Non-Newtonian Patient-specific Numerical Study of Left Atrial Hemodynamic*

Ischemic strokes due to cardiac thromboembolism are a leading cause of mortality in patients with atrial fibrillation (AF). During AF, disturbed atrial beating creates stagnant regions where clots can form, typically in the left atrial appendage (LAA). Blood has non-Newtonian rheology due to red blood cell aggregate (rouleaux) formation. Aggregation requires residence times of 1–10 s at shear rates <100 s^-1. These conditions are met in the LAA and, consonantly, rouleaux are observed clinically as spontaneous echocardiographic contrast. Yet, previous CFD studies consider Newtonian rheology. We explore how non-Newtonian rheology affects LA flow in 6 patient-specific anatomies from 4D-CT imaging (3 in sinus rhythm, 3 in AF). We implement a semi-implicit Carreau-Yasuda model incorporating hematocrit (Hct), shear, and residence time to account for rouleaux formation. We consider a wide range of Hct values (37-55) and compare vs. Newtonian simulations on the same patients. We found that non-Newtonian effects can significantly affect LAA blood viscosity even at low Hct, altering hemodynamics LAA stasis predictions by CFD.