Platelet activation and free emboli formation in flow past mechanical heart valves - numerical and in vivo studies

Numerical studies, in vitro and in vivo measurements were conducted, aimed at quantifying free emboli formation and procoagulant properties of platelets induced by flow past mechanical heart valves (MHV). 2D and 3D pulsatile turbulent flow simulations were conducted past a St. Jude Medical MHV in the aortic position, to study the effects of MHV implantation on valve thrombogenicity. Shear stress histories of platelets along turbulent trajectories exposed the platelets to elevated shear stresses around the leaflets, leading them to entrapment within the shed vortices formed in the wake of the valve. In vitro platelet studies were conducted in a model of left ventricular assist device (LVAD), using an innovative platelet activity state (PAS) assay. The platelet activation significantly increased as a function of the recirculation time past the valve, and as compared to controls. Transcranial Doppler (TCD) measurements were conducted in the carotid artery of sheep with implanted MHV, showing a marked increase in the number of HITS (high intensity transient signals) signifying the passage of free emboli generated by the valve. The HITS were analyzed to distinguish between gaseous and thrombi emboli. Finally, PAS measurements were conducted with sheep platelets, showing marked increase of platelet activation after valve implantation.