Analysis of cardio-pulmonary and respiratory kinetics in different body positions: impact of venous return on pulmonary measurements

Purpose The aim of the study was to compare the kinetics responses of heart rate (HR), pulmonary (V˙O2pulm), and muscular (V˙O2musc) oxygen uptake during dynamic leg exercise across different body positions (−6°, 45°, and 75°). Methods Ten healthy individuals [six men, four women; age 23.4 ± 2.8 years; height 179.7 ± 8.3 cm; body mass 73 ± 12 kg (mean ± SD)] completed pseudo-random binary sequence (PRBS) work rate (WR) changes between 30 and 80 W in each posture. HR was measured beat-to-beat by echocardiogram and V˙ O2pulm by breath-by-breath gas exchange. V˙O2musc kinetics were assessed by the procedure of Hoffmann et al. (Eur J Appl Physiol 113:1745–1754, 2013) applying a circulatory model and cross-correlation functions (CCF). Results For V˙O2pulm kinetics significant differences between −6° (CCF-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 V˙O2musc kinetics yield no significant differences across the postures. Conclusions The analysis of cardio-dynamic and respiratory kinetics, especially with an emphasis on muscular and cellular level, has to consider venous return and cardiac output distortions. Simplified observations of kinetics responses resulting in time constants and time delays only should be replaced by the time-series analysis for a more sophisticated evaluation. The results illustrate that isolated V˙O2pulm measurements without cardio-dynamic influences may not represent the kinetics responses originally revealed at muscular level.