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Dynamics of the blood pressure control system during head up tilt test

In physiological conditions, human blood pressure is controlled by the autonomous nervous system. The primary aim of this complex control system is to maintain the blood flow and pressure adjusted to the needs of the body. There are many factors that affect blood pressure, such as body position (hydrostatic pressure), variability of intrathoracic pressure, changing rate of venous return, flow autoregulation or baroreceptor hysteresis, to name just a few. To fulfill the aim of control a complex system of feedback loops is maintained. The main monitoring receptors include blood pressure receptors and chemoreceptors. The control is applied to the heart rate, heart contractility, vascular tonus, breathing rate and depth and, in the long time scale, to the body water balance.
The aim of the current work is to reveal the functional aspects of this control system that manifest themselves as simple linear correlations (including time delay) between various physiological observables. The measurements include correlation between respiratory signal, blood pressure and heart rate. The physiological data include the recordings of 12 patients who underwent the head-up tilt testing (HUTT) using the Westminster protocol. The diagnostic protocol is designed to show the dynamical characteristics of the blood pressure control system subject to various physiological stress factors (pressors) such as application of a postural stress, controlled breathing and hand grip test. Apart from the results concerning correlation, a basic structure of the control system is proposed and a minimalistic model of blood pressure control is built which mimics the correlation properties measured in the physiological data. Also various dynamical phenomena that occur in the rich measurement protocol are discussed.
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