Abstract
Shannon entropy (ShE) is a recognised tool for the quantization of the temporal organization of time series. Transfer entropy (TE) provides insight into the dependence between coupled systems. Here, signals are analysed that were produced by the cardiovascular system when a healthy human underwent a provocation test using the head-up tilt (HUT) protocol. The information provided by ShE and TE is evaluated from two aspects: that of the algorithmic stability and that of the recognised physiology of the cardiovascular response to the HUT test. To address both of these aspects, two types of symbolization of three-element subsequent values of a signal are considered: one, well established in heart rate research, referring to the variability in a signal, and a novel one, revealing primarily the dynamical trends. The interpretation of ShE shows a strong dependence on the method that was used in signal pre-processing. In particular, results obtained from normalized signals turn out to be less conclusive than results obtained from non-normalized signals. Systematic investigations based on surrogate data tests are employed to discriminate between genuine properties—in particular inter-system coupling—and random, incidental fluctuations. These properties appear to determine the occurrence of a high percentage of zero values of TE, which strongly limits the reliability of the couplings measured. Nevertheless, supported by statistical corroboration, we identify distinct timings when: (i) evoking cardiac impact on the vascular system, and (ii) evoking vascular impact on the cardiac system, within both the principal sub-systems of the baroreflex loop.
Highlights
IntroductionSystems controlling the human body produce complex signals that are usually not stationary and have properties specific to the individual subject from whom the recordings are obtained
Systems controlling the human body produce complex signals that are usually not stationary and have properties specific to the individual subject from whom the recordings are obtained. Such features hamper the use of many methods aimed at revealing physiological phenomena underlying the dynamics of the system. This applies to the two major cardiovascular signals: the time interval between subsequent heartbeats and the change in systolic blood pressure values (SBP), which are commonly used in the evaluation of the functional state of cardiovascular control maintained by the autonomic nervous system [1,2]
We describe couplings in the cardiovascular system evoked by the head-up tilt (HUT) test, which are detected and quantified by Transfer entropy (TE)
Summary
Systems controlling the human body produce complex signals that are usually not stationary and have properties specific to the individual subject from whom the recordings are obtained Such features hamper the use of many methods aimed at revealing physiological phenomena underlying the dynamics of the system. This applies to the two major cardiovascular signals: the time interval between subsequent heartbeats (the so-called RR-intervals) and the change in systolic blood pressure values (SBP), which are commonly used in the evaluation of the functional state of cardiovascular control maintained by the autonomic nervous system [1,2]. The change in body position results in a transition in neural activity from vagal predominance at rest into strong activation of the sympathetic system while standing [4,5,6]
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