August 15, 1995

Engineers and surgeon work to optimize designs of branching blood Vessels

For decades, surgeons have treated arterial disease with procedures such as bypass grafting and angioplasty, resulting in improved blood flow for over two hundred thousand people a year. However, many patients return to the operating room for the same procedure again, suffer debilitating side effects of the surgery, and live with pain or discomfort for the rest of their lives.

Dr. Clement Kleinstreuer, professor of mechanical and aerospace engineering at N.C. State University, aims to keep as many people as possible from repeat surgeries and enhance their quality of life. With the help of Dr. Joseph P. Archie Jr., a vascular surgeon at Wake Medical Center, and Dr. George A. Truskey, associate professor of biomedical engineering at Duke University, along with several graduate students at both universities, Kleinstreuer is designing ways to keep blood circulation unimpeded after surgery.

Genetics and lifestyle choices--for example, eating a high fat diet--can contribute to arterial disease. However, the obstruction of curved and branching blood vessels can also result from irregular blood flow produced after the initial surgery to correct the arterial blockage.

Excessive scar tissue formation and new plaque accumulation (restenosis) can become problems as soon as a few months after the procedure. More frightening, thrombosis, the formation of a blood clot on the walls of blood vessels, can occur within hours and require emergency surgery to prevent a massive stroke or amputation of the affected limb.

Non-uniform hemodynamics, disturbed blood flow, is the core of the team's research. Truskey links non-uniform hemodynamics to the focal occurrence of the disease; Kleinstreuer, an expert in modeling fluid dynamics by solving mathematical equations on multiple workstations and supercomputers, identifies areas of disturbed flow. Kleinstreuer and Archie analyze the data and submit their results to Truskey, who can then confirm their predictions with his actual measurements.

Recent test results from Truskey's work uphold Kleinstreuer's hypothesis about wall shear stress gradient (WSSG), a way of measuring sites susceptible to the disease processes. The measurements can reliably point to areas where excessive tissue growth and formation of atherosclerotic lesions inside the arteries are likely to occur.

"The results are very intriguing," Truskey said. Now that a correlation has been established, he said, the next step in the research is to determine exactly how the WSSG affects events leading to the initiation of arterial disease.

Though the causal mechanism is still unclear, Kleinstreuer is confident about what the results mean. "You minimize the wall shear stress gradient with optimal blood vessel geometries and you reduce potential for restenosis and thrombosis, which leads to high sustained patency rates," he said. Patency rates refers to the length of time the artery remains open.

A bypass requires an incision, and the body manufactures scar tissue then, especially around the suture line. According to Archie, surgeons have choices about how to put arteries back together but generally have no clear guidelines for designing the best surgical reconstruction and graft artery bypass configuration.

By altering the geometric shape of the affected artery, doctors can greatly reduce wall shear stress gradients, help prevent accumulation of plaque on the artery's inner wall, and lessen the amount of scar tissue that forms there after the initial surgery. The team's research, therefore, gives surgeons a tool for determining the optimal design for bypass grafts and artery reconstructions.

The team's current research focuses on two of the human body's main arteries: the carotid artery in the neck and the femoral artery in the leg. Disease of the carotid artery can result either in a sudden massive stroke or a partial stroke.

More common are problems with the femoral artery. According to Archie, the people who benefit most from this research are those who suffer severe atherosclerosis in the legs, mostly people in their sixties and older.

Though this mechanical approach will prove helpful to patients, Archie believes that the solution to most of these problems will be found in pharmaceuticals. "The early solutions are probably going to turn out to be pharmacological," he said, "but we're investigating a side not many people are looking at."

Kleinstreuer pointed out that research into genetics may also yield future treatments for people afflicted with hereditary arterial disease, but he believes that pharmaceutical and mechanical approaches will play a role as well. "I see a three-pronged approach, and we take one of the three," he said.

The mechanical approach complements the others and could provide new techniques in an area of medicine where no significant surgical discoveries have been made in decades.

"We want to analyze, to improve, to design, to optimize," said Kleinstreuer. "We want to help."

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