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1 Department of Physics, University of Turku, 20014 Turku; 2 Department of Pediatrics, Turku University Central Hospital, 20520 Turku; 3 Department of Clinical Physiology, Kuopio University Hospital, 70211 Kuopio; 4 Department of Clinical Pharmacology, Tampere University, 33521 Tampere; and 5 Tampere University Hospital, 33014 Tampere, Finland
The aim of this study was to characterize how different nonlinear methods characterize heart rate and blood pressure dynamics in healthy subjects at rest. The randomized, placebo-controlled crossover study with intravenous terbutaline was designed to induce four different stationary states of cardiovascular regulation system. The R-R interval, systolic arterial blood pressure, and heart rate time series were analyzed with a set of methods including approximate entropy, sample entropy, Lempel-Ziv entropy, symbol dynamic entropy, cross-entropy, correlation dimension, fractal dimensions, and stationarity test. Results indicate that R-R interval and systolic arterial pressure subsystems are mutually connected but have different dynamic properties. In the drug-free state the subsystems share many common features. When the strength of the baroreflex feedback loop is modified with terbutaline, R-R interval and systolic blood pressure lose mutual synchrony and drift toward their inherent state of operation. In this state the R-R interval system is rather complex and irregular, but the blood pressure system is much simpler than in the drug-free state.
nonlinear dynamics; complexity; dimensionality; entropy
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