Abstract
INTRODUCTION: One of the most widely used methods for studying the bioelectric activity of the heart is the electrocardiogram (ECG). An important diagnostic parameter that can be determined by the ECG is heart rate variability (HRV), which takes into account the difference between successive strokes of the heart. Changing HRV can be an indicator of a number of disease states, such as low HRV levels can show poor health that is not only associated with cardiovascular disease but also with other diseases such as internal, nervous, mental, and other disorders.
 OBJECTIVES: The subject of this article is the study of 24-hour ECG signals by applying non-linear graphical methods for HRV analysis. The non-linear graphical methods aim at obtaining graphical and quantitative information on the cardiovascular status of the study groups to complement the information obtained from traditional linear methods of analysis.
 METHODS: For the non-linear analysis of HRV, graphical methods were used: Poincaré plot and Recurrence plot were used, which are suitable for the examination of electrocardiographic signals. Two groups of people were investigated: 20 healthy controls and 20 patients with arrhythmia.
 RESULTS: Based on the nonlinear analysis of RR time series, the graphs of a healthy subject and a patient with arrhythmia were constructed using the Poincare plot. The graph of the healthy subject has the shape of a comet, while the graph of the patient with arrhythmia has the shape of a fan. The quantitative characteristics of patients with arrhythmia significantly change compared to the healthy subjects. The SD1 (p <0.003) and SD2 (p <0.0001) values decreased in patients with arrhythmia compared to the healthy controls. This reduction leads to reduction of the areas of the ellipse in the patients with arryhthmia. The ratio of SD1/SD2 (p <0.05) is lower for the healthy controls. The graphs obtained by the Recurrence plot of the investigated signals differ in healthy subjects and in patients with arrhythmia. For a healthy subject, the graph has a diagonal line and fewer squares showing a higher HRV. The graph of a patient with arrhythmia contains more squares, indicating periodicity in the investigated signal. The Recurrence Quantification Analysis showed that the values of the investigated parameters DET% (p <0.0001), REC% (p <0.0001) and ENTR (p <0.001) in patients with arrhythmia are increased.
 CONCLUSIONS: The importance of the graphical nonlinear methods used for the analysis of HRV consists in forming a parametric and graphical assessment of the patient's health status.
Highlights
One of the most widely used methods for studying the bioelectric activity of the heart is the electrocardiogram (ECG)
The graphs obtained by the Recurrence plot of the investigated signals differ in healthy subjects and in patients with arrhythmia
The importance of the graphical nonlinear methods used for the analysis of heart rate variability (HRV) consists in forming a parametric and graphical assessment of the patient's health status
Summary
One of the most widely used methods for studying the bioelectric activity of the heart is the electrocardiogram (ECG). As a result of the introduced new standard, the analysis of heart rate variability has become a rapidly evolving field in cardiology, in which is realized the use of mathematical methods of analysis in the diagnosis of a number of diseases. The advantage of this new direction is the possibility of detecting the most subtle variations in cardiac activity, so mathematical methods of HRV analysis are useful in assessing the general functional capabilities of the body in normal and early abnormalities that, in the absence of the necessary preventive procedures can progressively develop into a more serious illness (Smith et al, 2009)
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