Importance of Non-Newtonian Modeling of Blood Flow for Calcified Aortic Valves: Relevance to Sub-Clinical Thrombosis

Abstract

Objective: The Newtonian assumption for blood has often been applied for aortic valve studies where high shear rates can be seen on the ventricularis side. However, in the presence of calcifications this assumption is suspect due to large flow disturbances, especially on the fibrosa side. Regions of low wall shear stress (WSS) have been known to lead to thrombogenesis in the cardiovascular system, and thus accurate blood flow modeling is required to predict these locations. This study looked at the applicability of a Newtonian and non-Newtonian, Carreau, model for blood on a healthy and calcified aortic valve geometry. Methods: A reconstructed human aortic heart valve from an 82-year old patient at the early diastolic phase was commercially acquired (Valve - 012 - Heart Print catalog, Materialize Inc., Plymouth MI). Inlet velocity of 1 cm/s was prescribed and an 80-mmHg pressure condition for the outlet boundary condition. Fluent (ANSYS Inc, Canonsburg, PA) was used for solving the unsteady Navier-Stokes equations. Combinations of a healthy or calcified valve and a Newtonian or a non-Newtonian model were then simulated. Results: WSS showed little difference between Newtonian and non-Newtonian models in the healthy valve (Figure 1A and Figure 1B). However, there was an underestimation of WSS in the calcified case using the Newtonian model compared to non-Newtonian (Figure 1C and Figure 1D) in certain core leaflet regions. Conclusions: The Newtonian assumption can be used for modeling blood flow of a healthy aortic valve, but a non-Newtonian model must be used in cases of valve disease, such as severely calcified aortic valves. These irregularities affect determining WSS where a few dynes/cm2 mark the difference between regions being susceptible to potential thrombus or not. Figure 1. Results on the fibrosa side of the aortic valve A) WSS using a Newtonian model on a healthy valve. B) WSS using a non-Newtonian, Carreau, model on a healthy valve. C) WSS using a Newtonian model on calcified valve. D) WSS using a non-Newtonian, Carreau, model on a calcified valve