Estimating Muscular Oxygen Uptake Kinetics by Time-Series Analysis: Theoretical Approach and Practical Applications in Different Subject Groups

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Introduction: Oxygen uptake kinetics allows a valuable understanding of the integrated physiological processes during exercise. In the exercise steady-state, pulmonary oxygen uptake (V’O2pulm) reflects the rate of oxygen consumed by the tissues (V’O2musc). However, during non-steady-state these dynamics are dissociated. The interactions of muscular gas exchange with the dynamics of the circulation and remaining body O2 capacitances means that V’O2pulm kinetics are not simply a time-shifted version of V’O2musc, with an early distortion due to the dynamics of cardiac output (CO). The information about the distortive effects and the time delay between V’O2musc and V’O2pulm is essential for a proper estimation of V’O2musc. It is hypothesized that by determining the distortive effects in combination with a circulatory model application and time-series analysis will enable a reliable assessment of V’O2musc. Applying the proposed method in different subject groups will highlight the necessity to distinguish between V’O2musc and V’O2pulm kinetics. Methods: Different subject groups (young healthy (n=17; age: 28 ± 7 years; height: 174 ± 8 cm; weight; 71 ± 12 kg; maximal oxygen uptake: 52 ± 8 ml · min-1 · kg-1), healthy elderly (n=14; 72 ± 2 years; 176 ± 5 cm; 81 ± 2 kg; 25 ± 5 ml · min-1 · kg-1) and diseased (Diabetes-Type-II) elderly (n=27; 64 ± 10 years; 97 ± 20 kg; 176 ± 5 cm; 21 ± 5 ml · min-1 · kg-1), and astronauts (n=10; 48 ± 4 years; 176 ± 7 cm; 75 ± 16 kg; 40 ± 8 ml · min-1 · kg-1)) were subjected to pseudo-random binary sequence (PRBS) work rate changes between 30W and 80W for the kinetics analysis. Heart rate (HR) was assessed beat-to-beat by electrocardiography (ECG) and gas exchange was measured breath-by-breath for V’O2pulm. V’O2musc kinetics were estimated by the non-invasive approach of Hoffmann et al. (2013). Given a linear, time-invariant, first order (LTI) system the cross correlation function (CCF) of work rate (WR) and a second parameter (e. g. HR, V’O2musc) indicate the kinetic responses of this parameter by the maximum (CCFmax) and its lag (CCFlag). Higher CCFmax values denote faster system responses, and greater CCFlag values more time-delayed responses. Results: Significant differences were observed between V’O2pulm and V’O2musc kinetics (CCFmax) in young healthy (0.33 ± 0.08 vs. 0.41 ± 0.08; p<0.001), in healthy elderly (0.29 ± 0.06 vs. 0.31 ± 0.06; p<0.05) and in diseased elderly (0.28 ± 0.06 vs. 0.31 ± 0.06; p<0.01). In the astronauts group comparisons were performed between pre- (L-; [days]) and post-flight (R+; [days]) measurements. Here, CCFmax(V’O2musc) differed significantly between L-236 and R+6 (p=0.010), L-236 and R+21 (p=0.030) as well as L-72 and R+6 (p=0.043). For CCFmax(V’O2pulm) between L-236 and R+6 a significant difference was observed (p=0.011). Conclusion: The results of the different subject groups show a transient non-linear distortion in O2 exchange between muscle and lung which often result in a significant difference between V’O2musc and V’O2pulm kinetics during dynamic exercise. Especially, the astronauts group showed deviations between V’O2pulm and V’O2musc pre and post International Space Station missions, which would not be assessed by measurements on the mouth separately; and would therefore lead to deviant interpretations of the astronauts’ cardiopulmonary fitness level and their exercise training after return to Earth. This is likely due to the influence of cardiac output and venous return dynamics. Accounting for these distortions enables a more reliable assessment of V’O2musc kinetics. This will improve our understanding of the altered adaptations in chronic disease and ageing, in daily life, by training and extreme environments, and will allow us to better target the involved physiological systems by therapeutic interventions to maintain health and to counteract deconditioning. Reference: Hoffmann, U., Drescher, U., Benson, A. P., Rossiter, H. B. & Essfeld, D. (2013). Skeletal muscle V’O2 kinetics from cardio-pulmonary measurements: assessing distortions through O2 transport by means of stochastic work-rate signals and circulatory modelling. Eur J Appl Physiol, 113(7), 1745-1754.
Event titleResearch in Progress
Event typeOther
SponsorUniversity of California, Los Angeles
LocationLos Angeles, USA/United States of AmericaShow on map
Degree of RecognitionInternational