Skeletal muscle \( \dot{V} {\text{O}_2}\) kinetics from cardio-pulmonary measurements: assessing distortions through O₂ transport by means of stochastic work-rate signals and circulatory modelling

During non-steady-state exercise, dynamic changes in pulmonary oxygen uptake (\(\dot{V} {\text{O}_{\text{2pulm}}}\)) are dissociated from skeletal muscle \( \dot{V} {\text{O}_2}\) (\(\dot{V} {\text{O}_{\text{2musc}}}\)) by changes in lung and venous O₂ concentrations (CvO₂), and the dynamics and distribution of cardiac output (CO) between active muscle and remaining tissues (\( \dot{Q}_{\text{rem}}\)). Algorithms can compensate for fluctuations in lung O₂ stores, but the influences of CO and CvO₂ kinetics complicate estimation of \(\dot{V} {\text{O}_{\text{2musc}}}\) from cardio-pulmonary measurements. We developed an algorithm to estimate \(\dot{V} {\text{O}_{\text{2musc}}}\) kinetics from \(\dot{V} {\text{O}_{\text{2pulm}}}\) and heart rate (HR) during exercise. 17 healthy volunteers (28 ± 7 years; 71 ± 12 kg; 7 females) performed incremental exercise using recumbent cycle ergometry (\(\dot{V} {\text{O}_{\text{2peak}}}\) 52 ± 8 ml min⁻¹ kg⁻¹). Participants completed a pseudo-random binary sequence (PRBS) test between 30 and 80 W. \(\dot{V} {\text{O}_{\text{2pulm}}}\) and HR were measured, and CO was estimated from HR changes and steady-state stroke volume. \(\dot{V} {\text{O}_{\text{2musc}}}\) was derived from a circulatory model and time series analyses, by solving for the unique combination of venous volume and the perfusion of non-exercising tissues that provided close to mono-exponential \(\dot{V} {\text{O}_{\text{2musc}}}\) kinetics. Independent simulations showed that this approach recovered the \(\dot{V} {\text{O}_{\text{2musc}}}\) time constant (τ) to within 7 % (R ² = 0.976). Estimates during PRBS were venous volume 2.96 ± 0.54 L; \( \dot{Q}_{\text{rem}}\) 3.63 ± 1.61 L min⁻¹; τHR 27 ± 11 s; τ\(\dot{V} {\text{O}_{\text{2musc}}}\) 33 ± 8 s; τ\(\dot{V} {\text{O}_{\text{2pulm}}}\) 43 ± 14 s; \(\dot{V} {\text{O}_{\text{2pulm}}}\) time delay 19 ± 8 s. The combination of stochastic test signals, time series analyses, and a circulatory model permitted non-invasive estimates of \(\dot{V} {\text{O}_{\text{2musc}}}\) kinetics. Large kinetic dissociations exist between muscular and pulmonary \(\dot{V} {\text{O}_{\text{2}}}\) during rapid exercise transients.