Abstract
Multiscale entropy (MSE) quantifies the cardiovascular complexity evaluating Sample Entropy (SampEn) on coarse-grained series at increasing scales τ. Two approaches exist, one using a fixed tolerance r at all scales (MSEFT), the other a varying tolerance r(τ) adjusted following the standard-deviation changes after coarse graining (MSEVT). The aim of this study is to clarify how the choice between MSEFT and MSEVT influences quantification and interpretation of cardiovascular MSE, and whether it affects some signals more than others. To achieve this aim, we considered 2-h long beat-by-beat recordings of inter-beat intervals and of systolic and diastolic blood pressures in male (N = 42) and female (N = 42) healthy volunteers. We compared MSE estimated with fixed and varying tolerances, and evaluated whether the choice between MSEFT and MSEVT estimators influence quantification and interpretation of sex-related differences. We found substantial discrepancies between MSEFT and MSEVT results, related to the degree of correlation among samples and more important for heart rate than for blood pressure; moreover the choice between MSEFT and MSEVT may influence the interpretation of gender differences for MSE of heart rate. We conclude that studies on cardiovascular complexity should carefully choose between fixed- or varying-tolerance estimators, particularly when evaluating MSE of heart rate.
Highlights
A feature of heart rate variability, potentially useful for improving the stratification of cardiovascular risk or for detecting early alterations of the cardiovascular control, is the level of unpredictability of beat-by-beat dynamics
In the estimation of Sample Entropy (SampEn) or ApEn, each segment is represented as a point in an m-dimensional space and two segments are similar if their distance is lower than a given tolerance, r
We compared MSEFT and MSEVT in male and female groups of healthy volunteers, considering cardiovascular series often recorded in physiological and clinical studies: inter-beat interval (IBI, the inverse of heart rate); systolic blood pressure (SBP) and diastolic blood pressure (DBP); and we evaluated how the choice between fixed- and varying-tolerance affects the interpretation of Multiscale entropy (MSE) comparisons by gender
Summary
A feature of heart rate variability, potentially useful for improving the stratification of cardiovascular risk or for detecting early alterations of the cardiovascular control, is the level of unpredictability of beat-by-beat dynamics. One of the most popular estimators of heart-rate unpredictability is Sample Entropy (SampEn) [1], proposed as an unbiased variant of Approximate. In the estimation of SampEn or ApEn, each segment is represented as a point in an m-dimensional space and two segments are similar if their distance is lower than a given tolerance, r. Heart rate variability reflects part of the overall complexity of cardiovascular control. The latter regulates the amount of blood flow received by individual vascular beds, which compose a fractal
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