In this study the 24-h Heart Variability Signal s (HRV) of 206 patients with Aortic Stenosis (AS) and 68 healthy subjects (NRM) were analyzed, using dynamical nonlinear analysis to compare the complexity of the signals between these two groups during morning (7-12h), afternoon (15-20h) and night (0-5h
IntroductionDuring recent years, non-linear analysis of Heart Rate Variability (HRV) has been used to characterize healthy people and a variety of heart diseases and different levels of risk [1][2][3][4][5].Different non-linear measures are used for this purpose, like Hurst and Lyapunov exponents, entropies, complexity and information indexes, symbolic logic and dimensional analysis. This latter one, estimates the dimension of the attractor formed by the set of points in the phase space, which is a statistical measure of the self similarity of the geometry of points and is related to the number of independent variables (degrees of freedom) needed to generate a corresponding process. A method to measure the dimension of the attractor is the correlation dimension (CD) [6].CD has been determined for HRV in several circumstances and cardiac disorders. It has been observed that the CD of HRV has lower values in illness subjects compared with healthy ones [7][8][9].The HRV of healthy people has a circadian rhythm. The RR values during night are higher than during the day. Modifications in this rhythm in some pathologies and special situations has been reported [10 -12]. In healthy subjects non-linear indexes, such as CD and approximate entropy, also show a circadian rhythm, characterized by low values in the morning and high values at night [13]. Healthy females have higher CD values compared with healthy males [14].In this study we analyze the complexity of the HRV, by means of CD, in healthy subjects (NRM) and in patients with aortic stenosis (AS). This pathology is a narrowing or obstruction of the aortic heart valve, causing it to not open properly and to obstruct the flow of blood from the left ventricle to the aorta. Surgical repair or replacement of the valve is the preferred treatment for symptomatic AS. Using the dynamical analysis proposed by our group [6,15], we calculate the CD of HRV for NRM and AS, comparing t he results by gender and age.
2.Materials and methods
Analyzed dataHRV signals were obtained from 24-h ECG Holter recordings sampled at 256 Hz, from the database of the National Institute of Cardiology at Warsaw, Poland. HRV signals were selected from 206 AS patients and 68 signals from NRM group. All ECG had QRS<120 ms, no conduction abnormalities. All RR intervals were analyzed without any filtration and the number of ectopic beats was below 100 in all cases.
Correlation dimensionTo calculat e CD from the RR time series, Takens theorem is used [16]:The RR values are a data vector ( y i , i=1,… , N), where N is the number of beats. A m-dimensional phase-space is constructed, m is the embedding dimension, obtaining: