In-vitro performance of a single-chambered total artificial heart in a Fontan circulation

Journal of Artificial Organs - An in-vitro study was conducted to investigate the general feasibility of using only one pumping chamber of the SynCardia total artificial heart (TAH) as a...     

Jamming of prosthetic heart valves by suture trapping: experimental findings

The vulnerability of the Medtronic-Hall, Bj?rk-Shiley Monostrut, Duromedics, and St. Jude Medical valves to occluder immobilization by sutures was determined under static and pulsatile flow conditions. Variables were cardiac output, cross-sectional diameter of suture, type of suture (braided versus monofilament) and position of the offending suture along the circumference of the valve ring. Under static conditions, pressures, ranging from 40 to 340 mmHg and 10 to 170 mmHg, were required to decompress obstructed Medtronic-Hall and Bj?rk-Shiley Monostrut valves, respectively. As a result of different design characteristics and different occluder/orifice clearances the Medtronic-Hall valve showed its maximum opening pressure in case of interference with sutures at the axis of symmetry in both minor and major orifices, whereas for the Bj?rk-Shiley Monostrut valve this was reached in the minor orifice. Under pulsatile flow conditions, in case of interference with Prolene 2-0 suture, the Duromedics valve showed irregularly delayed opening and an opening pressure difference of 50 mmHg at a cardiac output of 8 L/min, whereas leaflet motion and pressure difference in the St. Jude Medical valve were undisturbed under similar conditions. The necessary pressure difference for opening the Medtronic Hall valve reached 44mmHg at a cardiac output of 8 L/min. High and low risk of extrinsic leaflet obstruction in the Duromedics and St. Jude Medical valves, respectively, is related to the design of the hinge mechanisms and the wedge angle of their leaflets (2 degrees versus 25 degrees). Precautionary principles in implantation of prosthetic heart valves are stressed to prevent the potentially lethal complication of occluder immobilization.(ABSTRACT TRUNCATED AT 250 WORDS)     

Combined Computational and Experimental Approach to Improve the Assessment of Mitral Regurgitation by Echocardiography

Mitral regurgitation (MR) is one of the most frequent valvular heart diseases. To assess MR severity, color Doppler imaging (CDI) is the clinical standard. However, inadequate reliability, poor reproducibility and heavy user-dependence are known limitations. A novel approach combining computational and experimental methods is currently under development aiming to improve the quantification. A flow chamber for a circulatory flow loop was developed. Three different orifices were used to mimic variations of MR. The flow field was recorded simultaneously by a 2D Doppler ultrasound transducer and Particle Image Velocimetry (PIV). Computational Fluid Dynamics (CFD) simulations were conducted using the same geometry and boundary conditions. The resulting computed velocity field was used to simulate synthetic Doppler signals. Comparison between PIV and CFD shows a high level of agreement. The simulated CDI exhibits the same characteristics as the recorded color Doppler images. The feasibility of the proposed combination of experimental and computational methods for the investigation of MR is shown and the numerical methods are successfully validated against the experiments. Furthermore, it is discussed how the approach can be used in the long run as a platform to improve the assessment of MR quantification.     

Design Modifications and Computational Fluid Dynamic Analysis of an Outflow Cannula for Cardiopulmonary Bypass

Cardiopulmonary bypass is a well-established technique during open heart surgeries. However, neurological complications due to insufficient cerebral oxygen supply occur and the severe consequences must not be neglected. Recent computational fluid dynamics (CFD) studies showed that during axillary cannulation the cerebral perfusion is strongly affected by the distance between the cannula tip and the vertebral artery branch. In this study we use two modifications of the cannula design to analyze the flow characteristics by means of CFD and experimental validation with particle image velocimetry (PIV). One approach applies a spin to the blood stream with a helical surface in the cannula cross section. Another approach uses radial bores in a constricted cannula tip to split the outflow jet. The additional helicity improves the perfusion of the cerebral vessels and suppresses the blood suction in the right vertebral artery observed with a standard cannula. The cannula with a helix throughout the entire length changes the blood flow from ?124 to 32 mL/min in comparison with an unmodified design and has the lowest prediction of blood damage. Separating the blood stream does not deliver satisfying results. The PIV measurements validate the simulations and correspond with the velocity distribution as well as vortex locations.     

Impact of Hyperthermal Rotary Blood Pump Surfaces on Blood Clotting Behavior: An Approach

The influence of heat dissipating systems, such as rotary blood pumps, was investigated. Titanium cylinders as rotary blood pump housing dummies were immersed in porcine blood and constantly tempered at specific temperatures (37–60°C) over a defined period of time. The porcine blood was anticoagulated either by low heparin dosage or citrate. At frequent intervals, samples were taken for blood analysis and the determination of the plasmatic coagulation cascade. Blood parameters do not alter at surface temperatures below 50°C. Hyperthermia-induced hemolysis could be confirmed. The plasmatic coagulation cascade is terminated at surface temperatures exceeding 55°C. The adhesion of blood constituents on surfaces is temperature and time dependent, and structural changes of adhesions and blood itself were detected.     

PulmoStent: <Emphasis Type="Italic">In Vitro</Emphasis> to <Emphasis Type="Italic">In Vivo</Emphasis> Evaluation of a Tissue Engineered Endobronchial Stent

Currently, there is no optimal treatment available for end stage tumour patients with airway stenosis. The PulmoStent concept aims on overcoming current hurdles in airway stenting by combining a nitinol stent with a nutrient-permeable membrane, which prevents tumour ingrowth. Respiratory epithelial cells can be seeded onto the cover to restore mucociliary clearance. In this study, a novel hand-braided dog bone stent was developed, covered with a polycarbonate urethane nonwoven and mechanically tested. Design and manufacturing of stent and cover were improved in an iterative process according to predefined requirements for permeability and mechanical properties and finally tested in a proof of concept animal study in sheep for up to 24 weeks. In each animal two stents were implanted, one of which was cell-seeded by endoscopic spraying in situ. We demonstrated the suitability of this membrane for our concept by glucose transport testing and in vitro culture of respiratory epithelial cells. In the animal study, no migration occurred in any of the twelve stents. There was only mild granulation tissue formation and tissue reaction; no severe mucus plugging was observed. Thus, the PulmoStent concept might be a step forward for palliative treatment of airway stenosis with a biohybrid stent device.     

Design Method of a Foldable Ventricular Assist Device for Minimally Invasive Implantation

To date, ventricular assist devices (VADs) have become accepted as a therapeutic solution for end‐stage heart failure patients when a donor heart is not available. Newer generation VADs allow for a significant reduction in size and an improvement in reliability. However, the invasive implantation still limits this technology to critically ill patients. Recently, expandable/deployable devices have been investigated as a potential solution for minimally invasive insertion. Such a device can be inserted percutaneously via peripheral vessels in a collapsed form and operated in an expanded form at the desired location. A common structure of such foldable pumps comprises a memory alloy skeleton covered by flexible polyurethane material. The material properties allow elastic deformation to achieve the folded position and withstand the hydrodynamic forces during operation; however, determining the optimal geometry for such a structure is a complex challenge. The numerical finite element method (FEM) is widely used and provides accurate structural analysis, but computation time is considerably high during the initial design stage where various geometries need to be examined. This article details a simplified two‐dimensional analytical method to estimate the mechanical stress and deformation of memory alloy skeletons. The method was applied in design examples including two popular types of blade skeletons of a foldable VAD. Furthermore, three force distributions were simulated to evaluate the strength of the structures under different loading conditions experienced during pump operation. The results were verified with FEM simulations. The proposed two‐dimensional method gives a close stress and deformation estimation compared with three‐dimensional FEM simulations. The results confirm the feasibility of such a simplified analytical approach to reveal priorities for structural optimization before time‐consuming FEM simulations, providing an effective tool in the initial structural design stage of foldable minimally invasive VADs.     

Real‐Time Visualization of Platelet Interaction With Micro Structured Surfaces

Improving the hemocompatibility of artificial implants by micro structuring their surfaces has shown promising results, but the mechanisms which lead to this improvement are not yet understood. Therefore, we built a test setup for real‐time visualization of platelet interaction with a plain and two micro structured surfaces. The micro structures, defined by the distance of the plain surface area between the structures, were chosen to be 3 and 30 μm, representing a positive and a negative effect on the hemocompatibility. The main part of the test setup was a flow chamber containing films of low density polyethylene (LDPE) with the differently structured surfaces. For different wall shear stresses, no considerable differences were observed in the platelet‐surface interaction for all surface types. Whereas, major differences in flow behavior were observed when comparing the surfaces to each other. The platelets “rolled” along the smooth surface, being in constant contact with the surface material. Although the platelets “rolled” over the surface with small structures as well, they were only in contact with the tips of the structure and therefore had less surface contact with the foreign material. The increased distance and height of the structures of the last surface led to a trapping of platelets between the structures. This resulted in a longer contact time with the foreign material as well as a larger contact area, which both increase the risk of platelet activation, adhesion, and finally clotting. Our results showed the mechanisms which lead to these effects and thus revealed why micro structuring of surfaces impacts the hemocompatibility. Furthermore, we established a test setup which can be used for future investigations on the platelet‐structure interactions.     

Estimation of Filling and Afterload Conditions by Pump Intrinsic Parameters in a Pulsatile Total Artificial Heart

A physiological control algorithm is being developed to ensure an optimal physiological interaction between the ReinHeart total artificial heart (TAH) and the circulatory system. A key factor for that is the long‐term, accurate determination of the hemodynamic state of the cardiovascular system. This study presents a method to determine estimation models for predicting hemodynamic parameters (pump chamber filling and afterload) from both left and right cardiovascular circulations. The estimation models are based on linear regression models that correlate filling and afterload values with pump intrinsic parameters derived from measured values of motor current and piston position. Predictions for filling lie in average within 5% from actual values, predictions for systemic afterload (AoP_mean, AoP_sys) and mean pulmonary afterload (PAP_mean) lie in average within 9% from actual values. Predictions for systolic pulmonary afterload (PAP_sys) present an average deviation of 14%. The estimation models show satisfactory prediction and confidence intervals and are thus suitable to estimate hemodynamic parameters. This method and derived estimation models are a valuable alternative to implanted sensors and are an essential step for the development of a physiological control algorithm for a fully implantable TAH.