Symmetric Cardiovascular Dynamics Generate Asymmetric Respiratory V'O₂ Responses to On- and Off-Step Changes in Workload

Introduction
Muscular V’O2 kinetics (V’O2musc) are frequently estimated on the basis of respiratory V‘O2 (V’O2resp) responses to step changes in workload. A typical approach to account for the distorting effects of cardiac output (CO) dynamics and venous return is to discard the initial response of about 20 s (“cardiogenic phase”) before data fitting.
Methods
In this study we investigated, by means of a mathematical model, the influence of CO dynamics, muscle perfusion, venous volume and venous volume distribution on respiratory V’O2 as well as the effect of discarding the first 20 s of V’O2resp on the quality of V’O2musc estimation.
Results
The results show marked asymmetries between on- and off V’O2resp kinetics and substantial errors in V’O2musc estimates in particular for the off-kinetics. Discarding cardiogenic phases only partly compensates for the distortion of muscular V’O2 generated by the transport through the venous system. Within the parameter settings chosen, errors in the estimate of V’O2musc time constants were up to 90% for off-kinetics and up to 40% for on-kinetics.
Discussion
The simulations suggest, that major improvements in estimated muscular V’O2 kinetics can only be obtained if the values of the relevant distorting parameters (muscle perfusion, venous volume and venous volume) are known. These values would allow a backward computation of muscular V’O2 dynamics from respiratory V’O2 data.
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