Abstract
In contrast to steady-state conditions, the dynamics of oxygen uptake at the muscle level (V'O2M) cannot be measured at the lung site (V'O2L). In comparison with V'O2M, the V'O2L time-courses are distorted, due to the venous volume between exercising muscles and the lungs (VvML) and the perfusion kinetics of the muscles. We developed a backward calculation method (BCM) allowing the estimation of muscle V'O2 kinetics on the basis of V'O2L and cardiac output measurements. The algorithm allows the adjustment of VvML and the perfusion of the non-exercising parts of the body (Q'R). As a criterion of an adequate estimation for tuning of VvML and Q'R we utilized normalized cross correlation function of workload and V'O2M. PURPOSE: To describe the characteristics of muscular V'O2-kinetics and the influence of cardio-vascular parameters during moderate exercise changes. METHODS: 19 volunteers (age: 27.9±6.8 years; height: 176±8 cm; weight: 70.9±11.7 kg; V'O2max: 3.8±1.0 L·min-1) were subjected to randomized changes between 30 W and 80 W on a cycle ergometer. V'O2L was measured breath-by-breath gas exchange. Stroke volume and heart rate were measured beat-to-beat by electrocardiography and impedance cardiography. The kinetics of V'O2M is characterized by the peak-value of the cross correlation function (CCFP). RESULTS: VvML and Q'R were estimated as 2.4±0.6 L and 3.4±1.8 L·min-1, respectively. CCFP correlated with relative V'O2max (r=0.513; p<0.05; n=19), maximal workload (r=0.470; p<0.05; n=19), relative maximal workload (r=0.519; p<0.05; n=19) and age (r=-0.465; p<0.05; n=19). CONCLUSIONS: Our algorithm allows a reliable estimation of venous volume between exercising muscles and the lungs and perfusion of the non-exercising parts of the body. The calculated V'O2 is more representative for aerobic muscle metabolism than the respiratory V'O2. The study was funded by the DLR (Deutsches Zentrum für Luft und Raumfahrt), Germany (FKZ 50WB0726).
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