Nonlinear and linear analysis of heart rate variability in Chagas disease

Abstract

Although heart rate variability (HRV) has been extensively studied in many diseases, there is scarce data on HRV in Chagas disease (ChD) patients. B slope is a nonlinear index derived from the power-law analysis of the 24-hour HRV; in healthy subjects, b index is typically near [minus ]1, suggesting a fractal behavior of HRV. A steeper b slope indicates a breakdown of this fractal pattern and is a strong predictor of mortality in dilated cardiomyopathy of both ischemic and nonischemic etiology. In ChD, power-law behavior of the HRV has never been studied. This transversal study intends to describe power-law and linear HRV patterns in different forms of ChD. ChD patients (n=165) and healthy controls without other diseases were submitted to a standardized protocol, including ECG, echo and 24h Holter monitoring. ChD patients were divided according to the LV systolic function (see table). The temporal series were carefully processed in order to obtain the HRV indexes SDNN, RMSSD and the b slope. If necessary, mathematical transformations were done to stabilize the variances and to normalize the data. The analysis of covariance was used to adjust the HRV indexes for significant confounding factors, as mean RR and age. HRV indexes can be deceivingly modified by significant covariates in more advanced ChD forms, indicating the necessity of controlling confounding variables, as mean RR and age. Adjusted HRV data showed that ChD patients present reduced RMSSD values and more negative beta slopes independently of the severity of the LV dysfunction, suggesting that dysautonomia is a primary phenomenon in ChD.