Bileaflet mechanical heart valves (BMHVs) are prone to thrombus formation in the hinge region due to high shear stress combined with stagnation regions. This thesis research addresses the hypothesis that models that isolate and mimic BMHV hinge geometries can be used to quantitatively characterize procoagulant potential using a novel in vitro blood flow system. Furthermore, these results can be correlated with digital particle image velocimetry (DPIV) measurements detailing flow fields for the same models. The significant findings were that: 1) recalcification of recirculating citrated blood markedly increases the magnitude of thrombus forming reactions and the sensitivity for their detection; 2) platelet activation, and the presence of adequate platelet numbers are essential for the activation of coagulation under conditions of high shear; and 3) thrombin formation can be inhibited by blocking the platelet receptors that facilitate platelet aggregation. The DPIV studies give some insight into why different channel geometries resulted in varying propensities for coagulation. The channel geometries with abrupt changes in diameter induced significantly higher levels of TAT and also formed jets that were subject to increased entrainment of the stagnant fluid in the chamber. This entrainment enables more mixing of the shear-activated platelets with the surrounding flow, which can propagate the coagulation cascade, thus increasing the chance for thrombus formation. The influence of abrupt changes in diameter was also evident in the BMHV human blood studies. The MP valve, which has a tortuous hinge pathway, induced significantly more TAT formation than the SJM Standard valve with a smoother hinge channel. Thus, BMHV hinge geometry should be as smooth and free of diameter changes as possible to eliminate stagnation regions that enable activated platelets to congregate and propagate the coagulation cascade. Leakage gap width also had a significant effect not only on procoagulant potential but also on platelet activation. Both the low and high leaker prototype valves had significantly higher levels of platelet activation compared to the SJM Standard valve, but only the low leaker valve demonstrated a higher propensity for coagulation. Thus, to minimize both platelet activation and thromboemboli formation, an optimal gap width should be maintained for BMHVs.

Objective: To document maternal and foetal morbidity and mortality in anticoagulated, pregnant patients with mechanical heart valves until 42 days postpartum. Methods: In a tertiary single-centre, prospective cohort, 178 consecutive patients at the cardiac-obstetric clinic were screened for warfarin use between 1 July 2010 and 31 December 2015. Of 33 pregnancies identified, 29 were included. Patients received intravenous unfractionated heparin from six to 12 weeks’ gestation and peripartum, and warfarin from 12 to 36 weeks. Maternal outcomes including death, major haemorrhage and thrombosis, and foetal outcomes were documented. Results: There were two maternal deaths, five returns to theatre post-delivery, eight patients transfused, six major haemorrhages, one case of infective endocarditis and three ischaemic strokes. Ten pregnancies had poor foetal outcomes (six miscarriages, three terminations, one early neonatal death). Twenty patients required more than 30 days’ hospitalisation, and 15 required three or more admissions. HIV positivity was associated with surgical delivery (p = 0.0017). Conclusions: Complication rates were high despite centralized care.

The performance of the heart after a valve replacement operation will greatly depend on the flow character downstream the mitral valve thus a better understanding of the flow character is essential. Most in vitro studies of the flow downstream of a MHV have been conducted with the valve in the aortic position. Researchers reported detailed measurements most of which were obtained by Laser Doppler Velocimetry (LDV) in rigid models of the aorta. Digital Particle Image Velocimetry (DPIV) has also been utilized to reveal intricate patterns of interacting shed vortices downstream of the aortic valve. The orientation of the valves may considerably affect the flow development and slight difference may produce significant differences in the ventricular flow fields. Two orientations, respectively anatomical and anti-anatomical, of the St. Jude Medical (SJM) bileaflet valve are presented and compared with the SJM Biocor porcine valve, which served to more closely represent the natural valve. In this effort, we employ a powerful tool to monitor the velocity field in a flexible, transparent LV and study the evolution of large eddies and turbulence through a complete cardiovascular cycle. Both time average and instantaneous results of velocity, vorticity, and turbulent kinetic energy distributions are presented. The presence and location of vortical structures were deduced as well as the level of coherence of these structures. The presence of three distinct flow patterns were identified, by the location of vortical structures and level of coherence, for the three configurations corresponding to significant differences in the turbulence level distribution inside the LV. / Master of Science

Mechanical heart valve (MHV) flows are characterized by high shear stress, regions of recirculation, and high levels of turbulent fluctuations. It is well known that these flow conditions are hostile to blood constituents, which could lead to thromboembolism. In the ongoing effort to reduce long-term complications and morbidity, it is imperative that we better understand the flow characteristics of the natural valve as well as that of the mechanical valve. In this study, we overcome many of the limitations imposed by other measurement techniques by employing a powerful, high-speed Time-Resolved Digital Particle Image Velocimetry (TRDPIV) system to map the flow field. We compare the flows downstream from a St. Jude Medical bileaflet MHV, a porcine mitral valve (MV), and a combination of both valves to simulate the technique of chordal preservation. Instantaneous velocity fields and vorticity maps are presented, which provide detailed information about the development of the flow. Time-averaged velocity, vorticity, and turbulent kinetic energy measurements are also discussed. Asynchronous leaflet behavior was observed in all cases involving the mechanical valve. Extensive vortex formation and propagation are present distal to the MHV, which leads to high levels of jet dispersion. The porcine mitral jet exhibits lateral oscillatory behavior, but it does not disperse like the MHV. In the MHV/porcine combination system, the native tissue limits vortex propagation and jet dispersion. The results presented provide insight on the hemodynamic characteristics of natural and MHVs, reveal the detrimental character of asynchronous leaflet opening, document the mechanism of vortex formation and interaction distal to the valve, and illustrate the importance of chordal preservation. These results may improve MHV replacement clinical practice and/or motivate and aid the design of MHVs that better mimic natural mitral flow patterns. / Master of Science

The thesis deals with CFD solution of flow through mechanical heart valves. The opening part includes a research on the negative impacts of implantation of artificial heart valves and the common approaches used in their modeling. This is followed by a detailed analysis of the dynamic mesh functionality and CFD solution is performed using the open-source library FOAM-extend. Part of the thesis focuses on evaluation of the geometry modifications from the perspective of forming the instabilities of the current and the course of force effects on the valve.

Among the four valves of the heart, the aortic valve (AV) is frequently affected by disease. When progressive dilatation of the valve produces a leak when the valve should close (regurgitation), repair may be possible. AV repair is a desirable option because, contrary to AV replace-ment using a prosthesis, it does not require life-long anticoagulation treatment, and retains the original tissues that naturally combat structural degradation. All the AV repair procedures developed by cardiac surgeons require a good stabilization of the ventriculo-aortic junction (VAJ) diameter, through annuloplasty or reimplantation, for long-term success. In the present work, a preliminary design for a new type of annuloplasty ring is proposed that surgeons could tailor to the each valve’s shape and suture inside the VAJ. The design consists in wrapping a commonly available surgical biomaterial into a ring of controlled radial flexibility. For sizing and material selection, several models of increasing complexity were created to account for the anisotropic, hyperelastic nature of all the materials involved. First, an analytical model was programmed in MATLAB to assess the radial flexibility of annuloplasty rings formed with different biomaterials and select those that could match the physiological VAJ radial flexibility between systolic and diastolic pressures. The same program was also used to reproduce the experimental radial and longitudinal stretches of the human VAJ from 0 to 140 mmHg pressures. The analytical models were used to calibrate the parameters of independent finite element (FE) models of the VAJ and ring. Finally, the FE approach was extended to simulate the ring after suturing inside the VAJ, to determine the radial flexibility of the assembly under pulsatile pressure. Supple Peri-Guard® bo-vine pericardium patches used in transverse orientation emerged as the best currently available material option for the proposed ring, although a material providing more physiological radial flexibility would be desirable.