Craig Miller MD

Thelma and Henry Doelger Professor of Cardiovascular Surgery Stanford University Stanford, California

The conventional surgery to repair an aneurysm of the descending portion of the thoracic aorta is a major operation. It requires a large incision between the ribs on the left side of the chest and has a substantial risk of death or serious complications, including stroke and lower body paralysis. After surgery, the hospital stay can range from one to two weeks followed by months of convalescence and rehabilitation. It is much more complex and risky surgery than coronary artery bypass graft or heart valve surgery.

Our surgically treated aneur-ysm patients are very grateful to be alive and free of the risk of aneurysm rupture but usually don't get back to feeling normal until three or more months after the operation. Despite this difficulty, surgical treatment of thoracic aorta aneurysms has saved innumerable lives since the late 1950s — and otherwise these aneurysms are almost universally fatal. Similar to a "blister" on a

worn-out car tire, the aneurysm may blow out unpredictably.

Unfortunately, most of the aneurysms do not cause symptoms, such as back or chest pain, until they are very large. They are commonly discovered only serendipitously, for instance when a chest x-ray is ordered to evaluate other symptoms. Patients with thoracic aortic aneurysms often do not have any warning that they have a life-threatening aortic problem until something catastrophic occurs.

Thanks to better diagnostic tests (mostly imaging techniques such as CT, MRI scans, and echocardiography) and longer life spans, the number of patients diagnosed with an aneurysm of the thoracic aorta has grown rapidly over the last decade. Patients at the highest risk of having an aneurysm are middle-aged to elderly people who have a history of high blood pressure; younger individuals born with "weak" aortas that they inherited; and those with a family history of aortic aneurysm or aortic dissection.

In the last two decades, minimally invasive techniques, often using catheters and smaller incisions, have been developed to treat more heart and cardiovascular problems. This treatment results in less pain and trauma to the patient and shorter hospital stays. Indeed, many heart and peripheral arterial problems that ten years ago required a week in the hospital can now be treated on an outpatient basis.

Because the surgery done to repair thoracic aortic aneurysms is so traumatic and the recovery process so long compared with that for many other cardiovascular problems, there is good reason to use minimally invasive techniques. Conventional surgical treatment of aneurysms requires replacing the weakened segment of the aorta with a Dacron tube graft.

To accomplish this using minimally invasive techniques and without opening the chest, the aneurysm must be covered with a tube placed inside the aorta. This blocks the high-pressure blood flow from entering the thin, weakened aneurysmal segment and eliminates the chance of aortic rupture. This inner graft, or "sleeve," must be anchored firmly on either end so it cannot migrate over time.

This device is called a "stent-graft," or covered stent. In a stent graft, an expanding metal stent is used to anchor the synthetic tube graft to the normal aortic wall. This stent is unlike the more commonly used uncovered stents, which are open metal frameworks that crush plaque against the arterial wall and open a bigger channel for more blood flow.

At Stanford University Medical School, we have been exploring the use of endovascular, or in-vessel, stent-grafts to treat various types of aneurysms of the descending thoracic aorta (Fig. 13.8) since 1992. To the surprise of some and the utter amazement of others, these pioneering efforts have been fairly successful. The clinical feasibility of using stent-grafts for descending thoracic aortic aneurysms has been firmly established, even though the learning curve was fairly steep. We learned which specific types of aneurysms are best suited to stent-grafting, which patients could be treated successfully, and many essential technical points about gaining access to the aorta, device design, and stent-graft deployment.

We conducted the first large-scale clinical trial of descending thoracic aortic endovascular stent-graft repair in 103 patients between 1992 and 1997 at Stanford University. The average age of the patients was sixty-nine years. Importantly, 60 percent of cases were judged by a cardiovascular surgeon to be otherwise inoperable.

In this preliminary first-generation study, a primitive self-expanding stent-graft device was used. This "home brew" stent-graft used self-expanding stainless steel covered with woven Dacron. The device was semirigid and quite large in diameter (10mm-15 mm, or more than one-half inch). Various types of aneurysms were treated, including atherosclerotic/degenerative aneurysms, a few aortic dissections, and others. Although the stent-graft was intended to be inserted in the groin by using a small incision and general anesthesia, the large size of this early device and/or arterial blockages in the pelvis and abdomen made this possible in only 58 percent of cases. A larger incision in the left flank was therefore necessary in 30 percent.

Immediate serious complications included fatal aortic perforation in one patient, obstruction of the aortic arch due to buckling of the stent graft in another, stent-

graft misdeployment outside the target zone in 3 percent, and a major peripheral arterial injury in 4 percent. The early mortality was 9 percent, which was quite good considering how old and sick many of these patients were.

Early neurological complications, which are the most dreaded complications of this type of major surgery, included paraplegia (paralysis of the legs and lower body) in 3 percent and stroke in 7 percent. The incidence of paralysis was similar to that for open surgical repair. The only risk factor associated with a higher probability of paraplegia was "more difficult surgical access," i.e., the need to insert the stent-graft via the abdominal aorta. We believe the stroke was caused by debris coming loose from the aorta and traveling to the brain in five of the seven cases, but two strokes were due to cerebral hemorrhage. No risk factors for stroke were identified.

The link between the design of our primitive device and stroke is indirectly substantiated by our more recent (1998) experience in an FDA Phase I trial in twenty-three carefully selected patients. We used a new commercial stent-graft called the Thoracic Excluder, built by W.L. Gore and Associates, Inc. In the entire FDA Phase I trial, which included twenty-eight patients, there were no strokes and no cases of paraplegia. We attribute this primarily to advanced design features, but more careful selection of patients could have also played a role. Nonetheless, the ability to avoid using large, stiff catheters and hardware inside the atherosclerotic ascending aorta and arch

Fig. 13.8: These illustrations show a stent-graft placed in the descending thoracic aorta. After the stent-graft has been placed inside the aneurysm, which can be seen bulging out to the right, blood flow can no longer enter the aneurysm sac. The aneurysm, which becomes a "blind pouch," then clots, and over time, it is hoped this blood clot will turn into scar tissue and the aneurysm sac will shrink. (Illustration courtesy of W.L. Gore and Associates, Inc.)

Fig. 13.8: These illustrations show a stent-graft placed in the descending thoracic aorta. After the stent-graft has been placed inside the aneurysm, which can be seen bulging out to the right, blood flow can no longer enter the aneurysm sac. The aneurysm, which becomes a "blind pouch," then clots, and over time, it is hoped this blood clot will turn into scar tissue and the aneurysm sac will shrink. (Illustration courtesy of W.L. Gore and Associates, Inc.)

is a key advance. Additionally, this new stent-graft conforms more easily to the curved aortic arch, is more flexible, and is considerably smaller. This gives us reason to expect that the incidence of stroke in the future will be much lower than before.

Given that 60 percent of the initial 103 cases treated with stent-grafts at Stanford University were deemed inoperable, our five-year clinical experience with the first-generation device indicates that endovascular stent-grafting of descending thoracic aortic aneurysms is feasible and relatively safe. The more refined devices available today cause much less trauma and enable more precise stent-graft deployment, which should further reduce the risks and make this procedure more reliable. These major design and technical advances, coupled with the lessons we have learned and more refined patient selection, should mean the results will be even better in the future.

Nonetheless, caution is necessary because very long-term follow-up is required before we can be completely confident this approach is a permanent solution. Only ten years or more of observation of greater numbers of patients will determine if stent-grafting is a durable and effective alternative approach to preventing aneurysm rupture. Until these long-term results are available, our Stanford multidisciplinary group believes younger, low-surgical-risk patients should opt for conventional open surgical graft replacement, which has a forty-year proven track record and is the gold standard. Conversely, stent-grafts are a reasonable option for patients who are not surgical candidates owing to advanced age or other coexisting medical problems and who otherwise cannot be offered any form of treatment.

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