Aneurysms result when the wall of a blood vessel becomes weak or damaged. Over time, the constant pressure of blood flowing through the weakened vessel can cause a section of it to slowly enlarge, creating a bulge. The majority of aneurysms are called AAAs because they are located in the abdominal aorta, a large blood vessel leading from the heart through the abdomen. Others can occur in the brain. They are called cerebral aneurysms.
The popular view is that aneurysms strike rarely, suddenly and unpredictably. In fact, most lethal AAAs are often easy to diagnose with an inexpensive ultrasound test, and can usually be treated. The question is when.
"Until now, all abdominal aortic aneurysms have been treated more or less the same way in that they are surgically repaired when the maximum diameter reaches a certain size – usually five centimeters," said David Vorp, Ph.D., assistant professor in the departments of surgery and bioengineering at the University of Pittsburgh School of Medicine and director of the Vascular Biomechanics and Vascular Tissue Engineering Research laboratories at the McGowan Institute. "We are looking at AAA in a more biomechanical way; that is to say that material failure, or a rupture, will occur when the stress at any point exceeds the strength of the material."
Dr. Vorp will be presenting these findings today and Saturday at "Bringing Basic Science into Clinical Practice," the 2003 Congress on Abdominal Aortic Aneurysms, an international symposium sponsored by the department of vascular surgery at the Karolinska Institute and Hospital. The symposium is designed for vascular surgeons, radiologists, researchers and other professionals interested in AAA treatment.
Using measurements derived from samples of vascular tissue removed during surgical repair combined with patient-specific information on risk factors such as smoking, family history, age, gender and AAA size, Dr. Vorp and his colleagues developed a statistical model to predict the local strength of one particular aneurysm wall. When this is combined with biomechanical analysis of stresses, a "rupture potential index" can be calculated with a range from zero to one, with zero representing a low risk of rupture, and one representing a high risk of rupture.
"This kind of knowledge may make it easier for a surgeon to decide whether it is time to operate," said Dr. Vorp, who also is a member of the division of vascular surgery at the University of Pittsburgh.
U.S. surgeons repair about 50,000 AAAs yearly, typically replacing the bulging area with a plastic or fabric tube. Some now use a less-invasive procedure that threads a tubular device called a stent-graft into the weakened area.
"But because surgical repair carries its own risks, being able to pinpoint when an AAA is likely to rupture gives surgeons a valuable tool," said Elena DiMartino, Ph.D., a research associate in Pitt's department of surgery who is a member of Dr. Vorp's team and worked on the study. "We know at a certain point that the aneurysm will rupture," she said. "We are trying to predict when." Only follow-up clinical studies can help to validate this new technology, Drs. Vorp and DiMartino say. While additional studies are planned soon, a clinical trial in patients is a more long-term project.
Over the past 20 years, the number of intact AAAs diagnosed in the United States has tripled to about 200,000 a year, partly because of the increased use of computed tomography, magnetic resonance imaging and other diagnostic scans. Most, however, are never diagnosed, resulting in the deaths of about 18,000 Americans a year from rupture – more than AIDS or brain cancer, and four times as many as cervical cancer. For more information on Dr. Vorp's research, visit http://www.surgery.upmc.edu/vorplab/default.htm.
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