Taking the Guesswork out of Surgical Planning How BME professor Alberto Figueroa’s patient-specific blood flow simulations help clinicians find the ideal surgical path

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by Aimee Balfe

Last summer, Alberto Figueroa’s BME lab achieved an important “first” – using computer-generated blood flow simulations to plan a complex cardiovascular procedure.

“I believe this is the first time that virtual surgical planning was done for real and not as a retrospective theoretical exercise ,” says Figueroa.

Using a patient’s medical and imaging data, Figueroa was able to create a model of her unique vasculature and blood flow, then use it to guide U-M pediatric cardiologists Aimee Armstrong, Martin Bocks, and Adam Dorfman in placing a graft in her inferior vena cava to help alleviate complications from pulmonary arteriovenous malformations (PAVMs).

“I believe this is the first time that virtual surgical planning was done for real and not as a retrospective theoretical exercise.” Alberto Figueroa

The PAVMs – abnormal connections between the patient’s veins and arteries – are a common complication of a procedure performed early in the lives of children born with only a single functioning ventricle. Called the Fontan procedure, the operation had rewired the patient’s pulmonary circulation so that the venous return bypassed the heart and was connected directly to the pulmonary arteries for transport to the lungs.

While these surgeries can be lifesavers, the long-term consequences depend heavily on how evenly blood flow is distributed between a patient’s lungs. Patients with ideal hemodynamics do well; those with less-than-perfect flow patterns suffer a sting of life-threatening complications.

Unfortunately, the 20-year-old on whom Figueroa and his team were working had suffered many such complications, among them, the PAVMs that left her with low blood oxygen and elevated cardiac output. The surgery aimed to improve these measures by better balancing the flow to her lungs.

Simulation & Outcome: A Perfect Match

“This endovascular procedure had only been attempted once in the country before,” says Figueroa. “What we brought onto the table was, instead of going in blind, we’d simulate multiple slightly different ways of doing the procedure to see if there was an optimal one.”

The medical team gave his lab a month. Armed with detailed anatomical data from CT scans, Doppler data on velocity in various vessels, invasive catheterization data that showed pressures at multiple locations, and perfusion data from nuclear medicine tests, Figueroa’s team got to work. They first created a hemodynamic model of the patient that matched each of these data points. They then simulated six different ways of placing the stent graft using U-M's high performance Flux computing cluster, provided by Advanced Research Computing, to see if there was an ideal outcome. To Figueroa’s delight, one placement proved far superior.

They shared their recommendation with the medical team, and four days later, the cardiologists placed the graft with millimeter precision. The results amazed everyone – except Figueroa.

“I’d asked them ahead of time to verify everything with an angiogram – using a catheter to flush dye through the patient’s vessel to illuminate the blood flow,” he says. “They did this before and after the procedure, and the results matched completely what our computer simulation had predicted.” (See image.)

“The clinicians were amazed, but we told them we were just solving Newton’s law.”  Alberto Figueroa

“The clinicians were amazed, but we told them we were just solving Newton’s law,”  he says modestly.

In truth, he says that having proof that his physics-based planning worked was the highlight of his year.

The low point was realizing that the successful procedure wasn’t enough to save a gravely ill young woman.

Before (left) and after (right) images from both angiograms (top) and the surgical simulations (bottom). Note the tight correlation between the simulations and angiograms as well as the significantly more even distribution of hepatic venous flow between the two lungs after the simulation-guided procedure. Credit: Kevin Lau, Alberto Figueroa.

Better Primary Surgeries

This patient’s passing hit the entire team hard, but Figueroa takes comfort in the belief that his simulations can allow surgeons to optimize initial procedures like the Fontan so that the complications this patient experienced – and the follow-up surgeries they require – may never happen at all.

Of the tens of thousands of patients undergoing Fontan operations each year, he says, roughly half experience major complications after 10 years. That’s because it’s almost impossible for surgeons to know exactly how to perform the procedure on patients with vessels of various sizes, shapes, and flows.

By accounting for these differences, Figueroa hopes his simulations will show surgeons where in the vasculature to make the surgical connections so that blood flow is ideally balanced between the lungs in each patient. He plans to continue working with Dr. Dorfman, who initiated the surgical planning collaboration, and U-M cardiac surgeon Edward Bove to do patient-specific Fontan planning.

And because his simulations add a layer of insight to any procedure where cardiologists and surgeons find that doing things the same way works in some patients and not others, Figueroa hopes they’ll soon become a ubiquitous planning tool, much like imaging is today.

Additional Applications

As promising as it is, surgical planning is only the tip of the iceberg for Figueroa. His lab also works to further develop its simulation software and to use it to understand disease progression, always with an eye toward devising better treatments.

In the software arena, his lab is working on enhancements that will account for dynamic changes in blood flow caused by anything from a change in posture to anesthesia.

One of the lab’s clinical fellows is studying how blood vessels remodel in response to the grafts used in thoracic aneurysm repair. Another is modeling aortic dissection, aiming to discover precisely how the flap that shears from the vessel wall moves, deforms the aorta, and affects blood flow. This understanding is a first step toward designing a device specifically for this condition.

His lab also hosts BME students who are developing tools to better understand blood flow in the brain, clot-development in veins, and the progression of hypertension, including which types of vessels sustain various degrees of damage over time. Figueroa has recently submitted a collaborative grant to explore the progression of pulmonary hypertension, as well.

The breadth and clinical relevance of his work are in many ways why Figueroa came to U-M from King’s College, London, two years ago. Named the Edward B. Diethrich M.D. Research Professor of Biomedical Engineering and Vascular Surgery, Figueroa, a PhD, was drawn by his 50/50 appointment in BME and vascular surgery at an institution where medicine and engineering are deeply integrated.

It’s because of this connection that the rapid-response surgical planning was made possible, he says. It’s also given him ready access to talented students from the medical and engineering schools – and to usually hard-to-reach study participants, like aortic dissection patients, to gain critical insight into this and other life-threatening conditions.