Transcatheter Heart Valve Downstream Fluid Dynamics in an Accelerated Evaluation Environment

Transcatheter aortic valve replacements (TAVRs) provide minimally invasive delivery of bioprosthetic heart valves (BHVs) for the treatment of aortic valve disease. While surgical BHVs show efficacy for 8-10 years, long-term TAVR durability remains unknown. Pre-clinical testing evaluates BHV durability in an ISO:5840 compliant accelerated wear tester (AWT), yet, the design and development of AWTs and their accuracy in predicting in vivo performance, is unclear. As a result of limited knowledge on AWT environment and BHV loading, durability assessment of candidate valves remains fundamentally empirical. For the first time, high-speed particle image velocimetry quantified an ISO:5840 compliant downstream AWT velocity field, Reynolds stresses, and turbulence intensity. TAVR enface imaging quantified the orifice area and estimated the flow rate. When valve area and flow rate were at their maximum during peak systole (1.49 cm² and 16.05 L/min, respectively), central jet velocity, Reynolds normal and shear stress, and turbulence intensity grew to 0.50 m/s, 265.1 Pa, 124.6 Pa, and 37.3%, respectively. During diastole, unique AWT recirculation produced retrograde flow and the directional changes created eddies. These novel AWT findings demonstrated a substantially reduced valve fully loaded period and pressure not matching in vivo or in vitro studies, despite the comparable fluid environment and TAVR motion.

[Analysis of flow in artificial stenosis models of mid-sized arteries using 3D PC-MRI]

Phase contrast MRI allows access to tri-directional encoded velocity information and therefore, measurement of flow in the human hemodynamic system. The aim of this work was to investigate whether this technology could be applied to support the grading of stenosis in mid-size arteries. Using a specially constructed flow phantom and a stenosis model with tube diameter of 5mm and 8mm and a stenosis of 50%, experiments at different flow rates (180-640 ml/min), slice thickness (1-4 mm), field strength (1.5 and 3.0 T), and multi-slice as well as 3D volume acquisition were performed. The observations were assessed visually and evaluated by signal-to-noise (SNR) ratios in regions before and after the stenosis. The obtained results show that examinations should be performed at high field (3.0 T) and at flow rates up to 500 ml/min without hampering the measurements by areas of signal loss. In comparison, no detectable differences in the flow patterns of the two acquisition schemes could be observed. However, the SNR was higher using the 3D volume acquisition and thick slices. In summary, 3D PC-MRI of mid-size vessels with stenosis is feasible for certain flow rates. The presented results could be seen as guidance for in vivo situations to assess if an examination of a patient is reasonable in terms of outcome.