Evidence that the information entropy estimating the nonlinear variability of human sinusal R-R intervals shows a circadian rhythm

The present study tries to detect whether the nonlinear variability of human electrocardiographic sinusal R-R intervals exhibits a circadian rhythm (CR). The sinusal R-R intervals were provided by the Holter ECG of 10 (5M, 5F, 23-30 years old) clinically healthy volunteers. Sinusal R-R intervals were analysed by measuring the information entropy (E) of their nonlinear variability each hour of the Holter recording. The hourly-qualified E measurements were analysed for CR via a three-component harmonic regression method. Hourly-qualified values of sinusal R-R intervals were seen to exhibit a variability which oscillates with a significant CR. Hourly-qualified values of heart rate (HR) were also seen to show a variability corresponding to a significant CR. Finally, hourly-qualified measurements of E were found to show a variability oscillating with a significant CR. Hourly-qualified measurements of E document that the nonlinear variability of the human sinusal R-R intervals is characterized by a time structure. The CR of E documents that such a time structure is the expression of a biological rhythm oscillating with a 24-h period. Accordingly, one can argue that the nyctohemeral variability of human sinusal R-R intervals results from a combination of chaos and periodicity. The chaotic component seems to be periodic, since it is structured in an oscillatory fashion over a 24-h scale.