A powerful alternative means to study the hemodynamics of bileaflet mechanical heart valves is the computational fluid dynamics method. It is well recognized that computational fluid dynamics allows reliable physiological blood flow simulation and measurements of flow parameters. To date, in almost all of the modeling studies on the hemodynamics of bileaflet mechanical heart valves, a velocity (mass flow)-based boundary condition and an axisymmetric geometry for the aortic root have been assigned, which, to some extent, are erroneous. Also, there have been contradictory reports of the profile of velocity in downstream of leaflets, that is, in some studies, it is suggested that the maximum blood velocity occurs in the lateral orifice, and in some other studies, it is postulated that the maximum velocities in the main and lateral orifices are identical. The reported values for the peak velocities range from 1 to 3 m/s, which highly depend on the model assumptions. The objective of this study is to demonstrate the importance of the exact anatomical model of the aortic root and the realistic boundary conditions in the hemodynamics of the bileaflet mechanical heart valves. The model considered in this study is based on the St Jude Medical valve in a novel modeling platform. Through a more realistic geometrical model for the aortic root and the St Jude Medical valve, we have developed a new set of boundary conditions in order to be used for the assessment of the hemodynamics of aortic bileaflet mechanical heart valves. The results of this study are significant for the design improvement of conventional bileaflet mechanical heart valves and for the design of the next generation of prosthetic valves.
To date, to the best of the authors' knowledge, in almost all of the studies performed around the hemodynamics of bileaflet mechanical heart valves, a heart rate of 70-72 beats/min has been considered. In fact, the heart rate of ~72 beats/min does not represent the entire normal physiological conditions under which the aortic or prosthetic valves function. The heart rates of 120 or 50 beats/min may lead to hemodynamic complications, such as plaque formation and/or thromboembolism in patients. In this study, the hemodynamic performance of the bileaflet mechanical heart valves in a wide range of normal and physiological heart rates, that is, 60-150 beats/min, was studied in the opening phase. The model considered in this study was a St. Jude Medical bileaflet mechanical heart valve with the inner diameter of 27 mm in the aortic position. The hemodynamics of the native valve and the St. Jude Medical valve were studied in a variety of heart rates in the opening phase and the results were carefully compared. The results indicate that peak values of the velocity profile downstream of the valve increase as heart rate increases, as well as the location of the maximum velocity changes with heart rate in the St. Jude Medical valve model. Also, the maximum values of shear stress and wall shear stresses downstream of the valve are proportional to heart rate in both models. Interestingly, the maximum shear stress and wall shear stress values in both models are in the same range when heart rate is <90 beats/min; however, these values significantly increase in the St. Jude Medical valve model when heart rate is >90 beats/min (up to ~40% growth compared to that of the native valve). The findings of this study may be of importance in the hemodynamic performance of bileaflet mechanical heart valves. They may also play an important role in design improvement of conventional prosthetic heart valves and the design of the next generation of prosthetic valves, such as percutaneous valves.
St. Jude Medical (SJM) bileaflet mechanical valves were approved by the Food and Drug Administration in 1977. The SJM valve design consists of two semicircular leaflets which pivot on hinges. Compared to other mechanical heart valve prostheses such as ball and cage and tilting disk prosthetic valves, it provides good central flow, the leaflets open completely, and the pressure drop across the valve is trivial. However, non-physiological hemodynamics around these valves may lead to red blood cells lysis and therombigenic complications. Also, the regurgitation-flow inSJM valves is almost twice that of the native valves in the aortic position. In this study, we suggest a new design for the stent (housing) of SJM valves in which 15% ovality is applied to the stent whereas its perimeter remains constant. In a pilot study, the hemodynamic performance of the proposed design is analyzed in the closing phaseand compared to that of conventional SJM models. Results show that while the elliptic SJM model offers a shorter closing phase (9.7% shorter), the regurgitation flow remains almost unchanged. In other words, even though the dynamic response of the valve is improved, the regurgitation flow is not decreased. Thus, a more efficient effective orifice area (EOA) is shown to be provided by the proposed model. The preliminary calculations presented in this study justify an improved hemodynamics of elliptic SJM valves compared to conventional models; the proposed design shows promise and merits further development.
Despite successful implantation of St. Jude Medical bileaflet mechanical heart valves, red blood cell lysis and thrombogenic complications associated with these types of valves are yet to be addressed. In our previous study, we proposed an elliptic housing where 10% ovality was applied to the housing of St. Jude Medical valves. Our preliminary results suggested that the overall hemodynamic performance of St. Jude Medical valves improved in both the closing and opening phases. In this study, we evaluated the hemodynamics around the leaflets in the opening phase using a more sophisticated computational platform, computational fluid dynamics. Results suggested both lower shear stress and wall shear stress values and an overall improved hemodynamic performance in the proposed design. This improvement is characterized by lower values of shear stress and wall shear stress in the regions downstream of the leaflets, lower pressure drop across the valve and smaller recirculation zones in the sinuses areas. The proposed design may open a new chapter in the concept of design and hemodynamic improvement of the next generation of mechanical heart valves.
Cerebral aneurysm (CA) is an abnormal dilation of the cerebral arterial wall, which accounts for more than half a million deaths each year worldwide. Flow diverters (FDs) represent one method recently developed in treating CAs. Typically, they do not need coiling (releasing micro-coils within the aneurysm) and act purely to prevent substantial blood inflow into the aneurysm. In collaboration with Evasc Neurovascular Enterprises (Vancouver, Canada), whose area of expertise is developing novel CA therapies, we have developed a novel FD for the treatment of bifurcation CAs with fusiform-like properties involving the confluence of the main and daughter branches. To the best of authors’ knowledge, currently there is no device for an effective treatment of such complex aneurysms. Through a stepwise design modification process and utilizing CFD modeling, we have developed a new design for the Evasc FD (eCLIPs) with improved hemodynamics, which is characterized by more than 30% reduction in the aneurysm inflow and wall shear stress (WSS) for the new implant design over eCLIPs for this subset of aneurysms. The new device design, modified-design eCLIPs (MD-eCLIPs), can represent the only device available for the treatment of such CAs with fusiform pathology.
In this study, the hemodynamic performance of the conventional St. Jude Medical (SJM) valve and our proposed design known as the oval SJM valve are studied and compared. These studies are based on a wide range of physiological heart rates, i.e., 70–130[Formula: see text]bpm, in the opening phase. We designed and developed a precise computational platform to assess the hemodynamics of bileaflet mechanical heart valves for laminar and turbulent regimes. Also, as one of the fundamental changes applied to the conventional SJM vales, the housing is considered oval similar to oval shape of annulus. Results clearly indicate hemodynamic improvements in the proposed design over the SJM valve. The improvements are characterized by lower shear stress and wall shear stress distributions around the valve and leaflets, and lower valve pressure drop compared to that of the conventional SJM model. The proposed design shows potential and merits additional development.
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