Approximately 20 million people worldwide
suffer annually from heart failure, a quarter of them in
America alone. In the United States, only 2,500
donor hearts are available each year. The DeBakey
Ventricular Assist Device (VAD) prolongs life until a
suitable transplant heart is available, and is used to
boost blood flow in patients suffering from hemodynamic
deterioration, that is, loss of blood pressure
and lowered cardiac output.
The use of computational fluid dynamics (CFD)
technology led to major design improvements in the
heart assist device, enabling its human implantation.
The DeBakey VAD is a miniaturized heart pump
designed to increase blood circulation in heart-failure
patients awaiting a transplant. A ventricular assist
device has to be small and efficient, generating a
5-liter-per-minute blood flow rate against
100 mm Hg pressure. Because blood is the operating
fluid, the design of a VAD requires that it propel the
blood gently, that is, it must minimize damage to the
red blood cells. In order to reduce red blood cell
damage, the pumping device must be designed to
avoid regions of high shear stress and separated flow
in the pump. In addition, the blood must be properly
washed out of the pump since the formation of blood
clots may appear within stagnation regions as a result
of previously damaged blood cells. Since the device
is small and the operating conditions severe, instrumentation
for making necessary flow measurements
is extremely difficult to design. Therefore it became
necessary to look at the flow by computational
means. The detailed computational flow analysis
now affords VAD designers with a view of the
complicated fluid dynamic processes inside their
devices.
Through the collaborative efforts of MicroMed
Technology Inc., Ames Research Center, and the
Johnson Space Center, the device has evolved from
early versions of the DeBakey VAD, which caused
thrombus formation (blood clotting) and hemolysis
(red blood cell damage). To solve these problems,
Ames scientists employed shuttle main engine
technology and CFD modeling capabilities, coupled
with high-performance computing technology, to
make several design modifications that vastly
improved the VAD's performance. A three-dimensional,
viscous, incompressible Navier-Stokes
code (INS3D) was used to analyze the flow. Several
design iterations were performed in order to increase
the hydrodynamic performances of this axial pump.
The research team investigated seven designs,
altering cavity shapes, blade curvature, inlet cannula
shapes, and impeller tip clearance size. They then
suggested three major design modifications to solve
the problems of cell damage resulting from their
exposure to high shear stress and interrupted regions
of blood flow in the DeBakey VAD (see figures 1
and 2).
The first improvement was the addition of an
inducer that spins with the impeller, drawing the
blood in and out of the device, thus preventing a
back flow. Additionally, the inducer provides enough
pressure rise to eliminate back flow in the impeller
hub region. The front edges of the blades were
slanted forward, allowing blood to flow at the correct
angle with the impeller, thereby increasing the
efficiency of flow through the device. Second, CFD
results suggested that the original design of the device
caused clotting in the front bearing area where the
blood passes over the flow straightener and meets the
impeller blades. Expanding the hub area's width
increased the circulation of blood, eliminating
stagnant sections where clotting was known to occur.
Additionally, researchers tapered the hub surface,
accelerating blood flow, and thus creating good wall
washing. And third, the exiting flow angle of the
blood was examined and the diffuser angle was
repositioned. Changing the diffuser blade angle aligns
it with the blood flowing through the device, creating
a smoother transition of blood over pump surfaces,
and reducing the shear stress that causes cell
damage.
Clinical tests conducted by MicroMed Technology
and Baylor College of Medicine have confirmed
the improvement in performance - hemolysis was
decreased tenfold (figure 3). In collaboration with
designers at MicroMed Technology, modifications
made through the use of CFD analysis have resulted
in a device that can perform for more than 100 days.
The longest successful trial period to date in a human
was 110 days, after which a donor heart was transplanted.
The team's ultimate goal is to make the VAD
a permanent alternative to heart transplant surgery.
Successful European trials of the device in humans
suggest its ability to provide long-term ventricular
assistance.
Point of Contact: C. Kiris
(650) 604-4485
ckiris@mail.arc.nasa.gov
Back To Top
Previous Paper
Return to Revolutionary Technology
Next Paper 
