Patient-specific bicuspid valve dynamics: overview of methods and challenges

Abstract

About 1-2 % of the babies are born with bicuspid aortic valves instead of the normal aortic valve with three leaflets. A significant portion of the patients with the congenital bicuspid valve morphology suffer from aortic valve stenosis and/or ascending aortic dilatation and dissection thus requiring surgical intervention when they are young adults. Patients with bicuspid aortic valves (BAVs) have also been found to develop valvular stenosis earlier than those with the normal aortic valve. This paper overviews current knowledge of BAVs, where several studies have suggested that the mechanical stresses induced on the valve leaflets and the abnormal flow development in the ascending aorta may be an important factor in the diseases of the valve and the aortic root. The long-term goals of the studies being performed in our laboratory are aimed towards potential stratification of bicuspid valve patients who may be at risk for developing these pathologies based on analyzing the hemodynamic environment of these valves using fluid-structure interaction (FSI) modeling. Patient-specific geometry of the normal tri-cuspid and bicuspid valves are reconstructed from real-time 3D ultrasound images and the dynamic analyses performed in order to determine the potential effects of mechanical stresses on the valve leaflet and aortic root pathology. This paper describes the details of the computational tools and discusses challenges with patient-specific modeling.

Figure 1

(a) Idealized bicuspid aortic valve geometries employed to study impact of geometry on stresses in valve leaflets (Jermihov et al., 2011) and the alterations in hemodynamics in the ascending aorta (Burken, 2012; Vigmostad et al., 2012). Type I depicts one oversized leaflet and a fusion of two undersized leaflets. Type II is two symmetric leaflets. Type III is a left-right fusion, with a second leaflet similar in shape to a normal TAV leaflets. Type IV employs the same geometry as type III, but with a raphe. (b) The maximum in plane principal stresses from our dynamic simulation during the fully closed position of the valve (c) The corresponding orifice opening of the various valve types in the fully open position. Reprinted from Cardiovascular Engineering and Technology, 2(1): 48–56, 2011 with kind permission from Springer Science and Business Media.

Figure 2

The idealized aortic arch geometry included three sinuses and the ascending aorta. The valve geometry in the fully open position was placed within the aorta, and flow was computed at the fully open position. The model includes the simulated ascending aorta ending at the proximal aortic arch and an extension at the outlet has been added to minimize end effects.

Figure 3

Cross-sections of velocity (vectors show in-plane velocity while contours show axial velocity) taken distal to the valve at the sinutubular junction (STJ). (a) Tricuspid aortic valve (b) BAV Type 1 (c) BAV Type 2 and (d) Type 2 rotated 90 degrees, (e) BAV Type 3. Note the strong jet on the inner wall with type 1 BAV, and an eccentric jet in type 3 geometry. Type 2 (symmetric valve leaflets) results in strong helical flow, with two sets of counter-rotating vortices (Burken, 2012; Vigmostad et al., 2012).

Figure 4

The most common BAV, a fusion of the right and left leaflets, results in a strong eccentric jet, which impacts the outer wall of the ascending aorta. Contours of dynamic pressure are shown (Burken, 2012; Vigmostad et al., 2012).

Figure 5

Examples of reconstructed (a) TAV and (b) BAV employed in the finite element analysis. In addition to the marked asymmetry of the leaflets as was shown in Figure 5, the complex geometry of the sinuses can be seen in this figure.

Figure 6

Typical results showing contours of maximum in-plane principal stresses, as computed from a patient-specific TAV model in the fully closed configuration. Note that the peak stress occurs in the non-coronary leaflet near the commissure, as has been reported in previous studies (Christie and Barratt-Boyes, 1991).

Figure 7

Preliminary results for a full FSI simulation of a patient-specific bicuspid aortic valve. (a) The original, closed configuration of the type II BAV plus aortic root, where L1 is adjacent to the non- and right-coronary sinuses, while L2 is attached to the left-coronary sinus. (b) The fully open position of the BAV, with contours of displacement. Note the relatively small EOA. (c) Axial flow contours (d) Axial flow at a slice within the aortic sinus (e) Flow along the center slice of the geometry, between the two open leaflets. Note the skewing of the jet, toward the left coronary sinus region.