The Next Frontier of Endovascular Aneurysm Repair

A priority is predicting improvement after endovascular treatment for aortic dissection.

Issue: May/June 2017

In certain patients with uncomplicated type B aortic dissection, thoracic endovascular aortic repair may be beneficial for preventing disease progression. The current challenge, however, is to not only understand who to treat but who should be treated better.

Recently, efforts have begun to incorporate factors related to imaging findings, physiology, likelihood of aortic remodeling and risk to better identify in which patients TEVAR is worth performing.

“We’re just entering the next phase. Once we get into this new field, we’re going to have to dig deeper out of necessity,” Michael Dake, MD, Thelma and Henry Doelger professor of cardiovascular surgery and medical director of the catheterization and angiography laboratories at Stanford Health Care, told Cardiology Today’s Intervention. “It’s going to have to be more sophisticated — another level of evaluation that actually looks at the physiology, the aortic wall itself and hemodynamic factors.”

Currently, most physicians are treating patients with uncomplicated type B aortic dissection with optimal medical therapy. Although TEVAR may be warranted in certain patients, the data do not offer a clear road map.

“We’re in a very early period of treating uncomplicated type B dissections,” Edward Y. Woo, MD, director of the MedStar Vascular Program, chairman of the department of vascular surgery and professor of surgery at Georgetown University, said in an interview. “Unfortunately, we’re unable to predict which patients will benefit from TEVAR with absolute certainty.”

Additionally, there is little consensus about how to approach aortic dissection, according to Joseph V. Lombardi, MD, professor and chief of vascular and endovascular surgery and the department of surgery at Cooper University Hospital in Camden, New Jersey.

“Many people have different opinions on what’s right and wrong about treating dissection, and that is reflective of the fact that there are so many variables,” he said. “There is a lot of room for discussion and debate, so until we get some hard data that points in certain directions with these presentations and variables, we’re going to have a variety of different opinions tailored to their own particular expertise.”

Importance of Imaging

Imaging is critical for treatment of acute dissection, according to Woo. It allows physicians to identify the primary pathology, including the primary entry tear and multiple fenestrations of the thoracic aorta. Moreover, physicians can see the native anatomy of the patient, such as their aortic arch and the sizing of the aorta.

Currently, CT angiography is the mainstay of diagnosis and procedural planning, but new imaging techniques such as 4D MRI are emerging, Lombardi said. Additionally, procedural imaging can be facilitated with an endovascular suite and fluoroscopy in the operating room. IVUS is also a valuable tool in allowing physicians to know where the wires and catheters are, as they can freely cross over into the true and false lumen without the physician’s awareness.

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Dake said physicians need “a different level of imaging” if predicting benefit from TEVAR is to improve.

“We are going to need technical advancements in imaging to crack open all of these physiologic and hemodynamic opportunities that may have prognostic importance,” he said. Dake also noted that these techniques should be noninvasive and not subject the patient to an intervention. “Can a CT scan actually measure pressure? Can MRI determine flow? These are works in progress that we haven’t utilized yet because these techniques weren’t developed in an accurate and reproducible enough manner to apply to these complex problems. But it’s just part of peeling the onion. Ultimately, it will have benefit for patients.”

Physiologic Features

At this point, the role of physiologic features in aortic dissection is complex and speculative, according to Dake. He said most physicians believe that a mismatch in pressures between the true and false lumen may be an indicator of which patients are at risk for disease progression.

“If these pressure imbalances between inflow and outflow reach a certain point, you may start to see a stagnation of blood and thrombus within the false lumen, and we know that patients with partial thrombosis of the false lumen are most vulnerable to disease progression compared with not only a completely thrombosed lumen but a completely patent false lumen as well,” Dake said.

However, he said, this is just the current working theory. These pressure imbalances may not play an important role in identifying which patients will do well after treatment, he said.

“A noninvasive method of accurately determining these physiological variables, pressures and flows is going to go a long way to helping us unravel what we should do next,” Dake said.

Aortic Remodeling

Favorable aortic remodeling can be a significant factor in treatment success and preventing future aneurysmal degeneration, according to experts Cardiology Today’s Intervention interviewed. Therefore, identifying which patients will have favorable remodeling is an important factor in making treatment decisions.

“We know that the ability of the aorta to plastically remodel after TEVAR is much greater in an acute or subacute setting than in a chronic setting, where the patient has developed aortic dilatation and possibly even an aneurysm as the indication for treatment,” Dake said. “We also know that to achieve remodeling in most cases, we need to have thrombosis of the false lumen, which usually portends a high likelihood of aortic remodeling in the acute or subacute phase. If there is still flow or patency of the false lumen, it means it’s still pressurized and probably unlikely to shrink and remodel.”

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Overall, Woo said the ability to seal the primary entry tear and as many fenestrations or connections between the true and false lumen as possible while also maximizing expansion of the true lumen, maximizing pressure of the true lumen and reducing pressure of the false lumen can help maximize aortic remodeling.

However, complete thrombosis of the false lumen is rare, according to Lombardi.

“Sometimes, the goal, particularly in patients with rupture, is to shut down the vulnerable areas of the aorta and worry about what’s left over in terms of false lumen flow in the visceral segment or the abdominal aorta,” he said. “With that, we are learning that not all patients need to have their false lumen thrombosed to have a good outcome.”

Risk Prediction

Although questions remain, researchers are making strides in using various risk factors to help predict which patients will do well after TEVAR.

In a recent study published in Circulation: Cardiovascular Imaging,Anna M. Sailer, MD, PhD, from the department of radiology at Stanford Health Care in California, Dake and other investigators identified five significant predictors of late adverse events in patients with uncomplicated type B aortic dissection:

connective tissue disease;
circumferential extent of the false lumen in angular degrees;
maximum aortic diameter in millimeters;
false lumen outflow in milliliters per minute; and
total number of intercostal arteries.

“We published [the Stanford Aortic Dissection Risk Calculator] an app in which physicians can plug in these factors, which then gives a high-, medium- or low-risk prognosis for their patient. Using this as opposed to isolated individual features of dissection seems to have more predictive power,” Dake told Cardiology Today’s Intervention.

These results could have important clinical implications should they be validated in further studies, Troy M. LaBounty, MD, and Kim A. Eagle, MD, both from the University of Michigan, wrote in an editorial accompanying the Circulation: Cardiovascular Imaging publication.

“This study suggests a possible path forward that could improve our ability to risk stratify and treat these patients,” LaBounty and Eagle wrote. “Such developments could result in a seismic shift in our approach to this disease, and we look forward to seeing where this path leads.”

Clearly, Woo said, the field is moving forward.

“We understand things better,” he said. “We’re beginning to treat patients with uncomplicated type B aortic dissection with TEVAR, particularly those whom we think we can predict will do well and in whom we think treating early with a minimally invasive procedure like TEVAR can potentially not only save their lives but also prevent the need for further intervention that is a lot more difficult and complex for the patient in the future. It’s very exciting.” — by Melissa Foster

References:
LaBounty TM, Eagle KA. Circ Cardiovasc Imaging, 2017;doi:10.1161/CIRCIMAGING.117.006323.
Sailer AM, et al. Circ Cardiovasc Imaging. 2017;doi:10.1161/CIRCIMAGING.116.005709.
Stanford Aortic Dissection Risk Calculator. http://web.stanford.edu/group/3dq/cgi-bin/typeb-aortic-dissection-and-risk-calculator/calc-absolute/index.html. Accessed May 1, 2017.

For more information:
Michael Dake, MD, can be reached at Falk Cardiovascular Research Center, 300 Pasteur Drive, Stanford, CA 94305.
Joseph V. Lombardi, MD, can be reached at lombardi-joseph@cooperhealth.edu.
Edward Woo, MD, can be reached at edwardy.woo@medstar.net.

Disclosures: Dake, Lombardi and Woo report no relevant financial disclosures.