Blood Damage
  Luke Herbertson, Jason Nanna, Karen Chau

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Mechanical heart valves

Mechanical heart valves can be very effective replacements for diseased or failing native valves.  These biomedical devices can be implanted directly into patients or as components within larger cardiovascular assist devices.  Tilting disc valves (Bjork-Shiley, MedHall, etc.) and bileaflet valves (St. Jude Medical, Carbomedics, etc.) are the most studied valves in our labs.  Very slight design modifications can have significant effects on the flow structures generated by the closure of these valves.  Thus, much of the research conducted in the Artificial Heart Lab involves exploring means by which to minimize hemolysis and valve failure.  The strength of vortices and regurgitant jets produced by valve closure are visualized using high speed videography and quantified through optical flow techniques such as PIV and LDV.
 
Mechanical heart valve induced cavitation is also investigated in our lab.  Cavitation formed due to the rapid pressure drop on the upstream side of the leaflet following impact with the housing can be detected acoustically with a hydrophone.  Various signal isolation techniques, some of which implement wavelets, are being explored in order to completely extract the cavitation signal from the statistically transient mechanical closure signal.  It is important to quantify the amount of cavitation produced by valves under physiologic loading conditions, because the collapse of the bubbles can be very damaging locally to red blood cells.  Cavitation has also been linked to stable bubble formation.  Once these stable bubbles have been introduced into the bloodstream, they can make their way into the cranial circulation, which increases the risk of stroke in mechanical heart valve patients.


           


Polymeric Valves
Varun Reddy

While mechanical heart valves (MHVs) remain the most widely implanted prosthetic valves, polymeric valves have inherent advantages. These include their unobstructed orifice and lower leaflet closing velocities which minimize their hemolytic, platelet activating and cavitation potential, relative to MHVs. Currently, most polymeric valves are used as part of left ventricular assist devices and total artificial hearts. There is interest to implant these valves clinically, and characterizing the fluid dynamics of polymeric heart valves will help achieve this goal. An in vitro laser Doppler velocimetry study was conducted on a 25 mm polymeric trileaflet heart valve in the aortic position. The valve is housed in an acrylic conduit with aortic sinuses that allows for optical access upstream and downstream of the valve. A sodium/iodide/glycerin fluid that matches the kinematic viscosity of blood and the index of refraction of the model is used as a blood analog.



  Artificial Heart and Ventricular Assist Devices

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Pediatric ventricular assist device (PVAD)
Breigh Roszelle, Isabella Valenti, Mike Perone, Andrew Hankinson



While heart transplantation has become a viable option for children, there is continual concern for organ availability, especially in infants under 1 year of age. As a result, there has been an increased movement to aid pediatric patients during the “bridge-to-transplant” period. Ventricular assist devices (VADs) have been successfully used in adult patients for several years; however, there have been limited options for pediatrics due to their smaller blood volume, the possible growth of the patient during the support period, and the size range of pediatric patients (from infant to adolescent). Therefore, in 2004 the National Institutes of Health initiated The National Heart, Lung and Blood Institute (NHLBI) Pediatric Circulatory Support Program. As part of this program, Penn State has been developing 12 and 25 cc devices for use in pediatric patients, based on the successful, pneumatic Pierce-Donachy adult device.

 

The 12 cc pulsatile pediatric ventricular assist device (PVAD) is a collaboration between the Departments of Bioengineering and Surgery at Hershey Medical Center and the Artificial Heart Lab at University Park. As part of this project the Artificial Heart Lab uses 2D particle image velocimetry (PIV) to observe the fluid dynamics within these devices including flow patterns and wall shear rates. This is important to the design since certain flow characteristics have shown a decrease in thrombus deposition within the device, an issue many VADs currently face. The desired properties include a strong rotation during diastole, low blood residence time, and no areas of stasis within the device. Several design considerations are currently being studied including valve selection orientation and position as well as possible weaning applications including changes in inflow timing and flow reduction.

   


PIV studies of a PVAD cannulae
Tobias Long

A challenge to the development of pediatric ventricular assist devices is the use of the aortic cannulae attached to the devices. Cannulae used for pediatric application have large pressure drops. Furthermore, during the development of the 12cc Penn State pediatric pulsatile VAD, particle image velocimetry (PIV) illustrated that a separation developed proximal to the aortic (outlet) mechanical heart valve (Bjork-Shiley Monostrut).  Hematocrit levels affected the extent of the fluid separation. An acrylic model was manufactured to model the aortic cannula (6mm diameter).  PIV data was collected to examine the flow as the fluid exits the outlet of the VAD and enters the aortic cannula. PIV data for Non-Newtonian blood analogs, with hematocrit levels of 20%, 40%, and 60%, were compared for a heart rate of 50 and 75 bpm. Areas of separation were evident in the cannulae contraction and their extent increased with hematocrit. Further cannulae models are being evaluated.

   



Adult 50 cc ventricular assist device
Varun Reddy


The Penn State 50 cc left ventricular assist device (LVAD) is a blood pump designed for smaller patients as a follow on to the clinically successful 70 cc Arrow LionHeart (TM) also developed at Penn State. In vitro fluid mechanic studies are performed on the device at University Park using particle image velocimetry and high speed video techniques. Flow field characteristics such as velocity and wall shear are measured, with the goal of reducing the hemolytic and thrombogenic potential of the pump. This research is used to accelerate the design process in conjunction with in vivo studies at the the Penn State Milton S. Hershey Medical Center.



   



  The Penn State PVAD anastomosis and its associated thrombosis
  Ning Yang

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The Penn State Pediatric Ventricular Assist Device (PVAD) is a pulsatile blood pump designed to provide mechanical circulatory support for children. While the fluid mechanics of the PVAD has been extensively studied, the effects of the PVAD outflow anastomosis on the aortic flow remain unknown. Our previous study on the adult left ventricular assist device (LVAD) anastomosis suggests that the outflow anastomosis can highly disturb the aortic flow, causing strong mixing of flow in the aorta and impingement jet on the aortic wall. As a result, turbulence flow may occur. Therefore, there is a need for a systematic study of PVAD outflow anastomosis on the aorta and its associated thrombosis.

In this study, we will study the aortic flow with different anastomotic configurations both numerically and experimentally. We will first develop three-dimensional CFD models to study the effects of the anastomosis on the aortic flow. OpenFOAM, an object-oriented open-source CFD toolkit, will be used to compute the flow field. Both continuous and pulsatile conditions will be considered. Implicit large eddy simulation (MILES) approach will be applied to study turbulent flow. Second, we will validate the numerical results with PIV and LDV flow measurements by developing a mock circulatory loop. The PVAD outflow graft will be connected to the outside wall of the ascending aorta, consistent with the numerical proximal model. A pulse duplicator will be used to simulate the heart. A mock circulatory loop will be set up with the PVAD parallel to the heart simulator. A variety of clinically relevant cardiovascular conditions will be simulated by the adjustment of the cardiac output (CO), heart rate (HR), and pressures. PIV and LDV will be used to measure detailed velocity field associated with the anastomosis. The comparison between the numerical and experimental data will help to fine tune the CFD models. 

  Blood Rheology
  Margaret Slattery, Ning Yang, Katie Chorpenning

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Blood is a suspension consisting of primarily red blood cells, along with other formed elements.  This gives blood viscoelastic behavior, with properties that are characteristic of both fluids and solids.  The concentration, aggregation and deformability of red blood cells as well as plasma viscosity and plasma protein concentration influence the viscosity and elasticity of whole blood.  Blood behaves as a Newtonian fluid at shear rates above 500 s-1.  At shear rates lower than 50 s-1 the viscosity of blood increases exponentially, due to the formation of large aggregates of erythrocytes.  As shear rate increases, the erythrocytes are dispersed and aligned in the direction of flow.   These parameters affect the flow properties, especially at low shear rates (< 50 s-1).  Newtonian blood analogs are often used for in vitro experiments, but it has been shown that these fluids produce higher values of wall shear stress and Reynolds stresses than non-Newtonian fluids.  The lower stresses seen in viscoelastic fluids are more conducive to thrombus formation.  When conducting in vitro testing to measure the fluid mechanics in cardiac devices, it is important to use a fluid that mimics properties of blood.  Shown below are viscoelastic blood analogs that we have developed to mimic blood at different values of hematocrit.

Thrombogenicity: Relation between cardic devices, fluid mechanics and platelet activation

The usefulness of cardiovascular prosthetics is limited by thromboembolic events. Even after many attempts to reduce thrombogencity by changing prosthetic design and modifying the surfaces of the blood contacting materials, it is estimated that ventricular assist devices still have a thromboembolic event rate of approximately 50%. Preventing thrombosis often involves an anticoagulation regime with its own related problems.

There is a need to predict thrombogencity of cardiovascular devices, both when in development and once they are implanted, but it is difficult to assess with existing assays. There are a variety of clinical markers to measure platelet function or thrombin level in response to the vascular implant, but they can be influenced by other existing pathologies and no marker specifically indicates actual thrombus formation. In the ventricular assist device, for example, the oscillatory stress field about artificial heart valves is known to activate platelets, but it is the subsequent exposure to low blood-flow that permits clot formation.

The goal of this research project is to develop techniques to better determine thrombogencity of cardiovascular prosthetics. An in vitro flow loop is being developed to simulate the complex flow patterns that are caused by cardiovascular prosthetics. Circulating whole blood through the loop, clinical measures of thrombogencity are used to assess platelet function and aggregation. Measures of platelet function and activation will be used to correlate thrombosis with flow stresses. Cross-correlating the assays may be a better indicator of thrombosis than any single assay. These studies will provide fundamental information necessary to develop tools for both cardiovascular implant development and patient monitoring devices.  


  Magnetic resonance imaging (MRI) flow visualization
  George Long

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