Heart Rate Variability (HRV) data exhibit long memory and time-varying conditional variance (volatility). These characteristics are well captured using Fractionally Integrated AutoRegressive Moving Average (ARFIMA) models with Generalised AutoRegressive Conditional Heteroscedastic (GARCH) errors, which are an extension of the AR models usual in the analysis of HRV. GARCH models assume that volatility depends only on the magnitude of the shocks and not on their sign, meaning that positive and negative shocks have a symmetric effect on volatility. However, HRV recordings indicate further dependence of volatility on the lagged shocks. This work considers Exponential GARCH (EGARCH) models which assume that positive and negative shocks have an asymmetric effect (leverage effect) on the volatility, thus better copping with complex characteristics of HRV. ARFIMA-EGARCH models, combined with adaptive segmentation, are applied to 24 h HRV recordings of 30 subjects from the Noltisalis database: 10 healthy, 10 patients suffering from congestive heart failure and 10 heart transplanted patients. Overall, the results for the leverage parameter indicate that volatility responds asymmetrically to values of HRV under and over the mean. Moreover, decreased leverage parameter values for sick subjects, suggest that these models allow to discriminate between the different groups.
IntroductionHeart Rate Variability (HRV) reflects the interaction between perturbations to the cardiovascular variables and the corresponding response of the cardiovascular regulatory systems [1]. The modeling of such variability can provide a quantitative and non-invasive method to assess the integrity of the cardiovascular system. HRV data display non stationarity and exhibit long memory and time-varying conditional variance (usually designated by volatility) among other nonlinear characteristics [2], that are well modelled by AutoRegressive Fractionally Integrated Moving Average (ARFIMA) models with Generalised AutoRegressive Conditional Heteroscedastic (GARCH) errors [3]. GARCH models assume that volatility depends only on the magnitude of the shocks and not on their sign, meaning that positive and negative shocks have a symmetric effect on volatility [4]. In [5] the authors consider an extension of GARCH models, Exponential GARCH (EGARCH) models, which allow for an asymmetric effect (leverage effect) and conclude that a model with leverage effect is more suited to describe the complex and nonlinear characteristics of HRV.
2.Data and Methods
DataThis study analyses HRV data from the Noltisalis database [6] which was collected by the cooperative effort of university departments and rehabilitation clinics in Italy. The dataset consists of 24 hour HRV recordings of 30 subjects: 10 healthy subjects (N, 22.5 ± 1.6 hours; 102115.2 ± 11365.4 beats; 42.2 ± 6.4 years), 10 patients suffering from congestive heart failure (C, 22.4±0.9 hours; 107170.5 ± 16689.3 beats; 53.6 ± 11.2 years) and 10 heart transplanted patients (T, 22.4 ± 0.7 hours; 116043.3 ± 119...