A model analysis of asymmetrical response of pulmonary VO2 during incremental and decremental ramp exercise

The responses of gas exchange and ventilatory variables exhibit obvious asymmetry between the incremental and decremental phases of trapezoidal ramp exercise (29). Computer simulation was performed to explore the possible mechanisms producing this asymmetry of pulmonary O2 uptake (VO2) dynamics. The model consisted of three compartments (the lungs, the exercising muscles, and the other organs and tissues), which were perfused by respective regional circulations. Exercise was simulated by increasing the pulmonary blood flow (Q) and the muscle. VO2 in proportion to work rate. The dynamic responses of Q and the muscle VO2 were assumed to obey first-order linear kinetics. The ratio of blood flow in the muscle compartment to total cardiac output was assumed to increase linearly with work rate. Our simulation demonstrated that the muscle blood flow and mixed venous O2 content exhibited significant asymmetry between the incremental and decremental phases of exercise. The model suggested that the asymmetry of VO2 between incremental and decremental ramp exercise was dependent on the redistribution ratio of blood flow to the working muscles. The validity of the model was then evaluated by comparing the simulated results with experimental data obtained.