Abstract
IntroductionSleep is a complex phenomenon characterized by important modifications throughout life and by changes of autonomic cardiovascular control. Aging is associated with a reduction of the overall heart rate variability (HRV) and a decrease of complexity of autonomic cardiac regulation. The aim of our study was to evaluate the HRV complexity using two entropy-derived measures, Shannon Entropy (SE) and Corrected Conditional Entropy (CCE), during sleep in young and older subjects.MethodsA polysomnographic study was performed in 12 healthy young (21.1±0.8 years) and 12 healthy older subjects (64.9±1.9 years). After the sleep scoring, heart period time series were divided into wake (W), Stage 1–2 (S1-2), Stage 3–4 (S3-4) and REM. Two complexity indexes were assessed: SE(3) measuring the complexity of a distribution of 3-beat patterns (SE(3) is higher when all the patterns are identically distributed and it is lower when some patterns are more likely) and CCEmin measuring the minimum amount of information that cannot be derived from the knowledge of previous values.ResultsAcross the different sleep stages, young subjects had similar RR interval, total variance, SE(3) and CCEmin. In the older group, SE(3) and CCEmin were reduced during REM sleep compared to S1-2, S3-4 and W. Compared to young subjects, during W and sleep the older subjects showed a lower RR interval and reduced total variance as well as a significant reduction of SE(3) and CCEmin. This decrease of entropy measures was more evident during REM sleep.ConclusionOur study indicates that aging is characterized by a reduction of entropy indices of cardiovascular variability during wake/sleep cycle, more evident during REM sleep. We conclude that during aging REM sleep is associated with a simplification of cardiac control mechanisms that could lead to an impaired ability of the cardiovascular system to react to cardiovascular adverse events.
Highlights
Sleep is a complex phenomenon characterized by important modifications throughout life and by changes of autonomic cardiovascular control
In order to quantify different aspects of the cardiovascular control mainly related to the organization of different subsystems, in the last years an increasing interest has been focused on the evaluation of the complexity of cardiovascular system, which cannot be adequately assessed by means of the classical linear tools
Shannon Entropy (SE)(3) and CCEmin were significantly reduced during rapid eye movement sleep (REM) sleep compared to Stage 1–2 (S1-2), Stage 3–4 (S3-4) and W (2.9960.32 vs 3.460.39, 3.6260.38 and 3.3260.32 respectively for SE (3); 0.8360.17 vs 1.0560.19, 1.1560.16 and 0.9860.17 respectively for Corrected Conditional Entropy (CCE) min) (Fig. 1 and 2)
Summary
Sleep is a complex phenomenon characterized by important modifications throughout life and by changes of autonomic cardiovascular control. In order to quantify different aspects of the cardiovascular control mainly related to the organization of different subsystems, in the last years an increasing interest has been focused on the evaluation of the complexity of cardiovascular system, which cannot be adequately assessed by means of the classical linear tools For such fine-grained analysis, the use of entropy-derived nonlinear indices, such as sample entropy, approximate entropy, corrected conditional entropy and Shannon entropy [7,8,9,10,11,12], has been proposed. There might be an overall deficiency in integration of control mechanisms, which in turn limits the capabilities of the system to adapt and react to perturbations [22,23,24], increasing the risk for major cardiovascular adverse events
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