Evolving management of type A aortic dissection
Standard classification of aortic dissection
Acute type A aortic dissection continues to be a catastrophic event that requires emergent surgical intervention. The process typically starts with an intimal tear in the midascending aorta, which allows blood to split the "at risk" media proximally toward the aortic root as well as distally into the remaining ascending aorta, into the arch, and, in the majority of patients, down the descending thoracic aorta to the abdominal aorta. If untreated, the early mortality is due to:
- Proximal acute aortic valve insufficiency leading to acute pulmonary edema
- Compromised perfusion of the coronary arteries leading to ischemia, myocardial infarction and lethal arrhythmias
- Malperfusion of the arch vessels, leading to large strokes
- Leakage and rupture of the ascending aorta, leading to pericardial tamponade or exsanguination
The surgical treatment of acute type A dissection must address the structural abnormalities that lead to death if uncorrected, most commonly within 48 hours of the onset of ascending intimal tear development. The aortic valve is resuspended, if feasible, and the aortic root can be stabilized by various techniques. Typically, with use of Teflon stent buttressing to prevent coronary dissections and malperfusion, the ascending aorta is replaced in its entirety as it is the at-risk aortic segment for rupture. The aortic arch also is stabilized to minimize the risk of cerebral malperfusion, with techniques ranging from open distal anastomosis buttressed by Teflon felt to total arch replacement.
"In large centers, the surgical outcome of acute repair has improved, with reports of acute mortality as low as 10 to 15 percent, although Medicare data suggest that in the United States, the overall average mortality from acute type A dissection remains higher than 25 percent," according to Alberto Pochettino, M.D., a cardiovascular surgeon at Mayo Clinic in Rochester, Minn.
Despite progress in the outcome of the proximal repair, the majority of patients who present with acute type A dissection are left with residual dissection beyond the end of the initial repair, typically at the distal arch, with dissection that extends down to the iliac bifurcation. Essentially, in these patients, the dissection is converted from a type A to a type B anatomically, which is then managed medically.
The early morbidity and mortality of a native type B dissection or a residual type B after repair of a type A are low, but up to 80 percent of these patients will experience aneurysmal degeneration of their remaining dissected aorta. Aneurysmal degeneration can be particularly accelerated in the proximal descending thoracic aorta, where expansion of the false lumen has been documented to be as much as 4 mm a year. As the descending aorta reaches a maximal diameter of more than 6 cm, the risk of rupture is significant and surgical intervention is indicated. "These late interventions can carry a high risk, especially if undertaken in an urgent-emergent setting," says Dr. Pochettino.
While significant advances have been implemented over the past decade to better treat such catastrophic disease, there remains much room for improvement. The first questions relate to the techniques used during the acute repair. As the patients are often unstable at presentation, it is important to recognize that survival is the first priority. Sound repair of the aortic root and complete replacement of the ascending aorta are imperative. Inadequate repair and replacement can account for some of the early mortality and lead to progression of root aneurysm and worsening aortic valve insufficiency, eventually requiring re-intervention.
Next, many surgeons underestimate the importance of stabilization of the aortic arch at the time of acute presentation. The variations in procedures used for arch stabilization are in part related to the lack of familiarity of many cardiac surgeons with surgical intervention on the aortic arch. While it is rarely necessary to perform a total arch replacement in the setting of acute dissection, prevention of devastating cerebral malperfusion by appropriate arch repair likely reduces acute mortality. Thus, the most commonly performed procedure is a buttress extended hemiarch technique.
The optimal treatment approach to acute presentation of an individual to a local emergency department remains controversial. It may be difficult to decide whether a potentially unstable patient is best served by being operated on expeditiously at an institution unfamiliar with aortic surgical interventions and a mortality of more than 25 percent or whether rapid transport to an experienced tertiary center with a mortality of less than 15 percent is worth the risk potentially incurred during the transport period. Data about timing between diagnosis and surgery suggest that, in most instances, several hours elapse before surgical intervention is carried out, and often delays are magnified in small communities where the cardiac surgical team is not readily available 24/7. The additional risk incurred during transport may be offset by the survival advantage of having the procedure performed in a large-volume aortic center.
But aside from the controversy of upfront mortality related to volume and expertise of the surgeons involved, the standard surgical technique leaves the patient with a type B dissection. Even if repair is carried out successfully with a good early outcome, all these patients need to be followed closely, and the majority require additional intervention.
One potential advance has been to address the residual type B dissection at the time of the acute repair. Certainly, this only makes sense when applied at a center where the acute early mortality is low, as adding more upfront surgical interventions may increase the morbidity and mortality of the primary operation. A technique designed to stabilize the dissected descending thoracic aorta involves placement of a stent graft in an antegrade fashion across the open arch at the time of primary proximal repair. Such techniques have been championed over the past five years by Dr. Pochettino. This technique is sometimes termed "a frozen elephant trunk," reflecting the fact that the stent graft is inserted in an antegrade fashion at the time of hypothermic circulatory arrest.
The idea of adding this technique to the acute treatment of type A dissection occurred following increased familiarity and success in treating complicated type B dissection with endovascular techniques. The main obstacle to generalized use of stent grafts in uncomplicated type B dissection is the low but significant risk of retrograde type A dissections. It would stand to reason that if the overall operative risk is not affected by stenting of the residual thoracic dissection up front, the benefits of acute stenting of type B dissection should be realized without the possibility of retrograde type A dissection.
Indeed, no significant differences in the acute mortality or morbidity were demonstrated in 55 acute DeBakey I dissections treated with additional antegrade stent grafting between June 2005 and June 2012, compared with 355 acute DeBakey I dissections treated with standard repair between June 1993 and June 2012. Overall, the acute mortality was 11 percent. During follow-up, no patients treated with antegrade stenting developed distal thoracic or thoracoabdominal dissecting aneurysms compared with the group operated on with the standard optimal proximal repair, in which almost 30 percent required open re-operation.
Mayo Clinic in Rochester, Minn., is a large-volume tertiary center where expertise is available to manage all aspects of aortic dissection, from the acute setting, where the standard technique can be complemented with a frozen elephant trunk, to the chronic setting, regardless of where the primary operation may have been performed. Cardiologists, geneticists, internists, cardiothoracic surgeons and vascular surgeons at the Thoracic Aorta Clinic work as a team to optimize diagnostic, medical and surgical options to care for patients with aortic aneurysms and dissections, who are recognized to have often challenging multisystem anatomic and physiologic problems.