Variations of atrioventricular block.

Atrioventricular (AV) block comprises a spectrum of cardiac conduction delays with varying clinical presentations. It is commonly encountered in both hospital as well as ambulatory settings, and recognition of the type of AV conduction delay is essential for appropriate subsequent management. The electrocardiogram is a key tool for identification of patients with AV conduction delays. Contrasting management strategies should be employed for differing levels of conduction block.

Digital particle image velocimetry investigation of the pulsating flow around a simplified 2-D model of a bileaflet heart valve.

BACKGROUND AND AIM OF STUDY: Strong interactions are believed to exist between the pulsating valvular flow and the valve leaflet motions. Hinge position, indicated by d/W (d = distance between the two axes of the hinge pivots; W = width of the testing section in the middle plane), plays a critical role in MHV performance. An optimized hinge position for a bileaflet heart valve can be identified as a design criterion for better valve performance. METHODS: A two-dimensional (2-D) digital particle image velocimetry (DPIV) system was used to map the transient flow field of a simplified 2-D model of a bileaflet heart valve with a hydraulic diameter enlarged three-fold under pressure waveforms which was expanded based on Womersley number and Euler number considerations. Six different hinge positions were investigated. RESULTS: At extreme hinge positions (d/W <0.2 or d/W >0.3), large-scale and long-duration stagnation of flow was found in the central orifice, and instability and highly disturbed flow was noted in plots of velocity vectors. CONCLUSION: The transient flow pattern in the vicinity of the valve was greatly affected by the hinge position of moving leaflets. An optimum d/W in the range 0.2-0.3 yielded good velocity field and opening and closing behaviors.

Computational fluid dynamics study of a protruded-hinge bileaflet mechanical heart valve.

BACKGROUND AND AIM OF THE STUDY: Following clinical experience with the Medtronic Parallel bileaflet mechanical heart valve, considerable interest has been shown in investigating fluid mechanics inside the hinge socket. Most of these studies involved hinges that are recessed into the valve housing, such as the St. Jude Medical (SJM), CarboMedics, Sorin and On-X bileaflet mechanical heart valves. The aim of this study was to investigate the flow fields of a protruded hinge under steady flow conditions, with the occluder in its fully open position. Computational fluid dynamics (CFD) simulation using the Fluent 4.4.7 commercial solver was applied in this investigation. This protruded hinge mechanism for pivoting the occluder is an in-house design from the Cardiovascular Dynamics Laboratory, Nanyang Technological University. METHODS: The Fluent 4.4.7 code was run on a Silicon Graphic Inc. computer (4-CPUx185 MHz) in the CFD simulation. A body-fitted coordinates (BFC) grid was generated to cover the entire valvular flow domain, including the interior of the hinge and leaflet. Clearance between the leaflet and pivot housing was 50-70 microm. In the vicinity of the protruded hinge, mesh cells were small compared with hinge dimensions. A power law distribution of grid points was applied to optimize the number of cells used to cluster the entire flow field. The overall computational flow domain of the valve channel, including the floating leaflet and immersed hinge, was approximately 170,000 cells in total. Inside the hinge socket, approximately 10,000 cells were generated. A comparative model with recessed hinge that resembled the SJM valve hinge design was modeled. Due to geometric difficulties, an unstructured grid scheme was applied. Great attention was focused within the hinge pocket, in particular to the clearance between the hinge pivot and leaflet. A total of 2 million cells was generated for the whole computational flow domain. RESULTS: Under steady flow conditions, with the leaflet fixed in an open position, the protruded hinge design yielded a pair of small vortices that formed behind the stoppers. A low-magnitude velocity was observed inside the hinge clearance. Vortices developed behind the protruded stopper. Migrating flow was noted beneath the leaflet clearance as a result of pressure difference across the leaflet. For the recessed hinge design, reverse flow dominated the inside of the hinge socket, and developed into a pair of vortices at high Reynolds number. CONCLUSION: The protruded hinge mechanism was designed to expose the overall hinge region to the mainstream flow for a positive washing effect. Flow in this protruded hinge design is, in general, found to be three-dimensional. Initial results under steady flow conditions showed low laminar and turbulent shear stress, while the hinge clearance was well washed.

CFD investigation of steady flow in bi-leaflet heart valve hinge.

This paper focuses on local flow patterns inside the hinge socket of a bi-leaflet mechanical heart valve (MHV), where experimental measurements are difficult due to the extremely small flow region of about 40 microm. The overall objective of this study is to simulate the steady flow in this confined micro channel within the hinge region of a partially protruded ball hinge concept. A CFD simulation of flow through a bi-leaflet heart valve hinge was carried out. Steady flow with the valve leaflet in the fully open position during the valve systole phase was studied using FLUENT 4.4.7 running on Silicon Graphics. Body Fitted Coordinates (BFC) grid distribution was applied in the overall flow domain and great care was taken at the mesh distribution within the hinge local area. The flow study focused on local flow patterns inside the hinge socket of the valve where experimental measurements in the actual size valve are not practical. CFD results show evidence of flow in local area of hinge and no evidence of stagnation. Flow migrates across the clearance, and small vortices are formed after the hinge stoppers. The results indicate that flow in the hinge region is complex and critical for the valve to function effectively.