Relationship Between Clinical Diagnostic Measures and Wall Shear Stress in Stenotic Coronary Arteries

Atherosclerosis is an important pathology affecting the coronary artery system which leads to the creation of stenoses that cause the narrowing or to the obstruction of the coronary arteries. Since coronary obstruction deprives the myocardium of oxygen-rich blood provoking a possible total heart failure, and because of the diffusion of this kind of pathology, in literature different methods of assessment of coronary stenosis have been proposed. The evaluation of the degree of stenosis is essential in order to find the proper treatment for the patient, therefore the physiological assessment of coronary artery disease has become progressively important in both clinical and research application. Until today, coronary angiography constitutes the basis of coronary artery disease diagnosis, but it can’t completely characterise the clinical significance of stenosis. For this reason, measurements of coronary pressure and flow have been developed to provide information complementary to the anatomic characterization obtained by angiographic examination. In this thesis, after a first overview of the main physiological measurements like Coronary Flow Reserve (CFR), Fractional Flow Reserve (FFR), Virtual Fractional Flow Reserve (vFFR) and Instantaneous Wave-free Ratio (iFR), much attention has been focused on the relationship between the assessment obtained by FFR and iFR measurements and the wall shear stress (WSS) distribution. In order to analyse the relationship between iFR and FFR measures and WSS, 80 cases of right coronary artery (RCA) affected by the presence of stenosis were examined. The angiographic images have been properly selected and segmented to reconstruct a 3D model of the coronary arteries, successively a volume mesh has been generated and, finally, the CFD simulations have been implemented using the frame count method to determine the inlet velocity as boundary condition. From the results which have been obtained, it emerges that in correspondence of low values of iFR and FFR, WSS values in the stenosis region are high indicating the presence of a disturbed flow induced by the narrowing of the vessel. Since WSS distribution can provide a general overview of the hemodynamic of the vessel, the analysis has been focused on the stenotic region and both on the first proximal and distal regions. WSS values result higher in the stenotic vessel with a peak in correspondence of the stenosis and lower before and after the stenosis. Therefore, a more uniform WSS distribution can be indicative of a healthy vessel; on the contrary, an uneven WSS distribution confirms inferior hemodynamic performance and, consequently, a possible critical hemodynamic situation. In conclusion, WSS can be a useful measure which could provide supplementary details improving the sensitivity of the physiological assessment given by iFR and FFR measurements.