RWTH Aachen University

Perfecting a Life-Saving Blood Pump

By Grant Ellis

According to the American Heart Association, in a typical year only 2500 donor hearts become available in the United States for the 40,000 people who could benefit from them. This unpromising ratio raises the question: Will the patient survive long enough to receive a transplant?

A tiny, four-ounce pump, whose design is being refined with the help of Apple technology, may hold the answer.

The MicroMed DeBakey Ventricular Assist Device (VAD), an axial flow blood pump, implanted in the patient’s chest could bypass his ailing heart, circulating up to five liters of blood a minute to sustain his life until a transplant becomes available. Professor Marek Behr of Aachen, Germany, works with this technology everyday. He uses finite element analysis (FEA) techniques and a Xserve cluster and Mac desktops to make blood pumps such as the MicroMed DeBakey VAD even more durable and patient-friendly.

“Implantable blood pumps can last for over a year,” says Behr. “That’s long enough for people to find a transplant donor. But our ambition is to make it viable as a permanent replacement.”

Building a Better Cardiovascular Pump

Behr directs the Chair for Computational Analysis of Technical Systems (CATS) at the RWTH Aachen University. His team of engineers receives CAD models of the DeBakey VAD from MicroMed Cardiovascular Inc. of Houston, Texas, and subjects them to engineering analysis. The DeBakey VAD is already a medical and commercial success — it has been implanted in nearly 400 patients. But MicroMed and CATS continue to look for ways to improve it.

The pump that CATS engineers are working on is a titanium tube approximately one inch in diameter and three inches long. It is capable of keeping a cardiac patient alive by pumping 300 liters of blood an hour. It pumps steadily, without a pulse.

“Previously, we always had to use machines like SGI or Sun for scientific work and desktop machines for office work. Finally, with Mac OS X, these two worlds have come together.”

“It’s a ventricular assist device — a bypass mechanism that works outside the heart,” says Behr. “Think of it as a jet engine that connects two cannulas, or tubes. Its inflow cannula is usually implanted into the left ventricle, and its outflow cannula is implanted in the aorta. It takes over the heart’s job of pumping blood from the ventricle into the aorta.”

The blades of the impeller, the pump’s only moving part, contain tiny magnets. A brushless DC motor stator surrounds and drives the impeller at around 10,000 RPM. An electrical conduit leads to an external battery. For long-term implantation, surgeons may prefer to reduce the risk of infection by using transcutaneous power transfer, in which power can be transferred through the skin between two electromagnetic coils.

Muscular Computing

The pump may be small, but it takes major computing muscle to run the FEA flow studies that Behr and his team are using to refine its design. CATS chose a 44-processor Xserve cluster to power the program.

“We convert the MicroMed CAD models to a finite element mesh,” says Behr, “and we use the mesh to simulate a fully developed flow field on the Xserve cluster using our own computational fluid dynamics (CFD) software. Each simulation is a series of about a thousand time steps, each step with five to ten million finite elements. We needed massive compute power for this process, and once we made our comparisons it was easy to choose the Apple cluster.”

CATS uses these compute-intensive simulations to explore the potential of each design modification, running a variety of flow profiles, flow rates, and impeller speeds to find the best way to improve the flow pump’s biocompatibility. Could it be reduced in size to make it more suitable for young patients? Would a change in the geometry of the impeller blades or the stators reduce hemolysis? Hemolysis — the release of hemoglobin into the bloodstream — can result from damage to red cells. It is a potential danger to internal organs and can be life-threatening in extreme cases.

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