Acute Aortic Dissection: A Case for Specialized Centers

BY Y. JOSEPH WOO, M.D.

Despite multiple advances in the surgical treatment of acute type A aortic dissection, reported perioperative mortality rates obtained from registry data still often exceed 20%. The classic estimated 1% hourly mortality of untreated type A aortic dissection, which originated from the 1972 metaanalysis by Anagnostopoulos, likely overestimates early lethality in the current era of fast-acting, titratable, intravenous beta-blockers that also decrease dP/dt.

For these reasons, efforts toward the identification and transfer of patients to advanced surgical centers specializing in the treatment of aortic dissection are warranted and may result in overall improved probability of perioperative survival.

On multiple fronts, specialized centers offer patients highly advanced care. Experienced anesthesiologists can rapidly obtain vascular access and induce general anesthesia without causing wide blood pressure swings, which are dangerous in the setting of aortic dissection.

They can accurately use transesophageal echocardiography (TEE) to provide diagnostic confirmation, assess aortic valve function, examine for pericardial fluid and early tamponade, and later, perform surface carotid duplex ultrasonography upon initiation of cardiopulmonary bypass (CPB) and cross-clamping to ensure continued adequate carotid flow and cerebral perfusion.

Experienced perfusionists are familiar with the routine use of deep hypothermic circulatory arrest with retrograde cerebral perfusion or selective antegrade perfusion. Also available are other infrastructural components, such as intraoperative continuous electroencephalography, that can improve patient outcomes.

Most important, specialized centers offer advanced surgical reconstructive procedures performed expeditiously by experienced surgeons. At a minimum, aortic dissection repair should entail five components:

• Safe establishment of effective CPB, with awareness of malperfusion syndromes.
• Reconstruction of the aortic root and valve.
• Replacement of the ascending aorta.
• Arch reconstruction.
• Consideration of the distal aorta.

Cardiopulmonary Bypass

Malperfusion syndrome at the time of presentation comprises a potentially catastrophic constellation of problems that can spiral into an unsalvageable situation.

Coronary malperfusion can usually be addressed by rapid establishment of CPB. Mesenteric and renal malperfusion syndromes often are reversed after reestablishment of proper antegrade flow in the true lumen of the distal thoracic aorta (DTA) after arch reconstruction. Malperfusion of the spinal cord is often managed with appropriate hypertension. Upper- and lower-extremity malperfusion present particular challenges when selecting cannulation strategies.

Usually, unilateral femoral artery cannulation is sufficient for establishing safe, effective CPB. Occasionally, conversion to contralateral femoral artery cannulation or, rarely, bilateral femoral artery cannulation is necessary. Right subclavian artery cannulation via a sewn-on graft is usually effective.

In extreme scenarios in which extremity arteries are all ineffective, the Seldinger method of cannulating the true lumen of the distal ascending aorta under TEE or epiaortic ultrasonographic guidance has been effective.

Confirming the effectiveness of CPB entails a combination of:

P First, the surgeon observing no evidence of local change in appearance of the cannulated vessel,

P Second, the anesthesiologist using TEE to document flow in both the true and the false lumens and surface duplex ultrasonography confirming proper antegrade carotid artery flow, and

P Third, the perfusionist confirming proper flow rates and arterial resistance levels as well as coordinated temperature decrements in both the nasopharyngeal and bladder temperature probes that mirror the blood temperature drops measured at the pump and to a lesser extent the pulmonary artery catheter.

Occasionally, a cerebral malperfusion scenario will occur after the initiation of CPB and cross-clamping, despite all cannulation alteration attempts. This may manifest as absence of significant carotid flow on duplex imaging and/or failure of the nasopharyngeal temperature to decrease. This challenging scenario may require a brief and temporary termination of CPB, removal of the aortic cross-clamp, rapid fenestration of the dissection flap in the arch, rapid deairing of the vasculature, reclamping, and reestablishment of CPB.

Aortic Root/Valve

At a minimum, the aortic valve must be resuspended and the sinotubular junction tightened. A more complex reconstruction of the root may be necessary if the dissection has propagated down to the aortic valve annulus or especially if there is a reentry tear into one of the sinuses of Valsalva. Not all root reentry tears necessitate full root replacement. A dissection around the right coronary artery (RCA) and, occasionally, even reentry tears involving the RCA ostium can be reconstructed without the need for coronary artery bypass grafting (CABG). In extreme situations in young patients, a valve-sparing root replacement can be performed. Once reconstruction is completed and the heart is reperfused, significant ventricular dysfunction may indicate a left main coronary problem. This can often be addressed with left internal thoracic artery to left anterior descending coronary artery bypass performed on the beating heart while still on CPB.

Ascending Aorta

Complete elimination of dissected ascending aorta is critical. This is rarely possible without removal of the cross-clamp to address the proximal arch, nor is it possible without resection of aorta down to the sinotubular junction and subsequent detailed aortic root reconstruction. In situations where concomitant CABG must be performed, positioning proximal anastomoses on the Dacron graft poses a geometric challenge and the long-term patency may be impacted by the Dacron vein interface.

Arch Reconstructi

This procedure can be approached as an open distal anastomosis, as a hemiarch reconstruction, or as a total arch replacement. Attempts should be made with felt and surgical adhesive to obliterate the dissection plane at the origins of the cerebrovasculature if possible. Thorough flushing of any debris and deairing are essential prior to reestablishment of antegrade flow. Immediate hemostasis at this suture line is important because the posterior arch suture line is virtually impossible to repair later.

Distal Aorta

Management of the distal aorta at the time of ascending aortic dissection repair has classically involved establishing proper antegrade flow and thus pressurizing the true lumen of the descending thoracic aorta by properly reconstructing the arch. Newer concepts include creation of an "elephant trunk" type of landing zone for future stenting of the DTA, and immediate placement of a stent into the proximal DTA antegrade from the open arch during arch reconstruction in an attempt to more effectively obliterate the false lumen of the DTA.

All of these issues demonstrate that surgical management of type A aortic dissection is complex and best approached by centers with specialization and experience in treating the multiple facets of this disease.