Pulsatile Pediatric Ventricular Assist Device Research Articles

Effect of the bileaflet inlet valve angle on the flow of a pediatric ventricular assist device: Experimental analysis.

Mechanical heart valves (MHV) and its fluid dynamics inside a pulsatile pediatric ventricular assist device (PVAD) can be associated with blood degradation. In this article, flow structures are analyzed and compared by an experimental investigation on the effect of bileaflet MHV positioned at varying angles in the inlet port orifice of a PVAD. Time-resolved particle image velocimetry was applied to characterize the internal flow of the device. St Jude Medical bileaftlet valves were used on the inlet orifice and positioned at 0°, 15°, 30°, 45°, 60°, and 90° in relation to the centerline of the device. Three planes with bidimensional velocity magnitude fields were considered in the analysis with visualization of diastolic jets, device wall washing patterns and flow circulation during emptying or systole of the pump. Also, the washing vortex area, and vertical velocity probabilities of regurgitant flows in the inlet valve were evaluated. The results show that a variation in the angle of the MHV at the inlet port produced distinct velocities, fluid structures, and regurgitant flow probabilities within the device. MHV positioned at an angle of 0° generated the strongest inlet jet, larger vortex area during filling, more prominent outgoing flow, and less regurgitation compared to the angles studied. The presence of unfavorable fluid structures, such as small vortices, and/or sudden flow structure interruption, and/or regurgitation, were identified at 45° and 90° angles. The 0° inlet angle had better outcomes than other angles due to its consistency in the multiple parameters analyzed.

Modeling, characterization and test of a pediatric ventricular assist device

This paper presents a 0D model for the pulsatile pediatric Ventricular Assist Device (pVAD), as well as its characterization and test results obtained by using an experimental hydraulic system that provides operational conditions close to the physiological conditions observed in pediatric patients. The measurements of the physical quantities used in characterization of the pVAD have been obtained by using the LabVIEW platform. The air chamber pressure signal is used to model the pneumatic actuation system behaviour, whereas the pressure-volume relationship inside the blood chamber is applied to model the pVAD compliance. The set of test and validation results demonstrated the correctness of the proposed model.

Continuous and Pulsatile Pediatric Ventricular Assist Device Hemodynamics with a Viscoelastic Blood Model.

To investigate the effects of pulsatile and continuous pediatric ventricular assist (PVAD) flow and pediatric blood viscoelasticity on hemodynamics in a pediatric aortic graft model. Hemodynamic parameters of pulsatility, along with velocity and wall shear stress (WSS), are analyzed and compared between Newtonian and viscoelastic blood models at a range of physiological pediatric hematocrits using computational fluid dynamics. Both pulsatile and continuous PVAD flow lead to a decrease in pulsatility (surplus hemodynamic energy, ergs/cm(3)) compared to healthy aortic flow but with continuous PVAD pulsatility up to 2.4 times lower than pulsatile PVAD pulsatility at each aortic outlet. Significant differences are also seen between the two flow modes in velocity and WSS. The higher velocity jet during systole with pulsatile flow leads to higher WSSs at the anastomotic toe and at the aortic branch bifurcations. The lower velocity but continuous flow jet leads to a much different flow field and higher WSSs into diastole. Under a range of physiological pediatric hematocrit (20-60%), both velocity and WSS can vary significantly with the higher hematocrit blood model generally leading to higher peak WSSs but also lower WSSs in regions of flow separation. The large decrease in pulsatility seen from continuous PVAD flow could lead to complications in pediatric vascular development while the high WSSs during peak systole from pulsatile PVAD flow could lead to blood damage. Both flow modes lead to similar regions prone to intimal hyperplasia resulting from low time-averaged WSS and high oscillatory shear index.

Flow Visualization of the Penn State Pulsatile Pediatric Ventricular Assist Device Cannulae and Change in Outlet Valve Placement

Due to the lack of long-term mechanical circulatory support options for children, Penn State is developing a pneumatically driven 12 cc pulsatile pediatric ventricular assist device (PVAD). The reduction in volume, however, necessary to accommodate pediatric patients leads to changes in the functional fluid mechanics. One area that has not been previously observed is the flow upstream and downstream of the inlet and outlet valves. In particular an area of blockage, that includes a large area of stagnant flow, has been observed upstream of the outlet valve that could cause an increase in blood damage. In order to measure the flow upstream and downstream of the ports, we deploy a 50 mm acrylic valve extension. The outlet valve is moved downstream of the outlet port in an attempt to eliminate a flow blockage region upstream of the valve. We mount the PVAD to a mock circulatory loop that models the systemic circulation under normal physiological conditions, with a 40% hematocrit blood analog as the fluid. Two dimensional particle image velocimetry is used to measure the flow. As expected, the flow patterns in the body of the device remain similar to those without the extension, except near the outlet port. Non-uniform flow is observed upstream of both the inlet and outlet valves and regurgitation is observed upstream of the inlet valve. The relocation of the outlet valve leads to a more uniform outflow and the blockage region is eliminated. The observations of non-uniform flow upstream of the inlet valve are a new and important observation when considering computational models. Also, the new outlet flow pattern associated with the relocation of the outlet valve reduces the potential for blood damage. Studies with a relocated valve in a clinical model are being considered.

A Parametric Study of Valve Orientation on the Flow Patterns of the Penn State Pulsatile Pediatric Ventricular Assist Device

Because of the shortage of organs for transplant in pediatric patients with end-stage heart failure, Penn State is developing a pneumatically driven 12 cc pulsatile pediatric ventricular assist device (PVAD). A major concern is the flow field changes related to the volume decrease and its effect on device thrombogenicity. Previous studies of similar devices have shown that changes in the orientation of the inlet valve can lead to improvement in the flow field. Herein, the fluid dynamic effects of orientation changes at both the inlet and outlet valves are studied. Using two-dimensional particle image velocimetry, we examine the flow field in vitro using an acrylic model of the PVAD in a mock circulatory loop. Regardless of valve orientation, the overall flow pattern inside the PVAD remains similar, but important differences were seen locally in the wall shear rates, which is notable because shear rates >500 s may prevent thrombus formation. As the inlet valve was rotated toward the fluid side of the PVAD, we observed an increase in inlet jet velocity and wall shear rates along the inlet port wall. A corresponding rotation of the outlet valve increases the wall shear rate along the outer wall near the device outlet. Wall shear rates were all higher when both valves were rotated toward the fluid side of the device, with the best rates found at orientations of +15 degrees for both the inlet and outlet valves. Overall, orientations of +15 degrees or +30 degrees of both the inlet and outlet valve resulted in an increase in wall shear rates and could aid in the reduction of thrombus formation inside the PVAD.

The Influence of Operational Protocol on the Fluid Dynamics in the 12 cc Penn State Pulsatile Pediatric Ventricular Assist Device: The Effect of End‐Diastolic Delay

The success of adult ventricular assist devices (VADs), coupled with the high transplant waiting list mortality of infants (40%) has prompted Penn State to develop a pediatric version of the clinically successful adult device. Although the primary use of this device will be bridge-to-transplant, there has been sufficient clinical data to demonstrate the efficacy of VADs in a bridge-to-recovery setting. However, removing the patient from the device, a process known as weaning, demands operation of the device at a lower beat rate and concomitant increased risk for thromboembolism. Previous studies have shown that the interrelated flow characteristics necessary for the prevention of thrombosis in a pulsatile VAD are a strong inlet jet, a late diastolic recirculating flow, and a wall shear rate greater than 500/s. In an effort to develop a strong inlet jet and rotational flow pattern at a lower beat and flow rate, we have compressed diastole by altering the end-diastolic delay time (EDD). Particle image velocimetry was used to compare the flow fields and wall shear rates in the chamber of the 12 cc Penn State pulsatile pediatric VAD operated at 50 beats per minute using EDDs of 10, 50, and 100 ms. Although we expected the 100 ms EDD to have the best wall shear profiles, we found that the 50 ms EDD condition was superior to both the 10 and 100 EDD conditions, due to a longer sustained inlet jet.

Hemodynamics of an End-to-Side Anastomotic Graft for a Pulsatile Pediatric Ventricular Assist Device

Numerical simulations are performed to investigate the flow within the end-to-side proximal anastomosis of a pulsatile pediatric ventricular assist device (PVAD) to an aorta. The anastomotic model is constructed from a patient-specific pediatric aorta. The three great vessels originating from the aortic arch--brachiocephalic (innominate), left common carotid, and left subclavian arteries--are included. An implicit large eddy simulation method based on a finite volume approach is used to study the resulting turbulent flow. A resistance boundary condition is applied at each branch outlet to study flow splitting. The PVAD anastomosis is found to alter the aortic flow dramatically. More flow is diverted into the great vessels with the PVAD support. Turbulence is found in the jet impingement area at peak systole for 100% bypass, and a maximum principal normal Reynolds stress of 7081 dyn/cm(2) is estimated based on ten flow cycles. This may be high enough to cause hemolysis and platelet activation. Regions prone to intimal hyperplasia are identified by combining the time-averaged wall shear stress and oscillatory shear index. These regions are found to vary, depending on the percentage of the flow bypass.

360: In Vivo Assessment of Thrombogenicity of the Penn State Pediatric Ventricular Assist Device

Penn State has recently completed the design and feasibility testing of a pulsatile pediatric Ventricular Assist Device (VAD) under the 5 year NHLBI contract program “Pediatric Circulatory Support”. The Infant VAD is a pneumatically-actuated pulsatile pump with a 14 ml stroke volume, based on the adult Thoratec (Pierce-Donachy) VAD. The Pediatric VAD may provide left, right, or biventricular support for long term bridge-to-transplant or weaning applications. The primary design objective has been to minimize thromboembolic risk; thrombogenicity has been assessed in 30 day chronic animal studies.

Flow Visualization of Three-Dimensionality Inside the 12 cc Penn State Pulsatile Pediatric Ventricular Assist Device

In order to aid the ongoing concern of limited organ availability for pediatric heart transplants, Penn State has continued development of a pulsatile Pediatric Ventricular Assist Device (PVAD). Initial studies of the PVAD observed an increase in thrombus formation due to differences in flow field physics when compared to adult sized devices, which included a higher degree of three-dimensionality. This unique flow field brings into question the use of 2D planar particle image velocimetry (PIV) as a flow visualization technique, however the small size and high curvature of the PVAD make other tools such as stereoscopic PIV impractical. In order to test the reliability of the 2D results, we perform a pseudo-3D PIV study using planes both parallel and normal to the diaphragm employing a mock circulatory loop containing a viscoelastic fluid that mimics 40% hematocrit blood. We find that while the third component of velocity is extremely helpful to a physical understanding of the flow, particularly of the diastolic jet and the development of a desired rotational pattern, the flow data taken parallel to the diaphragm is sufficient to describe the wall shear rates, a critical aspect to the study of thrombosis and design of such pumps.

The 12cc Penn State Pulsatile Pediatric Ventricular Assist Device: Fluid Dynamics Associated With Valve Selection

The mortality rate for infants awaiting a heart transplant is 40% because of the extremely limited number of donor organs. Ventricular assist devices (VADs), a common bridge-to-transplant solution in adults, are becoming a viable option for pediatric patients. A major obstacle faced by VAD designers is thromboembolism. Previous studies have shown that the interrelated flow characteristics necessary for the prevention of thrombosis in a pulsatile VAD are a strong inlet jet, a late diastolic recirculating flow, and a wall shear rate greater than 500 s(-1). Particle image velocimetry was used to compare the flow fields in the chamber of the 12 cc Penn State pediatric pulsatile VAD using two mechanical heart valves: Bjork-Shiley monostrut (BSM) tilting disk valves and CarboMedics (CM) bileaflet valves. In conjunction with the flow evaluation, wall shear data were calculated and analyzed to help quantify wall washing. The major orifice inlet jet of the device containing BSM valves was more intense, which led to better recirculation and wall washing than the three jets produced by the CM valves. Regurgitation through the CM valve served as a significant hindrance to the development of the rotational flow.

The 12 cc Penn State Pulsatile Pediatric Ventricular Assist Device: Flow Field Observations at a Reduced Beat Rate With Application to Weaning

Ventricular assist devices (VADs) have become a viable option for adult patients with end-stage heart failure during the bridge-to-transplant period and have recently shown promise in aiding in myocardial recovery. Because the number of available organs is insufficient, mechanical circulatory support systems such as VADs are also being developed for use in pediatric patients. During myocardial recovery, the system must be weaned from the patient to prepare for explant; for pulsatile devices, this often includes a reduction in flow rate, which can change the fluid dynamics of the device. These changes in flow need to be monitored because strong diastolic rotational flow, no areas of blood stasis, low blood residence time, and wall shear rates above 500 s, can help prevent thrombus deposition. Particle image velocimetry was used to observe the planar flow patterns and wall shear rates of the 12 cc Penn State Pneumatic Pediatric VAD (PVAD) at a normal operating condition and a reduced beat rate. At the reduced beat rate, the PVAD showed an earlier loss of rotational pattern, increased blood residence time, and an overall reduction in wall shear rate at the outer walls. Because this reduction in flow rate could lead to a possible increase in thrombus deposition, it may be necessary to look into other options for weaning a patient from the PVAD.

The Effect of Left Ventricular Function and Drive Pressures on the Filling and Ejection of a Pulsatile Pediatric Ventricular Assist Device in an Acute Animal Model

Penn State is currently developing a 12-mL, pulsatile, pneumatically driven pediatric ventricular assist device intended to be used in infants. After extensive in vitro testing of the pump in a passive-filling, mock circulatory loop, an acute animal study was performed to obtain data with a contracting ventricle. The objectives were to determine the range of pneumatic pressures and time required to completely fill and empty the pediatric ventricular assist device under various physiologic conditions, simulate reductions in ventricular contractility and blood volume, and provide data for validation of the mock circulatory loop. A 15-kg goat was used. The cannulation was achieved via left thoracotomy from the left ventricle to the descending aorta. The pump rate and systolic duration were controlled manually to maintain complete filling and ejection. The mean ejection time ranged from 280 ms to 382 ms when the systolic pressure ranged from 350 mm Hg to 200 mm Hg. The mean filling time ranged from 352 ms to 490 ms, for the diastolic pressure range of -60 mm Hg to 0 mm Hg. Esmolol produced a decrease in left ventricular pressure, required longer pump filling time, and reduced LVAD flow.

Pulsatile Pediatric Ventricular Assist Devices

A pulsatile pediatric ventricular assist device (PVAD) is being developed at The Pennsylvania State University to provide mechanical circulatory support in infants and children. The PVAD is based on the design of the adult-sized Pierce-Donachy VAD (Thoratec VAD). The infant-sized PVAD has a dynamic stroke volume of approximately 13 ml. A larger 25-ml size for children is also planned. The stroke volumes, beat rates, and pulsatility are comparable to normal physiologic values. The expected maximum duration of use is 6 months. The PVAD is intended to be placed paracorporeally, although the pump may be implanted for bridge-to-transplant applications. The pneumatically actuated PVAD uses a full-to-empty control mode, which allows maximum flexibility in its application for left, right, or biventricular assistance. The device may be used with atrial or ventricular inlet cannulation, with blood return to the aorta or pulmonary artery. In vitro testing is underway to measure hydrodynamic performance, hemolysis, and flow velocities using particle image velocimetry. In vivo implantation studies will be performed in juvenile goats or sheep after the completion of baseline studies to assess hematology and surgical fit.

WHOLE FLOWFIELD PARTICLE IMAGE VELOCIMETRY MEASUREMENTS IN A 10 CC PULSATILE PEDIATRIC ASSIST PUMP

The analysis of a whole flowfield particle image velocimetry study of the 10 cc Penn State pulsatile pediatric ventricular assist device (PVAD) is presented. The pressures and flow through the PVAD are controlled with a mock circulatory loop. The blood analog (Sodium Iodide/Xanthan gum) solution is a good match for the viscoelastic properties of blood across the shear rate range. An earlier version of the Penn State PVAD has shown a much higher incidence of clot formation than does the adult pump from which it was derived. Therefore, a focus of the current study is on the wall shear stress. We identify regions of low wall shear stress throughout the pump cycle and suggest how the pump design might be geometrically modified to increase the flow rate there. In addition, we analyze the flow in the valve regions, as predicting the result of downsizing the valves from the adult pump is not straightforward.

The Combined Use of Extracorporeal Life Support and the Berlin Heart Pulsatile Pediatric Ventricular Assist Device as a Bridge to Transplant in a Toddler

There is a very limited published material about experience with long-term pediatric mechanical circulatory support as a bridge to heart transplant. We report on a 2-year-old, 12 kg boy admitted with 2-week history of low-grade fever, ear pain, pulmonary edema, and congestive heart failure. Trans-thoracic echocardiography confirmed severe myocardial dysfunction with a left ventricular ejection fraction of 0.20 and percentage shortening of 13. After 2 days of ventilatory and inotropic support, the patient continued to deteriorate and subsequently required femoro–femoral extracorporeal life support (ECLS). This was later complicated by a progressive coagulopathy and massive bleeding. On day 17, a pulsatile pediatric paracorporeal biventricular assist device (VAD) (Berlin Heart) was implanted. The patient’s condition improved significantly with all coagulopathies corrected, and the patient was extubated 21 days later. After 109 days of bi-VAD support, the patient was successfully transplanted and discharged home 45 days post transplant. Our early experience with initial ECLS bridge to VAD and subsequently to transplant was encouraging. It allowed for additional time to select the ideal organ donor and optimize the recipient’s comorbid condition and multiorgan failure. VAD provides an additional armamentarium of circulatory support in pediatric patients with severe heart failure.

Evaluation of a Pulsatile Pediatric Ventricular Assist Device in an Acute Right Heart Failure Model

The development of pulsatile ventricular assist devices for children has been limited mainly by size constraints. The purpose of this study was to evaluate the MEDOS trileaflet-valved, pulsatile, pediatric right ventricular assist device (stroke volume = 9 mL) in a neonatal lamb model of acute right ventricular failure. Right ventricular failure was induced in ten 3-week-old lambs (8.6 kg) by right ventriculotomy and disruption of the tricuspid valve. Control group 1 (n = 5) had no mechanical support whereas experimental group 2 (n = 5) had right ventricular assist device support for 6 hours. The following hemodynamic parameters were measured in all animals: heart rate and right atrial, pulmonary arterial, left atrial, and systemic arterial pressures. Cardiac output was measured by an electromagnetic flow probe placed on the pulmonary artery. All results are expressed as mean +/- standard deviation and analyzed by Student's t test. A p value less than 0.05 was considered statistically significant. Base-line measurements were not significantly different between groups and included systemic arterial pressure, 80.6 +/- 12.7 mm Hg; right atrial pressure, 4.6 +/- 1.6 mm Hg; mean pulmonary arterial pressure, 15.6 +/- 4.2 mm Hg; left atrial pressure, 4.8 +/- 0.8 mm Hg; and cardiac output, 1.4 +/- 0.2 L/min. Right ventricular injury produced hemodynamics compatible with right ventricular failure in both groups: mean systemic arterial pressure, 38.8 +/- 10.4 mm Hg; right atrial pressure, 16.8 +/- 2.3 mm Hg; left atrial pressure, 1.4 +/- 0.5 mm Hg; and cardiac output, 0.6 +/- 0.1 L/min. All group 1 animals died at a mean of 71.4 +/- 9.4 minutes after the operation. All group 2 animals survived the duration of study. Hemodynamic parameters were recorded at 2, 4, and 6 hours on and off pump, and were significantly improved at all time points: mean systemic arterial pressure, 68.0 +/- 13.0 mm Hg; right atrial pressure, 8.2 +/- 2.3 mm Hg; left atrial pressure, 6.4 +/- 2.1 mm Hg; and cardiac output, 1.0 +/- 0.2 L/min. The results demonstrate the successful creation of a right ventricular failure model and its salvage by a miniaturized, pulsatile right ventricular assist device. The small size of this device makes its use possible even in small neonates.