Misinterpretation of Stenosis Severity in the Presence of Serial Coronary Stenoses

Diagnosis of the functional severity of an epicardial coronary stenosis using parameters like Fractional Flow Reserve, FFR (ratio of distal to proximal pressure of a stenotic region), might be affected in the presence of an additional downstream stenosis. In order to assess this effect, we have performed an in-vitro experiment which is used to validate a computational study. Three combinations of serial stenoses were tested: 80%-64%, 80%-80% and 80%-90% area stenosis (AS). The physiological mean hyperemic flow (flow at maximal arterial dilatation) values were obtained using an in-vitro experimental set-up. These flow rates were used as steady flow inputs by time-averaging the spatially averaged flow pulse over two cardiac cycles for the computational study. FFR values were calculated at hyperemic flow using both the experimental and numerical pressure data. As the downstream severity increased from 64% AS to 80% AS, hyperemic coronary flow decreased from 136.4 ml/min to 126.4 ml/min. Flow decreased further to 90.7 ml/min with a downstream severity of 90% AS. FFR of the intermediate stenosis increased from 0.76 to 0.79 and further to 0.88 as the downstream stenosis increased from 64% to 80% with a final severity of 90% AS. Similarly, numerically obtained FFR values increased to 0.83, 0.80 and 0.92 for the corresponding cases indicating an error within 7% of the experimental values. These results indicate that the presence of a downstream stenosis might lead to a clinical misinterpretation of the upstream stenosis severity. INTRODUCTION The severity of an epicardial coronary stenosis is commonly assessed using the pressure-derived Fractional Flow Reserve, FFR. Multiple lesions within the same coronary vessel represent a complex unstable flow characterized by fluid acceleration and wall shear-layer separation. Individual FFR of stenoses in series may fail to account for the fluid dynamic interactions between the two and hence inaccurately quantify disease severity leading to possible misdiagnosis. In this study, we sought to quantify this effect using a computational model and validate the results with experimental values. We assessed the effect of various downstream stenoses on the physiologically relevant case of an intermediate (80% AS) upstream stenosis. METHODS Experimental Set-up: Bench-top tests for the multiple stenoses models were carried out using a previously designed setup [1]. Using the pressure-drop (∆p) and flow curves for each serial stenoses combination, the physiological hyperemic flow rates and the corresponding mean distal coronary pressures were determined from the coronary flow reserve (CFR: ratio of flow at maximal vasodilation to flow at rest) distal perfusion pressure (CFR-Prh) curve [2] using the procedure outlined in [1]. Functional and hemodynamic parameters for the cases under study were then evaluated at hyperemic flow. Figure 1: Test Section of multiple stenoses Proceedings of the ASME 2013 Conference on Frontiers in Medical Devices: Applications of Computer Modeling and Simulation FMD2013 September 11-13, 2013, Washington, DC, USA