Analysis of Cardiorespiratory Kinetics – Practical Applications for the Assessment of Circulatory and Muscular Dynamics

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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 (_O2pulm) reflects the rate of oxygen consumed by the tissues (_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 _O2pulm kinetics are not simply a time-shifted version of _O2musc, with an early distortion due to the dynamics of cardiac output (CO). The information about the distortive effects and the time delay between _O2musc and _O2pulm is essential for a proper estimation of _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 _O2musc Hoffmann et al. Eur J Appl Physiol 113: 1745-1754, 2013). Therefore, we applied the proposed method within different conditions (different body postures; before and after endurance training; two different PRBS work rate amplitudes) to highlight the necessity to distinguish between _O2musc and _O2pulm kinetics for proper estimations of the involved physiological systems. Methods Three different subject groups (see Tab. 1) were subjected to PRBS work rate changes between 30W and 80W or 30W and 110W (group C only), respectively, for the kinetics analysis with a cycle ergometer. Heart rate (HR) was assessed beat-to-beat by electrocardiography (ECG) and gas exchange was measured breath-by-breath for _O2pulm. _O2musc kinetics were estimated by the non-invasive approach of Hoffmann et al. (2013). Tab. 1 - Groups Sample size Age [years] Height [cm] Body mass [kg] Peak _O2 [ml · min-1 · kg-1] A - Body positions n = 10 23.4 ± 2.8 179.7 ± 8.3 73 ± 12 40 ± 5 B - Endurance training n = 8 43 ± 9 179 ± 5 Pre: 89.1 ± 10.6 Post: 89.2 ± 8.8 Pre: 37.6 ± 4.8 Post: 41.2 ± 4.1 C - PRBS amplitudes n = 8 26 ± 4 177 ± 7 77 ± 11 54.2 ± 8.9 The body positions group (A) was tested on a tilt table across different postures (−6°, 45°, and 75°). The endurance training group (B) consisted of four subjects which were trained with a continuous (at 60% oxygen uptake reserve (_O2Reserve) and four subjects with an interval (at 90% V’O2Reserve) intervention method, each three times a week. These participants were tested before (Pre) and after (Post) the training intervention lasting six weeks. 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, _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. Differences in the physiological variables were analyzed either with a two-way repeated measure ANOVA or with Wilcoxon’s ranked samples test as appropriate. The alpha level was set to 0.05 for statistical significance. Results Group A: For _O2pulm kinetics significant differences between −6° (CCFmax-values: 0.292 ± 0.040) and 45° (0.256 ± 0.034; p < 0.01; n = 10) as well as between −6° and 75° (0.214 ± 0.057; p < 0.05; n = 10) were detected at lag ‘40 s’ of the CCF course as interaction effects (factors: Lag × Posture). HR and _O2musc kinetics yield no significant differences across the postures. Group B: Significant differences were found between Pre and Post in absolute _O2peak (3.3 ± 0.2 vs. 3.7 ± 0.2 L·min-1; p<0.05; n=8) and relative _O2peak (37.5 ± 4.8 vs. 41.2 ± 4.1 ml·min-1·kg-1; p<0.05). No significant differences (p>0.05) were observed in CCFmax and CCFlag of HR, _O2pulm and _O2musc. Group C: HR and _O2musc kinetics seem to be independent of WR intensity (p > 0.05). _O2pulm kinetics show prominent differences in the lag of the CCF maximum (39 ± 9s; 31 ± 4s; p < 0.05). Discussion The results of the different exercise conditions show a transient non-linear distortion in O2 exchange between muscle and lung which often results in a significant difference between _O2musc and _O2pulm kinetics during dynamic exercise. This is likely due to the influence of cardiac output and venous return dynamics. Accounting for these distortions enables a more reliable assessment of _O2musc kinetics. This will improve the 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 with appropriate training interventions. Acknowledgement The studies were supported by the DLR (Deutsches Zentrum für Luft- und Raumfahrt), Germany (FKZ 50WB1426).
Original languageEnglish
Title of host publicationHuman Physiology Workshop
Number of pages2
PublisherDeutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)
Publication date2016
Publication statusPublished - 2016
EventHuman Physiology Workshop - Köln, Germany
Duration: 10.12.201610.12.2016
Conference number: 1

ID: 2879358

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