Nonlinear characteristics of heart rate time series: influence of three recumbent positions in patients with mild or severe coronary artery disease

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Abstract
The aim of this study was to investigate the nonlinear characteristics of heart rate variability (HRV) for three recumbent positions: the supine, left lateral and right lateral decubitus positions. Recently, using a linear analyses method (for time and frequency domains), the effect of the right lateral decubitus position on vagal modulation has been found to increase parasympathetic activity and decrease sympathetic modulation. Little is known about the nonlinear dynamics of HRV for the three recumbent positions. Therefore, we studied the correlation dimension (CD), the largest Lyapunov exponent (LLE), the sample entropy (SampEn), the approximate entropy (ApEn) and the exponent alpha of the 1/falpha power spectrum as nonlinear characteristics of HRV. In response to the right lateral decubitus position, the CD, LLE, SampEn and ApEn increased significantly in both coronary artery disease (CAD) and control groups. In the linear analyses, the normalized high-frequency power (nHF) increased in the right lateral decubitus position. The CD, LLE, ApEn and SampEn correlated positively to the nHF. The alpha exponent did not correlate to either linear measure or CD, but correlated negatively to LLE, ApEn and SampEn. Among the three recumbent positions, it was found that the right lateral decubitus position can increase the complexity of the human physiological system and the vagal modulation of the cardiac autonomic nervous system the most.