Author + information
- †Cardiovascular Division, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts
- ‡Mechanical and Industrial Engineering Department, Northeastern University, Boston, Massachusetts
- §Department of Interventional Cardiology, San Giovanni Hospital, CLI Foundation, Rome, Italy
- ↵∗Reprint requests and correspondence:
Dr. Peter H. Stone, Cardiovascular Division, Brigham & Women’s Hospital, 75 Francis Street, Boston, Massachusetts 02115.
Appreciation of the pathobiology of coronary atherosclerosis and the factors responsible for its development and progression have expanded dramatically in the past few years, along with a growing interest in the in vivo assessment of factors that may predict future plaque outcomes and coronary events. It has been known for years that atherosclerosis develops and progresses preferentially in areas of low endothelial shear stress (ESS), located at the inner aspects of vascular curves, at bifurcations, and downstream from focal obstructions, but only recently have methodologies been developed that enable in vivo calculation of local shear stress patterns. In vivo hemodynamic assessments on the basis of imaging studies hold promise for facilitating clinical decision making concerning the presence and natural history of atherosclerosis (1) and for flow simulations to predict pressure drop to model fractional flow reserve measurements in the evaluation of need for a revascularization procedure (2).
Accurate measurement of local ESS is dependent on accurate reconstruction of the 3-dimensional (3D) geometry of the arterial lumen and accurate measurement of the coronary blood flow. These variables are then entered into a computational fluid dynamics (CFD) program to solve the fundamental equations of fluid flow (Navier-Stokes equations) to determine the local blood flow patterns along the course of the artery. The detailed local shear stress pattern along the lumen wall is calculated as the product of the velocity gradient at the lumen wall and blood viscosity. Most methodologies using intravascular ultrasound or optical coherence tomography (OCT) now reconstruct the coronary artery by censoring the side branches and focus on detailed blood flow patterns through the main artery. Because the side branches divert some portion of coronary blood flow, ignoring the blood flow through the side branches will lead to inaccurate measurements of the blood flow through the main coronary artery: flow in the main artery after the branching would be overestimated, resulting in overestimation of local ESS as well as a net pressure drop after the branching. The technical difficulty of quantifying the blood flow diverted into the side branches, however, has been so anatomically and computationally complex that the side branch blood flow has generally been ignored or investigation of main coronary segments with large side branches is avoided.
In this issue of the Journal, Li et al. (3) now introduce a different methodology with a goal of more accurately measuring the local blood flow in the main coronary artery by including the more complex component of the blood flow that is diverted through the side branches in the CFD computations. The preliminary adoption of a co-registration software, now commercially available, allowed for spatial combination of angiographic and OCT images. The investigators then fused OCT images of the main coronary artery, to identify the detailed lumen dimensions and the ostium size of the side branches, with coronary angiography that provided a 3D reconstruction of both the main artery and the side branches. They used 2 angiographic projections with a difference in angulation >25° and included the whole course of all side branches of >1-mm diameter. The OCT was co-registered and fused with the 3D angiography using side branches as landmarks to correct for longitudinal and rotational mismatch. The investigators then created a 3D reconstruction of the OCT-derived lumen by matching the centerline of the reconstructed coronary angiogram with the centerline of the OCT images. They performed CFD analyses using 2 models which were compared: the single-conduit model, which ignored the side branches; and the tree model, which included the lumina of the side branches derived from the 3D angiography.
The investigators observed that the calculation of mean blood flow rate through the coronary artery was significantly lower using the tree model than the single-conduit model, as expected, and the estimated pressure at the outflow was significantly higher in the tree model compared to the single-conduit model. The ESS in the tree model was an average of 4.64 Pa lower than in the single-conduit model.
The investigators’ goal is important, and the magnitude of ESS difference between the 2 methods is not trivial. Improving the computational model to include the blood flow through the side branches so the blood flow remaining in the main artery can be accurately measured would improve the accuracy of the calculations of local blood flow in the main artery, and consequently the calculations of local ESS and the local pressures. It is not surprising that there is not ideal agreement between the computations derived from the 2 computational models; the critical issue is whether the hemodynamic values from the tree model method are indeed more accurate than the values from the single-conduit model, or simply different approximations with comparable overall accuracy considering all the uncertainties in measurements, natural dynamic variations in flow, geometry, and pressure levels. The fundamental issue to this point is whether the anatomy of the side branches is accurately identified using the investigators’ methodology. OCT images certainly identify the cross-sectional lumen margins in a very accurate manner, but it is not clear that the lumen diameter of the side branch ostium is also measured in an accurate manner because the side branch ostium diameter is in a different plane than the OCT image acquisition plane. It is also not clear if the lumen dimensions of the side branches beyond the ostium are measured. If so, they could be on the basis of the angiographic images, or alternatively on the OCT images of the side branch take-off, obtained from interrogating the main artery. But either solution may well be inaccurate, especially because the branches are so narrow. Similarly, the length of the side branch would be necessary to determine the side branch blood flow and it is not clear this can be accurately assessed by coronary angiography. It is also not stated if the angle of the side branch from the main artery is included in the computational program. Validation of the absolute accuracy of the tree model method will be very difficult, but necessary, because the side branch size and blood flow are very small and direct measurement of actual flow and pressure would be very problematic. It will be necessary as well to validate this approach with fractional flow reserve during maximal hyperemia. It is also important to be aware of the incremental time required to perform the complex tree model methodology.
The proposed method may have clinical implications. More accurate assessment would be invaluable to more reliably determine the local ESS patterns responsible for the development and progression of coronary plaques and their risk of disruption leading to a new clinical event. More reliable assessment of the functional severity of an obstruction (distal coronary pressure to aortic pressure ratio, Pd/Pa) would also be very helpful to improve the physician’s decision-making process, especially in the presence of uncertain/intermediate lesions. Furthermore, OCT findings are extremely helpful to optimize percutaneous coronary intervention results (4). If the method is accurate then it would also be ideal to have the method readily available to other investigators to facilitate more in vivo investigation of the pathobiologic and clinical implications of local blood flow and local pressure. The time requirements for the ambitious post-processing 3D reconstruction may limit its online clinical application in the catheterization laboratory.
The field of intravascular assessment of local ESS and blood pressure has expanded dramatically in recent years, and more accurate, and more readily available, methodologies will be invaluable to measure these variables and inform clinical decision-making in a reliable, practical, and timely manner. The clinical applications are very important, both for local hemodynamic assessment for investigation of the pathobiology of atherosclerosis and for local pressure assessment. The methods proposed by Li et al. (3) may be very important to enhance the accuracy of our coronary invasive measurements and improve the accuracy of our insights compared to the more routine method of single-conduit model, but we need to be sure the new calculations are indeed more accurate, not just different.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Prati has received consultant fees from St. Jude Medical. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.
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