Comparative finite-element model analysis of ascending aortic flow in bicuspid and tricuspid aortic valve

Purpose: In bicuspid aortic valve (BAV) disease the role of genetic and haemodynamic factors influencing ascending aortic pathology is controversial. In order to test the effect of BAV geometry on ascending aortic flow, a Finite Element model analysis was undertaken. Methods: A surface model of aortic root and ascending aorta was obtained from magnetic resonance images of patients with BAV and tricuspid aortic valve using the segmentation facilities of the image processing code Vascular Modeling Toolkit (developed at the Mario Negri Institute). Simplified and however reliable analytical models of bicuspid (antero-posterior, AP, and laterolateral, LL, commissures) and tricuspid orifices were mathematically defined. Final models were turned into a volumetric mesh of linear tetrahedra for computational fluid dynamics simulations. Numerical simulations were performed with the Finite Element code LifeV (developed by the research centers MOX-Milan, INRIA-Paris, EPFL-Lausanne). Physiological inflow boundary conditions at inlet were imposed by means of a mathematically sound and well tested method based on the introduction of a Lagrange multiplier in the problem formulation. The flow velocity fields were assessed for four levels: aortic annulus, sinus of Valsalva, sinotubular junction, ascending aorta. Results: Comparison of finite-element model analysis of bicuspid and tricuspid aortic valve shows different blood flow velocity pattern. The flow in the AP bicuspid shows an asymmetrical distribution of velocity field towards the convexity of the mid-ascending aorta returning symmetrical in distal ascending aorta. In the LL bicuspid a less pronounced asymmetry is noted at the sinus of Valsalva level. On the contrary, tricuspid flow is symmetrical in each aortic segment. Moreover, aortic flow in bicuspid model gains a maximum velocity at the systole of 5.5 m/s for AP and of 5.7 m/s for LL, while in the tricuspid maximum velocity is 2.6 m/s. Conclusions: Comparison between models shows an asymmetrical and higher flow velocity in the bicuspid models, in particular in the AP configuration. The asymmetry is more pronounced at the aortic level known to be more exposed to aneurysm formation in bicuspid patients. This supports the hypothesis that haemodynamic factors may contribute to ascending aortic pathology in this subset of patients.