Hydrodynamic and coagulation characteristics of a re-engineered mechanical heart valve in an ovine model
Jordaan, Christiaan Johannes
MetadataShow full item record
Introduction A valve with haemodynamic properties mimicking a natural heart valve and having the durability that will exceed the life expectancy of the recipient patient without requiring lifelong anti-coagulation, would be considered by most as the Holy Grail of prosthetic heart valve design. Although mechanical heart valves have a superior durability compared to biological valves, the thrombogenicity of mechanical heart valves necessitates lifelong anti-coagulation therapy, balancing bleeding risk with thrombosis and emboli. The explantation of two UCT valves that had remained in pristine condition decades after implantation and the reviewing of historical data after implantation in children without anti-coagulation in the 1960s, led to the idea of re-engineering a poppet valve to possibly be used without anti-coagulation. This idea was revisited during the development of the Glycar Valve. Objective During the planning phase of this study three main objectives were considered: 1. To understand the principles of heart valve functioning with the resulting influence on thrombosis; to apply these principles while designing a mechanical heart valve that will be easy and affordable to produce and that can safely be used without anti-coagulation. This included an in-depth literature review of heart valve design, fluid-structure interaction within the valve as well as valvular thrombosis. 2. To use computational fluid dynamics followed by pulse duplication testing in the in vitro evaluation of a prototype mechanical heart valve (the Glycar valve) and to compare the findings to the commercially available Carbomedics bi-leaflet valve. 3. To study the Glycar valve in vivo in the ovine model, evaluating overall function and specifically, to assess the thrombogenicity of the valve without the use of anti-coagulant or anti-platelet therapy, in comparison to the Carbomedics bi-leaflet valve. Methods An extensive review of mechanical valve design, coagulation and available mechanical valve research and development methodology was performed . Thereafter several modifications were made to the original UCT valve in order to create the Glycar valve. The flow across the valve during systole was streamlined, reducing areas of flow acceleration across the valve and the poppet surface, reducing the viscous shear rate. The diastolic flow profile was changed and areas of stagnation were eliminated around the valve leaflets. Regurgitation jets were eliminated, which negated the problems associated with the ‘washing jets’ seen in bi-leaflet valves. A two-part CFD analysis (dynamic and non-dynamic) was performed on the Glycar valve to understand the flow patterns generated within the Glycar valve and across the valve components. Pulse duplication analysis was performed on the Glycar valve and the valvular performance during five simulated physiological conditions were compared to four different commercially available heart valves in the aortic position. In the in vivo study the bio-interaction of the Glycar valve was tested in the ovine model in the absence of anti-coagulation in comparison with a bi-leaflet valve. Two groups of five Glycar valves and one Carbomedics bi-leaflet valve were implanted in the pulmonary valve position in juvenile sheep. Group 1 was followed for six months and Group 2 for twelve months after implantation. Results The Glycar valve was centred on a CAD design, which was based on flow-dynamic principles. CFD confirmed acceptable flow-patterns - both during systole and diastole - with a greater than expected EOA (1.39 cm2) and a low transvalvular gradient (1.5 mmHg). Systolic flow patterns showed a low incidence of flow separation and recirculation, minimal areas of stasis and turbulence, reduced vortex formation and a surface shear stress that does not exceed the platelet activation threshold. The Glycar valve had comparative hydrodynamic properties and characteristics compared to the Carbomedics bi-leaflet valve in a simulated pulsatile environment. Pulse duplication comparison of the Glycar valve to commercially available mechanical and biological valves demonstrated similar pressure drops, Qrms, energy losses and EOA’s. However, at higher cardiac outputs (>8 L/min) the poppet valve developed significant regurgitation. The current Glycar valve design in the pulmonary position in the ovine model proved to be reliable and thrombo-resistant in the absence of anti-coagulation in the short term as well as in the long term follow-up. None of the valves, control valves included, showed any macroscopic or microscopic thrombi. Biochemistry and hematology did not demonstrate hemolysis, activation of coagulation or platelet activity. Histology showed no thrombi on the sewing cuff, housing, poppet or struts. None of the sheep had embolic events and no pulmonary embolic events or sequelae could be identified. Cardiac echocardiography confirmed normal prosthetic function in all valves except those with infective endocarditis. Conclusion The Glycar valve proved to be a suitable alternative to the traditional mechanical bi-leaflet valve design. The improvements made to the Glycar valve showed acceptable results in both the CFD analysis and pulse duplication testing, exceeding the minimum standards required by ISO 5840:2015 certification. In the ovine model the Glycar valve demonstrated acceptable haemodynamics and no trombo-embolic events were recorded in the absence of anti-coagulation or anti-platelet drugs. Future recommendations This prosthesis should be tested in a more aggressive coagulation model at systemic pressures or in the more thrombogenic tricuspid valve position. Improvement in the poppet design is required to address the regurgitation experienced at flows exceeding 8 L/min. Fatique testing of the final valve design.