The pressure is on
Aneurysms form when a weak segment in an arterial wall balloons under the pressure of blood flow. Just like a weak spot in an inner tube that begins to bulge, the weakened wall of the artery will eventually reach a point where it can no longer sustain this pressure, which is especially severe for people with hypertension. The result is a rupture in the diseased part of the arterial wall, which leads to massive internal hemorrhaging and most often death.

Occurring in arteries throughout the body, aneurysms can form in the brain, heart, intestine, neck, and other areas, but they most often occur in the aorta, which carries oxygenated blood from the heart to the organs. As the name indicates, the abdominal aortic aneurysm forms in a section of the aorta that runs through the abdomen, between the renal arteries and the iliacs which supply blood to the legs.

While much of how an aneurysm initially forms is clear to scientists, the progression of the condition is far from understood. Regardless of where aneurysms are located, it is difficult to predict their rate of development and when—if ever—they will rupture. Often, they expand sporadically, leaving doctors unsure when exactly treatment would be optimal. Equally confounding, a relatively small aneurysm in one patient can unexpectedly rupture, while a comparatively larger and older aneurysm in another patient may never rupture. Since treatment can be risky in many cases, doctors are continually faced with weighing the risks between simply monitoring aneurysms and intervening surgically.

Seeing inside aneurysms
In the preliminary phase of Santago’s work, the aim has been to take some of the mystery out of exactly how blood flow affects an aneurysm. Using medical imaging data provided by Dr. Mark A. Farber, Associate Professor in the Department of Surgery at the University of North Carolina at Chapel Hill, Santago created a virtual solid model of an actual aneurysm and visualized pulsatile blood flow as well as wall stress contours based on the computer simulation work of Dr. Kleinstreuer’s former and present graduate students Zhonghua Li and Christopher Basciano, respectively.

Blood flow patterns in an abdominal aortic aneurysm (Z. Li and C. Kleinstreuer, J. Medical Engineering & Technology, Vol. 30 (5); pp. 283-97).

“Once we’ve created a solid model which consists of many nodes, we can then figure out what happens at each of these nodes during a simulation. If we want blood to flow through the artery in a typical pulsing fashion, we can simulate this, outputting the data to a results file,” explains Santago.

Santago then loads the results file into EnSight, a visualization program from Computational Engineering International (CEI), to create a visualization. An important goal of his work, however, goes beyond simply creating a visualization of the current data. He has used EnSight’s ability to produce command files to create a tool that will serve graduate students in future AAA research. Any researcher will be able to essentially plug in his or her own results file to get a customized visualization.

“The idea is to get this to the point where anyone can take an instruction manual and the file and quickly adapts it to their own use,” says Santago.

The visualization has been created to clearly show the local flow rates and velocities over time as well as the Von Mises stresses. These results are all shown for the entire aneurysm in a sagittal view and are also shown for four key points in axial views. In addition to accurately representing these characteristics, part of Santago’s work was to devise graphics that also were the most user-friendly. For example, he worked with a number of color schemes before establishing the final version, which was perfected to represent the workings of an aneurysm in colors that allow for optimal contrast between blood flow and arterial walls. Seemingly minor efforts such as these will save future researchers valuable time and allow them to perceive the finest level of detail more clearly.

Simulating surgery
Santago’s work has now progressed to a second phase in which he is studying the effectiveness of inserting endovascular stent grafts to protect the arterial walls, which, in contrast to open surgery, is a relatively new but increasingly common practice used to repair AAAs. In this procedure, a tube of pliable material known as a graft is inserted inside the artery to shield the diseased area. Since the graft itself is not strong enough, a stent of wire mesh is added to serve as reinforcement, adding strength in much the same fashion as reinforcement rods in concrete.

Arterial wall pressure for a non-stented and stented AAA (A. Santago, Senior Project, MAE Dept., NCSU, December 2007).

EnSight is used to simulate the addition of a stent graft. “The pressure in an aneurysm can be very high, so the stent graft is put in place to shield and protect the diseased wall. Our simulations have shown that wall stress can be reduced by a factor of 20, and the pressure reduced by a factor of 8.5,” explains Dr. Kleinstreuer, who heads the research project.

Visualization work will continue in a third phase that also deals with stent grafts. While highly effective under ideal circumstances, stent grafts are subject to migration. In other words, the life-saving device can sometimes slip by just a few millimeters, causing a sudden rush of blood to flow into the previously protected aneurysm. Unfortunately, the sudden flow results in a rapid increase in pressure on the diseased arterial walls and generally causes a rupture. Less severe, seepage can also occur around the stent graft, which leads to an accumulation of blood in the aneurysms. While slower, this can also lead to dangerous levels of pressure on the weakened arterial wall. Furthermore, the stent wires forming the mesh surrounding the tubular graft may experience micro-cracks due to material fatigue caused by the pulsatile blood flow. That may lead to broken wires which poke holes into the graft material, causing major endoleaks.

The next phase of research at NCSU will investigate the factors and conditions that can lead to stent-graft fatigue or slippage to make certain that stent-graft implants for AAAs don’t simply delay ruptures. The research will continue to study the effects of blood flow on AAA-rupture risk and on the stent graft itself, using EnSight visualizations to discover AAA weak spots, the ideal configuration for stent-graft placement and other factors that will help prevent failure. In addition to the computational analyses and fluid-structure-interaction visualizations, laboratory testing with synthetic replicas of patient-specific AAAs will further validate the computer simulation results and evaluate AAA-rupture risk data as well as improved stent-graft designs.

As doctors continue to treat AAAs with stent grafts, the condition’s death toll is falling. And thanks to the efforts of Anthony Santago and Dr. Clement Kleinstreuer and his team, endovascular surgeons may be able to perfect treatments and techniques so that even more patients suffering from AAAs will one day live longer, healthier lives.