The Impact of Myocardial Bridging on Fractional Flow Reserve

Arterial stenosis is a problem of immediate significance, as cardiovascular disease is the number one leading cause of death worldwide. Fractional flow reserve (FFR) was proposed to evaluate the functional severity of coronary plaque-induced stenosis more accurately. FFR relies on invasive pressure measurements, while computational fluid dynamics (CFD) studies have been demonstrated to be useful tools to predict FFR less invasively. Myocardial bridging (MB) is an abnormality of the epicardial coronary artery where a segment of artery tunnels through the myocardium. MB presents as a ‘dynamic’ stenosis, in contrast to the ‘fixed’ stenosis caused by plaque: in systole, the artery is compressed due to the heart compression force, while in diastole the compression is non-significant. The objective of the project is to replicate the MB compression phenomenon via fluid-structure interaction (FSI) analysis and identify its impact on FFR. The relationship between ‘fixed’ stenosis and FFR was analyzed as a reference firstly, followed by the introduction of a pressure wire and surface roughness, to determine their impacts on CFD-derived FFR. Secondly, both commercial software and in-house code solver were used to perform FSI study and investigate the mechanism of bridging. With increasing severity of the ‘fixed’ stenosis – 0% to 70% diameter reduction, FFR decreased from 0.96 to 0.55. The presence of the pressure wire led to an overestimation of FFR by 3%-38% in various degrees of stenosis model, while the impact of the surface roughness on FFR was not apparent. Mild MB was studied via COMSOL simulations, while moderate and severe MB models were computed with the in-house code solver. The combination effect of the pressure wire and the upstream plaque in the mild MB was not additive, which was larger than the separate effect caused by each factor. With the increasing of the compression of MB – 44% to 60% diameter reduction, FFR decreased slightly, where the values were larger than 0.92. However, FFR dropped noticeably from 0.84 to 0.75 when the compression of MB increased from 72% to 87%. Furthermore, an expansion was observed in the severe MB model due to a greater inner pressure than outer compression pressure. In conclusion, the flow dynamics of MB were quite different compared to the plaqueinduced or ‘fixed’ stenosis. The use of traditional FFR to evaluate the functional severity of MB should be applied carefully and the cut-off value needs to be amended accordingly