Thermal Analysis of the PediaFlow Pediatric Ventricular Assist Device

Scaling the Low-Shear Pulsatile TORVAD for Pediatric Heart Failure

This article provides an overview of the design challenges associated with scaling the low-shear pulsatile TORVAD ventricular assist device (VAD) for treating pediatric heart failure. A cardiovascular system model was used to determine that a 15 ml stroke volume device with a maximum flow rate of 4 L/min can provide full support to pediatric patients with body surface areas between 0.6 and 1.5 m. Low-shear stress in the blood is preserved as the device is scaled down and remains at least two orders of magnitude less than continuous flow VADs. A new magnetic linkage coupling the rotor and piston has been optimized using a finite element model (FEM) resulting in increased heat transfer to the blood while reducing the overall size of TORVAD. Motor FEM has also been used to reduce motor size and improve motor efficiency and heat transfer. FEM analysis predicts no more than 1°C temperature rise on any blood or tissue contacting surface of the device. The iterative computational approach established provides a methodology for developing a TORVAD platform technology with various device sizes for supporting the circulation of infants to adults.

Steady and transient flow analysis of a magnetically levitated pediatric VAD: time varying boundary conditions

A magnetically levitated impeller within a pediatric ventricular assist device operates under highly transient flow conditions. In this study, computational analyses were performed to investigate the hydraulic performance and fluid forces on the impeller under the steady and dynamic flow conditions, including: 1) time-varying boundary conditions (TVBC) considering a pulsed pump flow rate and pulsed left ventricular pressure; 2) transient rotational sliding interfaces (TRSI) to capture virtual blade rotation. Under steady flow conditions, the pressure generation for 0.5-6 l/min over 6000-10000 rpm was 20-140 mmHg; experimental validation agreed to within 6-27%. Under transient flow conditions, the outflow pressure of the pump increased with higher inlet pressure during the TVBC simulation. During TVBC, the pressure rise across the pump decreased as a function of higher flow rates and increased as a function of lower flow rates. The radial fluid forces varied directly with the flow rate by demonstrating larger forces at higher flow rates. For TRSI simulations, pressure fluctuations due the blade passage frequency were found to have 12 peaks per revolution, having magnitude ranges of 
0.7 and 1.0 mmHg for 8 000 and 10 000 rpm, respectively. At 8 000 rpm, the fluid forces ranged from 1.15-1.17 N (axial) and 0.02-0.11 N (radial). Transient simulations model implant scenarios more realistically and provide critical information about the fluid conditions in the pump.

Antaki J, Verkaik J, Snyder S, Ricci M. Computational Fluid Dynamics-Based Design Optimization for an Implantable Miniature Maglev Pediatric Ventricular Assist Device

Computational fluid dynamics (CFD)-based design optimization was applied to achieve the finalized design of the PediaFlow^® PF4, a magnetically levitated rotodynamic pediatric ventricular assist device. It features a streamlined blood-flow path with a single annular fluid passage between the rotor and the stationary housing. The resulting impeller is composed of a first-stage mixed-flow section having four blades at the conical nose region followed by a second-stage fully axial-flow section with three blades within the annular gap region. A stator with three inwardly-directed vanes is provided at the conical tail region to recover pressure and straighten the flow. CFD predictions of head and efficiency characteristics agreed remarkably well with the validation experimental data: with overprediction of head by <7 mmHg over the entire operational range and a slight overprediction in best efficiency by ∼1%. The new optimized PF4 extended the maximum flow range of the previous PF3 device by more than 100% to over 2.3 liter per minute (LPM) for the same range of operating speeds, and doubled the maximum hydraulic efficiency to ∼27%. Evaluation of hemolysis was performed by a Lagrangian particle-tracking technique with analysis of regional contributions to the overall blood damage. The simulation revealed that hemolysis increases with an increase in both the flow rate and rotor speed but not necessarily with just an increase in flow rate at a constant rotor speed. At the flow rate of 1.0 LPM and a head of 138 mmHg, PF4 has a hemolysis index of 0.0032 compared to 0.0058 produced by PF3 at the same flow rate with a head of 48 mmHg. Numerical simulation of radial fluid forces performed by the CFD model with an eccentric rotor revealed the presence of negative fluid stiffness that was monotonically related to both flow and speed. Finally, conjugate heat transfer analysis predicted temperature rise adjacent to the motor to be inversely proportional to the length, but not exceeding ∼2 °C over the intended range of operation. In conclusion, CFD-based design optimization greatly expedited and facilitated the completion of the PediaFlow^® flow path and contributed to the system-wide optimization to produce a miniature maglev pump with exceptional hemocompatibility.

Biocompatibility assessment of the first generation PediaFlow pediatric ventricular assist device

The PediaFlow pediatric ventricular assist device is a miniature magnetically levitated mixed flow pump under development for circulatory support of newborns and infants (3-15 kg) with a targeted flow range of 0.3-1.5 L/min. The first generation design of the PediaFlow (PF1) was manufactured with a weight of approximately 100 g, priming volume less than 2 mL, length of 51 mm, outer diameter of 28 mm, and with 5-mm blood ports. PF1 was evaluated in an in vitro flow loop for 6 h and implanted in ovines for three chronic experiments of 6, 17, and 10 days. In the in vitro test, normalized index of hemolysis was 0.0087 ± 0.0024 g/100L. Hemodynamic performance and blood biocompatibility of PF1 were characterized in vivo by measurements of plasma free hemoglobin, plasma fibrinogen, total plasma protein, and with novel flow cytometric assays to quantify circulating activated ovine platelets. The mean plasma free hemoglobin values for the three chronic studies were 4.6 ± 2.7, 13.3 ± 7.9, and 8.8 ± 3.3 mg/dL, respectively. Platelet activation was low for portions of several studies but consistently rose along with observed animal and pump complications. The PF1 prototype generated promising results in terms of low hemolysis and platelet activation in the absence of complications. Hemodynamic results validated the magnetic bearing design and provided the platform for design iterations to meet the objective of providing circulatory support for young children with exceptional biocompatibility.