Fluid Flow Studies Help Understanding of Aneurysms

By Ernie Tretkoff

Studies of fluids can lead to better understanding of aortic aneurysms, researchers reported at the APS March Meeting.

Anne-Virginie Salsac of the University of California, San Diego, described how changes in blood flow patterns contribute to the growth and rupture of abdominal aortic aneurysms.

Abdominal aortic aneurysms are abnormal dilations of the major blood vessel that supplies blood to the lower part of the body. They affect about 3% of the population over age 50. These bulges in the arterial wall often have no symptoms until they rupture, at which point as many as 80% of them are fatal. Men are more likely to be affected by this condition than women, and smoking, high blood pressure, and insufficient exercise are also risk factors. This is a major health issue, said Salsac, but much remains unknown about the problem.

Currently there is no technique to predict the expansion rate of abdominal aortic aneurysms or the critical size when they rupture, said Salsac.

Studies have shown that the mechanical forces exerted by the blood flow on the arterial wall play an important role in controlling the biological processes in the arterial wall cells, said Salsac. "Blood shear force controls the cell function and thus the structure and integrity of the arterial wall," she said.

But no one had actually measured these forces before. Salsac made the first measurement of the spatial and temporal distribution of forces along the wall of an abdominal aortic aneurysm. She measured these forces in a model of an aneurysms, while systematically changing the size and shape of the aneurysm.

Salsac found that because of the aneurysm’s bulged shape, the fluid flow separates from the arterial wall, and a large vortex develops. The vortex ring then hits the blood vessel downstream where the aneurysm narrows. This creates a region of very high stress on the arterial wall in the downstream part of the part of the bulge, while leaving regions of low force on the upstream artery wall.

The abnormally low shear stresses upsteam in the dilated region actually lead to endothelial cell dysfunction, weakening the arterial wall, and possibly causing the aneurysm to grow, reported Salsac.