Bicuspid aortic cusp fusion morphology alters aortic three-dimensional outflow patterns, wall shear stress, and expression of aortopathy

Abstract

Background: Aortic 3-dimensional blood flow was analyzed to investigate altered ascending aorta (AAo) hemodynamics in bicuspid aortic valve (BAV) patients and its association with differences in cusp fusion patterns (right-left, RL versus right-noncoronary, RN) and expression of aortopathy.

Methods and results: Four-dimensional flow MRI measured in vivo 3-dimensional blood flow in the aorta of 75 subjects: BAV patients with aortic dilatation stratified by leaflet fusion pattern (n=15 RL-BAV, mid AAo diameter=39.9±4.4 mm; n=15 RN-BAV, 39.6±7.2 mm); aorta size controls with tricuspid aortic valves (n=30, 41.0±4.4 mm); healthy volunteers (n=15, 24.9±3.0 mm). Aortopathy type (0-3), systolic flow angle, flow displacement, and regional wall shear stress were determined for all subjects. Eccentric outflow jet patterns in BAV patients resulted in elevated regional wall shear stress (P<0.0125) at the right-anterior walls for RL-BAV and right-posterior walls for RN-BAV in comparison with aorta size controls. Dilatation of the aortic root only (type 1) or involving the entire AAo and arch (type 3) was found in the majority of RN-BAV patients (87%) but was mostly absent for RL-BAV patients (87% type 2). Differences in aortopathy type between RL-BAV and RN-BAV patients were associated with altered flow displacement in the proximal and mid AAo for type 1 (42%-81% decrease versus type 2) and distal AAo for type 3 (33%-39% increase versus type 2).

Conclusions: The presence and type of BAV fusion was associated with changes in regional wall shear stress distribution, systolic flow eccentricity, and expression of BAV aortopathy. Hemodynamic markers suggest a physiological mechanism by which the valve morphology phenotype can influence phenotypes of BAV aortopathy.

Keywords: aortic diseases; bicuspid aortic valve; hemodynamics; magnetic resonance imaging.

Figure 1

A: Quantification of ascending aorta (AAo) hemodynamics in three analysis planes (S1, S2, S3) to compute the systolic flow angle, θ, between the net systolic flow, Q, and unit normal vector, n. B: The location of the top 15% of velocities at peak systole was mapped onto an aortic lumen chart. A symmetric flow profile is reflected by a central location of the maximum velocities (red shaded area) with the highest velocity (V_Max) at the center of the vessel. Flow profile asymmetry results in an off-center location of the top 15% of velocities. The flow displacement, d, calculated as the distance (in mm) from the vessel centroid to the velocity-weighted centroid. C: Example images illustrating the three different aortopathy types in our patient cohort. PA: pulmonary artery, DAo: descending aorta, CCA: common carotid artery, LSA: left subclavian artery, A: anterior, P: posterior, L: left, R: right, BT: brachiocephalic trunk.

Figure 2

Top row: 3D streamline visualization of peak systolic blood flow in patients with BAV (C,D) compared to an aorta size matched control subject (B) and a healthy volunteer (A). Note the presence of distinctly different 3D outflow flow jet patterns (black dashed arrows) in the ascending aorta (AAo) for patients B-C. Bottom row: 3D flow patterns in the left ventricular outflow tract (LVOT) and ascending aorta (AAo) distal to the aortic valve. Note the different systolic AV outflow flow jet patterns (red color indicating high velocities > 1m/s) and wall impingement zones which correspond to variable exertion of high wall shear forces between different valve groups (C,D) and aorta size matched controls (B) and healthy volunteers (A).

Figure 3

Segmental systolic wall shear stress (WSS) measurements at the sinotubular junction (analysis plane S1), the mid ascending aorta (S2), and the distal ascending aorta (S3). The individual data points represent mean systolic WSS for patients with bicuspid aortic valves (C,D), aorta size matched controls (B), and healthy volunteers (A) across eight anatomical locations (“A” anterior, “LA” left anterior, “L” left, “LP” left posterior, “P” posterior, “RP” right posterior, “R” right, “RA” right anterior). Error bars represent the standard deviation of inter-individual WSS variation. * indicates statistically significant differences for RL-BAV and RN-BAV cohorts compared to aorta size matched controls (p < 0.0125 after Bonferroni correction).

Figure 4

Flow profile asymmetry maps schematically illustrating flow eccentricity using the locations of the upper 15% of systolic velocities for all participants within each cohort at the the sinotubular junction (analysis plane S1). RN-BAV patients showed outflow asymmetry towards the right posterior wall compared to RL-BAV, whose flow profile was directed towards the right-wall. (“A” anterior, “L” left, “P” posterior, “R” right). Note that each subject’s profile map was normalized to their sinotubular junction diameter.

Figure 5

Distribution of flow angle and outflow asymmetry (flow displacement d) for all BAV patients as a function of aortopathy phenotype. The numbers represent mean flow angle or displacement for each aortopathy type 1, 2 or 3. Note that statistical comparisons were not performed due to the small number of subjects with type 3 and type 1.