Temporal asymmetries of short-term heart period variability are linked to autonomic regulation

Abstract

We exploit time reversibility analysis, checking the invariance of statistical features of a series after time reversal, to detect temporal asymmetries of short-term heart period variability series. Reversibility indexes were extracted from 22 healthy fetuses between 16th to 40th wk of gestation and from 17 healthy humans (aged 21 to 54, median ϭ 28) during graded head-up tilt with table inclination angles randomly selected inside the set {15, 30, 45, 60, 75, 90}. Irreversibility analysis showed that nonlinear dynamics observed in short-term heart period variability are mostly due to asymmetric patterns characterized by bradycardic runs shorter than tachycardic ones. These temporal asymmetries were 1) more likely over short temporal scales than over longer, dominant ones; 2) more frequent during the late period of pregnancy (from 25th to 40th week of gestation); 3) significantly present in healthy humans at rest in supine position; 4) more numerous during 75 and 90°head-up tilt. Results suggest that asymmetric patterns observable in short-term heart period variability might be the result of a fully developed autonomic regulation and that an important shift of the sympathovagal balance toward sympathetic predominance (and vagal withdrawal) can increase their presence. heart rate variability; autonomic nervous system; head-up tilt; fetal maturation; nonlinear dynamics THE VARIABILITY OF HEART PERIOD (usually approximated as the temporal distance between two consecutive R peaks on the ECG, R-R) has been proven to be nonlinear in healthy fetuses between 38th and 40th week of gestation (8) and in healthy humans (1, 4), mostly during experimental conditions periodically forcing cardiovascular regulation (i.e., controlled breathing) (15, 16). However, this finding has not been translated yet into a notion actually helpful in pathophysiology. The main reason is that, until now, the detection of nonlinear dynamics has not been linked to a clear temporal correlate (i.e., a pattern associated with nonlinear dynamics).Time irreversibility analysis checks the invariance of the statistical properties of a time series after time reversal. This analysis might be helpful to translate the involved concept of nonlinear dynamics into a simple, comprehensible notion useful in pathophysiology, since it clearly indicates a time domain scheme responsible for nonlinear dynamics. Indeed, time irreversibility analysis is capable of detecting a specific class of nonlinear dynamics, that is, those characterized by a temporal asymmetry. In other words, when a series is detected as irreversible using simple tests in the two-dimensional phase space (6, 9, 17), it can be stated that the nonlinear behavior is the result of the presence of asymmetric patterns (i.e., waveforms characterized by the upward side shorter or longer than the downward side), thus directly linking the abstract concept of nonlinear dynamics to a clear, easily imaginable, feature (17).The aim of this study is twofold. The first aim is to link the presence of temporal asymme...

Clinical impact of evaluation of cardiovascular control by novel methods of heart rate dynamics

“…Several indices, such as the Lyapunov exponent (Eckmann & Ruelle 1985), the Hausdorff dimension D (Eckmann & Ruelle 1985;Babyloyantz & Destexhe 1988), the correlation dimension D 2 (Grassberger & Procaccia 1983a;Eckmann & Ruelle 1985), Kolmogorov entropy K (Grassberger & Procaccia 1983b), nonlinear predictability (Porta et al 2000), the wavelet transform modulus maxima (Ohashi et al 2003), time asymmetry/irreversibility parameters (Costa et al 2005;Porta et al 2008) and the permutation entropy (Frank et al 2006), have been used to estimate the complexity of time series, but the clinical applicability of these methods has not been well established. The below-mentioned measurements of entropy are the nonlinear complexity measures of HRV that are most widely studied in clinical settings.…”

Asymmetric features of short-term blood pressure variability

“…Two other descriptors of blood pressure asymmetry are based on the analysis of the number of SBP increases (n i ) and reductions (n r ) corresponding to the numbers of points above and below the identity line of the Poincaré plot, respectively. 17,24 However, as both n i and n r are strictly dependent on the length of the recording, in further analysis these numbers are also normalized:…”

“…For heart rate asymmetry the following previously reported variables were calculated with the use of Poincaré plot analysis of RR intervals and in-house software written in Python (Python Foundation, USA): 12,16,17 SD1 2 -the variance quantifying the dispersion of all points in the plot across the identity line-this is the measure of the short-term HRV and is used only for C1 d and C1 a quantification, see below; SD1 d 2 -the part of SD1 2 contributed by the points in the Poincaré plot above the identity line-these points correspond only to decelerations of heart rate (prolongations of RR intervals). 12,16 SD1 a 2 -the part of SD1 2 contributed by the points in the Poincaré plot below the identity line-these points correspond only to accelerations of heart rate (shortenings of RR intervals).…”

“…N d , called also Porta's index, is the ratio of the number of heart rate decelerations to the number of all changing cardiac cycles, that is, the total number of all decelerations and accelerations of the heart rate. 17,24 Analogously, N a can be calculated for heart rate accelerations; however, N d +N a equals 100%, and therefore to avoid redundancy, only N d will be shown in the results section.…”

“…12,16 At supine rest, the prolongations of cardiac cycles usually have a significantly larger contribution to shortterm HRV than shortenings, and this unidirectional phenomenon is present in the majority (480%) of electrocardiography (ECG) recordings. 12,16 Porta et al 17 have also shown that the number of prolongations of RR intervals is significantly smaller than the number of shortenings.…”