Computational Fluid Dynamics Characterization of Blood Flow in Central Aorta to Pulmonary Artery Connections: Importance of Shunt Angulation as a Determinant of Shear Stress-Induced Thrombosis

Abstract

The central aortic shunt, consisting of a Gore-Tex (polytetrafluoroethylene) tube (graft) connecting the ascending aorta to the pulmonary artery, is a palliative operation for neonates with cyanotic congenital heart disease. These tubes often have an extended length, and therefore must be angulated to complete the connection to the posterior pulmonary arteries. Thrombosis of the graft is not uncommon and can be life-threatening. We have shown that a viscous fluid (such as blood) traversing a curve or bend in a small-caliber vessel or conduit can give rise to marked increases in wall shear stress, which is the major mechanical factor responsible for vascular thrombosis. Thus, the objective of this study was to use computational fluid dynamics to investigate whether wall shear stress (and shear rate) generated in angulated central aorta-to-pulmonary artery connections, in vivo, can be of magnitude and distribution to initiate platelet activation/aggregation, ultimately leading to thrombus formation. Anatomical features required to construct the computer-simulated blood flow pathways were verified from angiograms of central aortic shunts in patients. For the modeled central aortic shunts, we found wall shear stresses of (80-200 N/m(2)), with shear rates of (16,000-40,000/s), at sites of even modest curvature, to be high enough to cause platelet-mediated shunt thrombosis. The corresponding energy losses for the fluid transitions through the aorta-to-pulmonary connections constituted (70 %) of the incoming flow's mechanical energy. The associated velocity fields within these shunts exhibited vortices, eddies, and flow stagnation/recirculation, which are thrombogenic in nature and conducive to energy dissipation. Angulation-induced, shear stress-mediated shunt thrombosis is insensitive to aspirin therapy alone. Thus, for patients with central aortic shunts of longer length and with angulation, aspirin alone will provide insufficient protection against clotting. These patients are at risk for shunt thrombosis and significant morbidity and mortality, unless their anticoagulation regimen includes additional antiplatelet medications.

Catheter-Directed Thrombolysis for Occluded Central (Ascending Aorta-to-Pulmonary Artery) Shunts: Importance of Shear Stress-Induced, Platelet-Mediated Thrombosis

“…The simulated central shunts were constructed using a 3D modeling software computer package (Autodesk Inventor, San Rafael, CA, USA). Once the centerline of a shunt was determined from the angiogram, a constant-diameter, circular cross-section was swept along its course to form the tubular graft ( Figure 2, case 2; Figure 3, case 4) [29].…”

“…The flow calculations were carried out using the CFD computer program Fluent 14.5 (Ansys, Inc., Lebanon, NH, USA), which solves the 3-D Navier-Stokes and continuity equations using the finite volume method [29,33] Computer simulation of an ascending aorta to main pulmonary artery shunt (blue) superimposed on the angiogram for case 2 (see Table 1). The shunt is 3. at the luminal wall.…”

“…We recently used computational fluid dynamics (CFD) to demonstrate that a viscous fluid (e.g., blood) traversing an "angulated" vessel (or conduit) can produce marked increases in fluid shear stress near the luminal wall (i.e., wall shear stress) where platelets tend to reside [29]. Moreover, it is known that high fluid shear stress can activate platelets, causing them to aggregate and/or bind to coagulation proteins in the blood to form microparticles that serve as precursors for thrombus formation [30].…”

“…Our goal was to identify a relationship between shunt hemodynamics and thrombosis. An understanding of shear stress-mediated thrombosis is clinically important as this process does not involve thromboxane A 2 production and hence is insensitive to aspirin (ASA) [29,31].…”

Numerical and Physical Models of Distal Anastomosis in Coronary Artery Bypass Grafting

“…However, only a few of these studies have in vitro physical models to verify their numerical models [15][16][17][18][19][20][21][22]. The physical model of CABG is very valuable to assess the flow fields parameters practically.…”