A patient-specific study of type-B aortic dissection: evaluation of true-false lumen blood exchange

Abstract

Background: Aortic dissection is a severe pathological condition in which blood penetrates between layers of the aortic wall and creates a duplicate channel - the false lumen. This considerable change on the aortic morphology alters hemodynamic features dramatically and, in the case of rupture, induces markedly high rates of morbidity and mortality.

Methods: In this study, we establish a patient-specific computational model and simulate the pulsatile blood flow within the dissected aorta. The k-ω SST turbulence model is employed to represent the flow and finite volume method is applied for numerical solutions. Our emphasis is on flow exchange between true and false lumen during the cardiac cycle and on quantifying the flow across specific passages. Loading distributions including pressure and wall shear stress have also been investigated and results of direct simulations are compared with solutions employing appropriate turbulence models.

Results: Our results indicate that (i) high velocities occur at the periphery of the entries; (ii) for the case studied, approximately 40% of the blood flow passes the false lumen during a heartbeat cycle; (iii) higher pressures are found at the outer wall of the dissection, which may induce further dilation of the pseudo-lumen; (iv) highest wall shear stresses occur around the entries, perhaps indicating the vulnerability of this region to further splitting; and (v) laminar simulations with adequately fine mesh resolutions, especially refined near the walls, can capture similar flow patterns to the (coarser mesh) turbulent results, although the absolute magnitudes computed are in general smaller.

Conclusions: The patient-specific model of aortic dissection provides detailed flow information of blood transport within the true and false lumen and quantifies the loading distributions over the aorta and dissection walls. This contributes to evaluating potential thrombotic behavior in the false lumen and is pivotal in guiding endovascular intervention. Moreover, as a computational study, mesh requirements to successfully evaluate the hemodynamic parameters have been proposed.

Figure 1

The reconstructed surface of the aortic dissection. (a) is one axial slice of the CT scan; (b) is the reconstructed surface of the aortic dissection; (c) shows the positions of the entries along the flap (indicated by arrows).

Figure 2

Boundary settings of the model. (a) describes the boundary conditions at each inlet and outlets; (b) displays the cross-sectional velocity profile of the inlet at the ascending aorta. Data extracted from [27].

Figure 3

Grid and time step sensitivity study. (a) displays the studying point: the upper panel shows the cutting slice, and the lower one is the shape of that slice (viewing from the top) with an arrow pointing towards the studying point. (b) and (c) display the pressure and velocity magnitude variations at that point for grid independency study and for time step sensitivity study, respectively.

Figure 4

Flow pattern within the dissected aorta. The larger pictures on the left in (a) and (b) display the flow pattern at systolic peak and in the middle of diastole, respectively, by drawing lines that are tangential to the instantaneous velocity vectors. These lines are contoured by velocity magnitude. The smaller pictures on the right show the flow distribution by drawing the velocity magnitude contour at three slices, each of which crosses the entries that connect true and false lumen. Arrows in (a) indicates the positions and numbers of those entries.

Figure 5

Mass flow rate over a cardiac cycle (5~6 s). The upper panel of (a) displays the mass flow rate at the inlet; while, the lower panel represents the variations of the mass flow rate at the three entries and the outlet of the celiac trunk; (b) represents the positions of the entries, the inlet of the aorta, and the outlet of the celiac trunk.

Figure 6

Flow patterns at the entries and the outlet of the celiac trunk. (a) displays the velocity vectors at these entries and outlet of the celiac trunk at the systolic peak; (b) displays the results at mid-diastole.

Figure 7

Pressure drop curves over a cardiac cycle and the pressure distribution at systolic peak. (a) displays the time-various pressure drop curves, which measure the pressure difference from the inlet at the ascending aorta to the end of the thoracic aorta; (b) and (c) respectively displays the pressure distribution at systolic peak for the dissected aorta and for the true lumen only.

Figure 8

Pressure distributions along a slice that crosses primary entry and re-entry 1. The small image on the left bottom of each picture is the cardiac pulse wave, and the point on it indicates the time of the corresponding snapshots.

Figure 9

Wall shear stress distribution of the aortic dissection system. The left panel shows the results at systolic peak; and the right panel displays the results at mid-diastole.

Figure 10

Snapshots of turbulence kinetic energy distributions along a slice that crosses primary entry and re-entry 1. The small image on the left bottom of each picture shows the corresponding time of the snapshots in a cardiac cycle.

Figure 11

Wall shear stress distribution of the aortic dissection system: a steady state test with systolic peak inflow. (a) displays the result of the base mesh with a turbulence model; (b) is the result of the same mesh but based on a laminar model; while, (c) is the result based on laminar flow on a purely tetrahedral mesh without the prismatic boundary layer.