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

Abstract

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.