of the dog gastrocnemius-plantaris muscle rose with the contraction frequency up to a maximal value and then decreased as contraction frequency was increased further. PVO2 was constant over most of the contraction frequency range. Reducing perfusion pressure/blood flow reduced VO2max with a constant PVO2. During these maneuvers the diffusion conductance, DCO2 (VO2/PVO2), changed with VO2. Raising the perfusion pressure/flow with a pump increased VO2 with a small rise in PVO2 so that DCO2 also increased. Removing tension from the muscle between contractions elevated VO2 and DCO2 without a change in perfusion pressure. Hypoxemia decreased VO2 with a decrease in PVO2; DCO2 remained constant. A three-compartment mathematical model, based on microsphere measurements of regional flow, was used to illustrate how regional flow variations may exist, and how they are poorly revealed in the mixed whole-muscle venous blood. The model shows VO2.g-1 strongly related to flow. As VO2.g-1 increased as Q.g-1 increased, extraction decreased, and DCO2 increased.