Greg W. Burgreen

Effects of turbulent stresses upon mechanical hemolysis: experimental and computational analysis.

Experimental and computational studies were performed to elucidate the role of turbulent stresses in mechanical blood damage (hemolysis). A suspension of bovine red blood cells (RBC) was driven through a closed circulating loop by a centrifugal pump. A small capillary tube (inner diameter 1 mm and length 70 mm) was incorporated into the circulating loop via tapered connectors. The suspension of RBCs was diluted with saline to achieve an asymptotic apparent viscosity of 2.0 ± 0.1 cP at 23°C to produce turbulent flow at nominal flow rate and pressure. To study laminar flow at the identical wall shear stresses in the same capillary tube, the apparent viscosity of the RBC suspension was increased to 6.3 ± 0.1 cP (at 23°C) by addition of Dextran-40. Using various combinations of driving pressure and Dextran mediated adjustments in dynamic viscosity Reynolds numbers ranging from 300–5,000 were generated, and rates of hemolysis were measured. Pilot studies were performed to verify that the suspension media did not affect mechanical fragility of the RBCs. The results of these bench studies demonstrated that, at the same wall shear stress in a capillary tube, the level of hemolysis was significantly greater (p < 0.05) for turbulent flow as compared with laminar flow. This confirmed that turbulent stresses contribute strongly to blood mechanical trauma. Numerical predictions of hemolysis obtained by computational fluid dynamic modeling were in good agreement with these experimental data.

Multilaboratory Particle Image Velocimetry Analysis of the FDA Benchmark Nozzle Model to Support Validation of Computational Fluid Dynamics Simulations

This study is part of a FDA-sponsored project to evaluate the use and limitations of computational fluid dynamics (CFD) in assessing blood flow parameters related to medical device safety. In an interlaboratory study, fluid velocities and pressures were measured in a nozzle model to provide experimental validation for a companion round-robin CFD study. The simple benchmark nozzle model, which mimicked the flow fields in several medical devices, consisted of a gradual flow constriction, a narrow throat region, and a sudden expansion region where a fluid jet exited the center of the nozzle with recirculation zones near the model walls. Measurements of mean velocity and turbulent flow quantities were made in the benchmark device at three independent laboratories using particle image velocimetry (PIV). Flow measurements were performed over a range of nozzle throat Reynolds numbers (Re(throat)) from 500 to 6500, covering the laminar, transitional, and turbulent flow regimes. A standard operating procedure was developed for performing experiments under controlled temperature and flow conditions and for minimizing systematic errors during PIV image acquisition and processing. For laminar (Re(throat)=500) and turbulent flow conditions (Re(throat)≥3500), the velocities measured by the three laboratories were similar with an interlaboratory uncertainty of ∼10% at most of the locations. However, for the transitional flow case (Re(throat)=2000), the uncertainty in the size and the velocity of the jet at the nozzle exit increased to ∼60% and was very sensitive to the flow conditions. An error analysis showed that by minimizing the variability in the experimental parameters such as flow rate and fluid viscosity to less than 5% and by matching the inlet turbulence level between the laboratories, the uncertainties in the velocities of the transitional flow case could be reduced to ∼15%. The experimental procedure and flow results from this interlaboratory study (available at http://fdacfd.nci.nih.gov) will be useful in validating CFD simulations of the benchmark nozzle model and in performing PIV studies on other medical device models.

Fluid dynamic characterization of operating conditions for continuous flow blood pumps.

As continuous flow pumps become more prominent as long-term ventricular assist devices, the wide range of conditions under which they must be operated has become evident. Designed to operate at a single, best-efficiency, operating point, continuous flow pumps are required to perform at off-design conditions quite frequently. The present study investigated the internal fluid dynamics within two representative rotary fluid pumps to characterize the quality of the flow field over a full range of operating conditions. A Nimbus/UoP axial flow blood pump and a small centrifugal pump were used as the study models. Full field visualization of flow features in the two pumps was conducted using a laser based fluorescent particle imaging technique. Experiments were performed under steady flow conditions. Flow patterns at inlet and outlet sections were visualized over a series of operating points. Flow features specific to each pump design were observed to exist under all operating conditions. At off-design conditions, an annular region of reverse flow was commonly observed within the inlet of the axial pump, while a small annulus of backflow in the inlet duct and a strong disturbed flow at the outlet tongue were observed for the centrifugal pump. These observations were correlated to a critical nondimensional flow coefficient. The creation of a map of flow behavior provides an additional, important criterion for determining favorable operating speed for rotary blood pumps. Many unfavorable flow features may be avoided by maintaining the flow coefficient above a characteristic critical coefficient for a particular pump, whereas the intrinsic deleterious flow features can only be minimized by design improvement. Broadening the operating range by raising the band between the critical flow coefficient and the designed flow coefficient, is also a worthy goal for design improvement.

A design improvement strategy for axial blood pumps using computational fluid dynamics.

During the initial stages of concept development of non traditional axial flow pumps, numeric simulation offers an attractive advantage. Computational fluid dynamics (CFD) provides the rationale to evolve the design numerically such that undesirable flow features may be significantly mitigated before a physical prototype is fabricated. The initial design of a novel axial flow blood pump is shown through CFD analysis to exhibit large regions of reverse flow. Such fluid dynamic behavior not only decreases the pumps hydrodynamic efficiency, but, more significantly, increases its overall potential for blood trauma and thrombogenesis. The design improvement strategy consists of creating a geometric model of the blood wetted surfaces and changing the associated geometric parameters such that more desirable fluid dynamic behavior is systematically attained with each incremental modification. The fluid flow through each new pump design is analyzed by numerically solving the incompressible Navier-Stokes equations in rotating coordinates. Marked improvements in the major fluid dynamic aspects of the axial flow pump were observed over an evolutionary sequence of four generations of pump design.

Efficient, Physiologically Realistic Lung Airflow Simulations

One of the key challenges for computational fluid dynamics (CFD) simulations of human lung airflow is the sheer size and complexity of the complete, multiscale geometry of the bronchopulmonary tree. Since 3-D CFD simulations of the full airway tree are currently intractable, researchers have proposed reduced geometry models in which multiple airway paths are truncated downstream of the first few generations. This paper investigates a recently proposed method for closing the CFD model by application of physiologically correct boundary conditions at truncated outlets. A realistic, reduced geometry model of the lung airway based on CT data has been constructed up to generation 18, including extrathoracic, bronchi, and bronchiole regions. Results indicate that the new method yields reasonable results for pressure drop through the airway, at a small fraction of the cost of fully resolved simulations.

Effect of impeller design and spacing on gas exchange in a percutaneous respiratory assist catheter.

Providing partial respiratory assistance by removing carbon dioxide (CO2 ) can improve clinical outcomes in patients suffering from acute exacerbations of chronic obstructive pulmonary disease and acute respiratory distress syndrome. An intravenous respiratory assist device with a small (25 Fr) insertion diameter eliminates the complexity and potential complications associated with external blood circuitry and can be inserted by nonspecialized surgeons. The impeller percutaneous respiratory assist catheter (IPRAC) is a highly efficient CO2 removal device for percutaneous insertion to the vena cava via the right jugular or right femoral vein that utilizes an array of impellers rotating within a hollow-fiber membrane bundle to enhance gas exchange. The objective of this study was to evaluate the effects of new impeller designs and impeller spacing on gas exchange in the IPRAC using computational fluid dynamics (CFD) and in vitro deionized water gas exchange testing. A CFD gas exchange and flow model was developed to guide a progressive impeller design process. Six impeller blade geometries were designed and tested in vitro in an IPRAC device with 2- or 10-mm axial spacing and varying numbers of blades (2-5). The maximum CO2 removal efficiency (exchange per unit surface area) achieved was 573 ± 8 mL/min/m(2) (40.1 mL/min absolute). The gas exchange rate was found to be largely independent of blade design and number of blades for the impellers tested but increased significantly (5-10%) with reduced axial spacing allowing for additional shaft impellers (23 vs. 14). CFD gas exchange predictions were within 2-13% of experimental values and accurately predicted the relative improvement with impellers at 2- versus 10-mm axial spacing. The ability of CFD simulation to accurately forecast the effects of influential design parameters suggests it can be used to identify impeller traits that profoundly affect facilitated gas exchange.

Simulations of Cyclic Breathing in the Conducting Zone of the Human Lung

Computational fluid dynamics (CFD) simulations were performed to predict the air flow in the human lung during cyclic breathing. The study employed a morphologically complex computational geometry generated using a combination of patient-specific CT-scan data for the extrathoracic and upper airway regions and a representative branching geometry for the lower airways that is available in the open literature. The geometry extended throughout the entire conducting zone and includes 16 partially resolved airway generations. For each generation beyond the third, only a fraction of the airway branches were retained, resulting in truncated flow outlets (for inspiratory flow) in generations 414. The inhalation and exhalation air flow boundary conditions were prescribed based on a physiologically realistic ventilation pattern, which was obtained using a whole-body model of human physiology. The flow was driven by specifying time-varying volumetric flowrates applied at each of the distal boundaries, while the oral boundary was maintained at constant (atmospheric) pressure. The study investigated the effectiveness of three different mass flow distribution schemes to drive the air flow. It was found that prescribed mass flow distribution fractions based on the square of the airway cross-sectional area produced the best results in terms of a uniform distal pressure distribution, while all methods produced reasonable results in terms of mass flow distribution throughout the lung airway geometry.Copyright

CFD-Based Design Optimization of the Outlet Stator of a Rotodynamic Cardiac Assist Device

Two designs of an outlet stator for an axial flow left ventricular assist device are analyzed at nominal operating conditions. The original stator assembly (design 1) has significant flow separation and reversal. A second stator assembly (design 2) replaces part of the original tubular outer housing with a converging-diverging throat section with the intention of locally improving the fluid dynamics. Both stator designs are analyzed using computational fluid dynamics (CFD) analysis and experimental flow visualization. The computational and experimental methods indicate persistent regions of flow separation in design 1 and increased surface washing in design 2. However, the improved fluid dynamics of design 2 is accompanied by a significant decrease in pressure recovery as compared to design 1. To resolve these apparently conflicting end point behaviors, CFD-based design optimization is used to evolve the outer housing shape of a third stator assembly to simultaneously reduce flow stasis and maintain a high level of pressure recovery. During the optimization process, several insightful guidelines for designing efficient outlet stators of axial flow turbomachines become evident.

In vitro and in vivo evaluation of a novel integrated wearable artificial lung

BACKGROUNDnConventional extracorporeal membrane oxygenation (ECMO) is cumbersome and is associated with high morbidity and mortality. We are currently developing the Pittsburgh Ambulatory Assist Lung (PAAL), which is designed to allow for ambulation of lung failure patients during bridge to transplant or recovery. In this study, we investigated the in vitro and acute in vivo performance of the PAAL.nnnMETHODSnThe PAAL features a 1.75-inch-diameter, cylindrical, hollow-fiber membrane (HFM) bundle of stacked sheets, with a surface area of 0.65 m2 integrated with a centrifugal pump. The PAAL was tested on the bench for hydrodynamic performance, gas exchange and hemolysis. It was then tested in 40- to 60-kg adult sheep (n = 4) for 6 hours. The animals were cannulated with an Avalon Elite 27Fr dual-lumen catheter (DLC) inserted through the right external jugular into the superior vena cava (SVC), right atrium (RA) and inferior vena cava (IVC).nnnRESULTSnThe PAAL pumped >250 mm Hg at 3.5 liters/min at a rotation speed of 2,100 rpm. Oxygenation performance met the target of 180 ml/min at 3.5 liters/min of blood flow in vitro, resulting in a gas-exchange efficiency of 278 ml/min/m2. The normalized index of hemolysis (NIH) for the PAAL and cannula was 0.054 g per 100 liters (n = 2) at 3.5 liters/min, as compared with 0.020 g per 100 liters (n = 2) for controls (DLC cannula and a Centrimag pump). Plasma-free hemoglobin (pfHb) was <20 mg/dl for all animals. Blood left the device 100% oxygenated in vivo and oxygenation reached 181 ml/min at 3.8 liters/min.nnnCONCLUSIONnThe PAAL met in vitro and acute in vivo performance targets. Five-day chronic sheep studies are planned for the near future.

Numerical simulations of flow pattern and particle trajectories in feline aorta for hypertrophic cardiomyopathy heart conditions

ABSTRACT Numerical simulations of pulsatile flow in a feline aorta for hypertrophic cardiomyopathy (HCM) heart conditions are performed to predict flow details and to evaluate possible thrombus trajectory patterns. The study demonstrates that average flow rate boundary conditions (QBC) at artery outlets act as a resistance-type boundary condition for pulsatile flow. For simulations when the exact artery outflows are not known, specification of estimated values from physiological conditions is a plausible approach. This boundary condition is further improved using an iterative method (I-QBC) to accurately satisfy outflow conditions when expected outflow is known. The approach is validated against experimental data for the prediction of iliac artery flow and wall stresses in a human abdominal aorta. The feline aorta simulations including Lagrangian particle transport are performed on grids with up to 11M cells for a generalized feline aorta. It is found that LES on larger grids performs significantly better than URANS for the prediction of vortical structures. Simulations for both healthy and HCM conditions show similar flow patterns in the upper abdominal aorta. However, the HCM condition shows the presence of large recirculation regions in the thoracic aorta resulting in 50% lower flow through the iliac arteries and increased entrapment of fluid-borne particles near the trifurcation region compared to the healthy condition.