Physiological aspects on intracardiac blood flow
Abstract: Magnetic resonance imaging enables detailed in vivo study of complex flow through 3D, time-resolved phase contrast imaging (4D flow). 4D flow may reveal previously unknown aspects of the physiology of intracardiac blood flow, and possibly elucidate pathophysiological processes that have yet to manifest themselves outside the heart.The purpose of this thesis is to examine the physiological role of intracardiac flow phenomena, in order to better understand the normal function of the heart and provide a backdrop against which future studies into disease processes can be compared. The thesis consists of four studies with the following aims:I) to quantify atrial blood kinetic energy (KE),II) to evaluate the relationship between left ventricular vortex ring formation and myocardial motion,III) to quantify the hemodynamic force exchange between blood and myocardium in both ventricles, andIV) to compare hemodynamic forces in dyssynchronous, failing left ventricles with normal findings.Study I revealed significant differences between left and right atrial KE. Systolic KE was determined to a large extent by atrial volume and the systolic velocity of the atrioventricular plane, which differs between the left and right sides of the heart. Diastolic KE increased more in the left atrium, likely reflecting suction-initiated filling driven by left ventricular recoil. Rotational blood flow KE decay was slower than non-rotational KE decay, indicating a potential enhancement of cardiac function through energy preserving macroscopic flow structures.Study II demonstrated a strong spatiotemporal coupling between the vortex ring boundary and the endocardial boundary, suggesting that the normal development of the left ventricle is guided by the shear layer generated by the vortex ring. The coupling may act as a dynamic optimization mechanism to facilitate fluid transport at varying levels of cardiac output. In contrast, failing ventricles show no connection between the vortex ring and endocardium, suggesting that the adaptive coupling of the myocardium to the shear layer is disrupted.Studies III and IV demonstrated that biventricular hemodynamic forces are similar between healthy hearts regardless of size, and reflect fundamental aspects of flow redirection; this homogenous appearance is significantly altered in heart failure. The findings elucidate a previously unknown ’slingshot’ force that appears to result from contraction of the septum and RV free wall. In left ventricular dyssynchrony, several different hemodynamic force patterns were seen, which could indicate different degrees of detrimental effects on myocardial function and hence predispose to varying treatment response.Together, the findings in the thesis have advanced the knowledge on intracardiac flow at rest. To further validate their physiological importance, it is necessary to evaluate intracardiac flow during physical exercise.
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