Analysis of stationary periods of heart rate via symbolic dynamics

Abstract

In recent literature the reduced heart rate variability has been recognized as an adverse sign in several heart diseases. In the present paper a new measure of variability based on the research of stationary periods of heart rate in symbolic dynamics is introduced. Stationary periods are defined in terms of accelerations and decelerations of the instantaneous rate in strips of four consecutive qualified beats. Using this definition we compare the amount of stationary periods of the electrocardiographyc sinusal R-R intervals both in clinically healthy and in transplanted subjects to the ones in atrial fibrillation. The R-R sequences are provided by the Holter ECG of 12 recipient subjects transplanted from at least 5 years, of 18 healthy subjects and 18 subjects presenting only atrial fibrillation. Despite the comparability of the mean heart rate of the three groups, the amount of stationary periods was found to be significantly larger in transplanted than in healthy. The amount of stationary periods in fibrillating subjects turns out to have an intermediate value; this value can be explained assuming that in the symbolic dynamics the R-R intervals appears to be a sequence of independent random variables. The larger amount of stationary periods in transplanted subjects can suggest a reduced or absent autonomic nervous regulation.