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In order to analyze the information carried by arterial blood pressure (ABP) variability, a multivariate parametric model of interactions involving systolic ABP (SAP), diastolic ABP (DAP), pulse pressure (PP), heart period (HP), and respiration is proposed. The model defines SAP as sum of the preceding DAP and PP values; DAP model accounts for arterial baroreflex, diastolic runoff; PP reflects changes in stroke volume related to respiration and HP, afterload; equation residuals reveal other vascular and cardiac output modulations. The model was applied to data from nine young volunteers (aged 29 [+ or -] 6 years) during supine cycling at 10%, 20%, and 30% of their maximum effort. Significant basal values and changes across the epochs of the experiment were found in all hemodynamic parameters describing fast, beat-by-beat responses; in SAP and PP total power, DAP low- and high-frequency power (LF, HF), PP very low frequency (VLF), and LF and HF power. A primary role of vascular control through DAP and PP was emphasized by the considered feedbacks and the model residuals. The model proved to be able to assess beat-by-beat cardiovascular interactions and offer a comprehensive view of arterial tree control. Index Terms--Arterial pressure variability, cardiovascular control, closed loop, exercise, multivariate modeling.

Objective: Traditional definition of sample entropy (SampEn), here referred to as global SampEn (GSampEn), provides a conditional entropy estimate that blurs the local statistical properties of the time series. We hypothesized that a local version of SampEn (LSampEn) might be more powerful in the presence of determinism than GSampEn. Methods: LSampEn was computed by calculating the probability of the current sample conditioned on each reference pattern and averaging it over all reference patterns. The improved ability of LSampEn compared to GSampEn was demonstrated by simulating deterministic periodic, deterministic chaotic, and linear stochastic dynamics corrupted by additive noise and over real cardiovascular variability series recorded from 16 healthy subjects (max–min age range: 22–58 years) during incremental bicycle ergometer exercise. Results: We found that: i) LSampEn is more robust in describing deterministic periodic or nonlinear features in the presence of additive noise than GSampEn, ii) in association with a surrogate approach, LSampEn is more powerful in detecting nonlinear dynamics than GSampEn, iii) LSampEn and GSampEn are equivalent in the presence of stochastic linear dynamics, and iv) only LSampEn can detect the decrease of complexity of heart period variability during bicycle exercise being a likely hallmark of sympathetic activation. Conclusion: LSampEn preserves the GSampEn capability in characterizing the complexity of short sequences but improves its reliability in the presence of deterministic patterns featuring sharp state transitions and nonlinear dynamics. Significance: Variations of complexity can be measured with a greater statistical power over short series using LSampEn, especially when nonlinear features are present. [ABSTRACT FROM AUTHOR]