Bioprosthetic pulmonary valve hemodynamics in tetralogy of fallot patients
Keywords: Mechanical Engineering.

Tetralogy of Fallot (ToF) is a congenital heart defect affecting 1 in 2500 live births annually in the United States and typically requires surgical repair at a few months of age. Subsequently, most ToF patients require pulmonary valve replacement (PVR) surgery to restore valve function in their early teen years. The bioprosthetic valves used during PVR fail early and unpredictably in many ToF patients and there is currently little understanding of what hemodynamic and anatomic factors may lead to early valve dysfunction. This work analyzes the impact of patient-specific features such as RVOT anatomy, cardiac output, valve placement, and valve orientation on the hemodynamics local to the bioprosthetic valve. We developed a physiological flow loop to study hemodynamics in 3D printed anatomical right ventricular outflow tract (RVOT) models with an implanted surgical valve. Full 3D, three-component, phase-averaged velocity fields were obtained over the cardiac cycle for all models using 4D flow MRI. The effects of RVOT anatomy were evaluated by comparing a healthy control model to a diseased model with a dilated main pulmonary artery (MPA) with two valve orientations: the native pulmonary valve orientation and an orientation rotated 180 degrees from native. Flow features, such as recirculation, vortex formation, and reversed flow regions, differed significantly with the RVOT anatomy and valve orientation. In particular, the dilated MPA geometry with the rotated orientation had increased secondary flow strength and recirculation, which demonstrated the compound effect of RVOT anatomy and valve orientation on the hemodynamics. We further used the diseased model to assess the impact of varying cardiac output, which corresponds to the clinical decision of valve sizing. We discovered multiple adverse flow and leaflet features produced by a cardiac output of 2 L/min, indicating that valve oversizing may produce a hemodynamic environment that predisposes the valve for failure. The effects of valve position were examined in two RVOT models with an acute angle between the right ventricle (RV) and MPA, one with the valve aligned with the RV and one with the valve aligned with the MPA. Despite substantial differences in the hemodynamic environment between the models, we found that in both cases, rotating the valve orientation alleviated the amount of jet impingement on the vessel walls, which could prevent adverse vessel remodeling. The 4D flow MRI data from the healthy control model with the native orientation were used to validate a design-based mechanistic valve model using the immersed boundary method for fluid-structure interaction simulations. This process demonstrated how the experiments conducted in this work provide high quality data for comparisons with computational results. The simulations successfully captured the jet angle and reversed flow regions present in the experimental data. Overall, this work demonstrated that RVOT anatomy, cardiac output, valve orientation, and valve position have a significant impact on the hemodynamics local to bioprosthetic valves in ToF patients. This emphasizes the need for a better clinical understanding of how the hemodynamic environment impacts valve function. Ultimately, this research could aid clinicians in determining the optimal pulmonary valve placement for long-term performance on a patient-specific basis, enabling personalized care for ToF patients