Open repair for thoracoabdominal aortic aneurysms precipitated by chronic aortic dissection

Chronic dissection of the thoracoabdominal aorta may require surgical repair for aneurysm, malperfusion, or rupture. Endovascular repair is made difficult by a noncompliant dissection septum, visceral vessels arising from different lumens, and the common use of diseased aortic landing zones. Thus, open repair remains the gold standard in terms of favorable outcomes and durability. During thoracoabdominal aortic repair, we use a multimodal strategy to prevent spinal cord and visceral or renal artery ischemia; key modalities include cerebrospinal fluid drainage, left heart bypass with and without visceral protection, cold renal protection, and aggressive reimplantation of intercostal or lumbar arteries. Patients with chronic dissection require lifelong surveillance, as there is a significant risk for subsequent intervention on unrepaired aortic segments.


INTRODUCTION
Aortic dissection begins as a tear in the intima that leads to a separation of the layers of the media and the propagation of blood along a false lumen. Acute aortic dissection of the thoracoabdominal aorta is a complex condition that can be further complicated by rupture and organ malperfusion. After three months, aortic dissection is typically considered chronic and is susceptible to aneurysm formation induced by a pressurized false lumen and other factors; subsequently, although most patients with chronic aortic dissection are initially managed medically, surgical intervention may become necessary [1,2] .
In this manuscript, we describe the classification of aortic dissection, along with indications for repair, and highlight operative techniques for open repair of aneurysms related to chronic dissection of the thoracoabdominal aorta.

Classification
Aortic dissection is classified by the anatomical distribution and time from onset, which guide treatment strategy. The most commonly used anatomical classification systems include the Stanford classification, the DeBakey classification, and the Society of Thoracic Surgeons/Society of Vascular Surgery (STS/SVS) system [ Figure 1].
The Stanford classification characterizes the dissection on the basis of its location: dissection involving the ascending aorta is defined as type A, and dissection involving the descending thoracic or thoracoabdominal aorta only is defined as type B [3] . The DeBakey classification further characterizes an aortic dissection on the basis of its distal extent [4] : DeBakey type I aortic dissection is extensive, involving both the ascending aorta and the descending thoracic or thoracoabdominal aorta, whereas type II is strictly limited to the ascending aorta; DeBakey type III dissection originates distal to the left subclavian artery and is further subdivided into type IIIA, which is limited to the descending thoracic aorta, and type IIIB, which extends below the diaphragm [4] .
The STS/SVS classification was designed to characterize dissections of the aortic arch better and is based on the location of the originating tear. The aorta is divided into several zones (0 to 9); a type B dissection has an entry tear originating in zones 1 to 9 (distal to the innominate artery). The entry tear and distal extent of dissection zones are denoted by a subscripted zone number. Thus, extensive aortic dissection limited to thoracoabdominal aorta would be described as type B 3,9 [5] . The STS/SVS system also classifies aortic dissection by the time from the initial insult: Hyperacute is defined as < 24 h; acute, 1-14 days; subacute, 15-90 days; and chronic, > 90 days [5] .
Thoracoabdominal aortic aneurysms (TAAAs) are classified by the extent of repair, according to the E. Stanley Crawford classification system [ Figure 2] [6] . A Crawford extent I TAAA involves the descending thoracic aorta to the suprarenal aorta. An extent II TAAA involves the descending thoracic aorta to the aortoiliac bifurcation. An extent III TAAA involves the distal descending thoracic aorta (T6 aorta) to the aortoiliac bifurcation. Finally, an extent IV TAAA involves the aorta from the diaphragm to the aortoiliac bifurcation.

Acute aortic dissection
The initial management for acute aortic dissection is anti-impulse therapy (e.g., beta-blockers) to limit hemodynamic stress on the aortic wall and minimize propagation of the dissection by reducing heart rate, blood pressure, and the velocity of left ventricular contraction [7] . Our hemodynamic targets are a heart rate < 70 beats per minute and systolic blood pressure of 100-120 mmHg. If the patient continues to be hypertensive with adequate heart rate control, vasodilator therapy is indicated. Acute DeBakey type I or type II aortic dissection is a surgical emergency and requires the replacement of the dissected ascending aorta. Arch replacement with an elephant trunk may be indicated when the aortic arch is aneurysmal, the primary intimal tear is in the arch or proximal descending aorta, there is evidence of malperfusion, or the patient is young and has a heritable thoracic aortic disease (e.g., Marfan syndrome). The elephant trunk is an extension of Dacron graft that may be free-floating or combined with a thoracic endovascular stent (frozen elephant trunk); it is placed within the true lumen of the proximal segment of the descending thoracic aorta and incorporated into the distal anastomosis of the arch replacement. The elephant trunk facilitates subsequent descending thoracic or thoracoabdominal aorta repair by moving the extent of aneurysm repair distally, thereby avoiding distal aortic arch manipulation through a left thoracoabdominal approach. Patients who have undergone surgery to repair DeBakey type I aortic dissection tend to develop subsequent aortic dilatation in the chronic phase that necessitates downstream aortic repair.
The management of uncomplicated, acute DeBakey type III dissection is controversial. This type of aortic dissection also is managed with anti-impulse therapy. However, the natural history of DeBakey type III aortic dissections that are managed medically is poor: 60% of patients develop aneurysmal rupture or dilatation requiring surgical repair within 5 years [8][9][10][11] , and the 5-year survival rate of patients with untreated, chronic DeBakey type III dissection is only 60%-80% [12][13][14] . Predictors of aneurysm formation after DeBakey type III dissection include uncontrolled hypertension, an initial aortic diameter > 4.0 cm, an entry tear > 10 mm, partial thrombosis or continued patency of the distal false lumen, or a false lumen > 2.2 cm wide [5,7,[15][16][17] . Thus, there is a movement toward using thoracic endovascular aortic repair to induce aortic remodeling and obliteration of the false lumen to prevent late aortic complications [5] . If a DeBakey type III aortic dissection is complicated by malperfusion or rupture, our preferred strategy is an endovascular aortic repair to cover the primary entry tear, redirect flow to the true lumen, and depressurize the false lumen [5] . In rare circumstances, a complicated, acute DeBakey type III dissection may require open repair if endovascular repair is not technically feasible (e.g., if endovascular landing areas are inadequate) [18] .

Chronic aortic dissection
A chronically dissected aorta is susceptible to dilation and late aneurysm formation that necessitates repair [ Figure 3]. The American College of Cardiology and American Heart Association guidelines for patients with aortic dissection of the thoracoabdominal aorta recommend open aortic repair when aortic diameter exceeds 5.5 cm (class I recommendation; level of evidence B); the diameter-based threshold is lowered in patients with rapid aortic growth (> 0.5 cm/year) or heritable thoracic aortic disease [7] . Symptomatic aneurysms can signal impending rupture and should be urgently repaired, irrespective of aortic dimensions [7] . Other indications for repair include malperfusion syndromes or symptoms due to compression of surrounding structures [7] . Patients with aneurysms precipitated by chronic dissection are different from those with degenerative atherosclerotic aneurysms: they are typically younger, have fewer comorbidities, and are more likely to have heritable thoracic aortic disease [19] . In general, open thoracoabdominal aortic repair of chronic dissection is preferred over experimental endovascular repair, a technique that is not yet widely available. In open repair, the thickened and stiff dissection septum is excised, and a common lumen is created by replacing it with an aortic graft. As a result, only the aneurysmal or symptomatic aortic segment is replaced, and a portion of residual chronic aortic dissection often remains.
Conversely, endovascular treatment of chronic dissection within the thoracoabdominal aorta is limited by the thickened and noncompliant dissecting septum, which prevents the elimination of false lumen blood flow even if the main entry tear is covered. Retrograde blood flow into the false lumen may persist at the distal landing zone, and the stent graft may introduce new entry tears [20,21] . Techniques to mitigate the influence of the noncompliant dissecting septum include: (1) using coils to embolize or plugs to obliterate the false lumen; or (2) creating a common aortic lumen by fenestration, stent-graft-assisted balloon aortic septal fracture, or provisional extension with a bare-metal stent to induce complete thromobosis [22][23][24][25] . Even so, the aortic diameter does not improve in 70% of patients with chronic dissection who undergo endovascular repair [26][27][28] . Open repair of aneurysms precipitated by chronic aortic dissection remains the gold standard.

SURGICAL TECHNIQUE AT THE BAYLOR COLLEGE OF MEDICINE
Our operative technique for open repair of Crawford extent II TAAAs precipitated by chronic aortic dissection involves the placement of a four-branched replacement graft. We prefer to use a prefabricated branched graft because (1) the visceral and renal arteries are often displaced in a chronic aortic dissection; (2) eliminating diseased aortic tissue mitigates the risk for the subsequent formation of an island patch aneurysm; and (3) reducing the anastomotic tension decreases the likelihood of late pseudoaneurysm formation.

Preoperative imaging
Meticulous analysis of preoperative imaging is the mainstay of TAAA repair. We look for calcification, dissection, and mural thrombus at potential aortic clamping and cannulation sites. Hypothermic circulatory arrest may be necessary if no suitable proximal aortic clamping site is available because of the aneurysm or a contained rupture. At our institution, we prefer a staged approach for treating patients with arch aneurysms (especially those related to chronic aortic dissection), in which we perform total aortic arch replacement by using an elephant trunk extension technique to set up the subsequent thoracoabdominal aneurysm repair. We rarely use hypothermic circulatory arrest during thoracoabdominal aortic repair, given the ensuing potential for coagulopathy and lung injury. The origins of important branching vessels, including the arch vessels and intercostal, lumbar, visceral, renal, and iliofemoral arteries, are assessed with respect to the true and false lumens. The spatial orientations of the renal and visceral arteries are considered in decisionmaking about the use of an island patch or individual branching grafts. Anatomic variations of the aorta, its branches, and the visceral organs also should be reviewed.

Preparation, monitoring, and adjuncts for spinal cord protection
Standard intravenous access is obtained by including a central line, a pulmonary artery catheter, and an arterial line in the right upper extremity for hemodynamic monitoring. A double-lumen endobronchial tube is used to facilitate single-lung ventilation. We have found that a bronchial blocker provides inferior lung isolation and results in a higher incidence of postoperative pulmonary contusion due to manual retraction of the lung. The patient's temperature is passively cooled to mild hypothermia (32-33 °C). Core temperature is measured in the nasopharynx. Cerebral oximetry is used to monitor cerebral perfusion throughout the procedure.
We use a multimodal strategy to prevent spinal cord, visceral artery, and renal artery injuries during TAAA repair. The mean arterial pressure (MAP) is kept at 70-90 mmHg, measured from the right-sided upper extremity. In most cases, a left heart bypass (LHB) circuit with a cell saver for autotransfusion is used; however, a complete cardiopulmonary bypass circuit is needed for cases requiring hypothermic circulatory arrest (18-20 °C). In addition, cerebrospinal fluid (CSF) drainage is used for Crawford extent I and II TAAA repairs, extent III repairs when iliac artery replacement is anticipated, and extent IV repairs when previous descending thoracic aneurysm or extent I repairs are present. The CSF pressure is kept below 15 mmHg, and CSF is drained at a maximum rate of 10 mL/h or 25 mL over 4 h. Importantly, we use passive mild hypothermia, sequential cross-clamping, aggressive reimplantation of suitable intercostal or lumbar arteries, cold renal solution to protect the kidneys whenever possible, and isothermic blood from the LHB circuit to perfuse the celiac axis and superior mesenteric artery (SMA) [29][30][31] .

Positioning, exposure, and left heart bypass
The patient is placed in a modified right lateral decubitus position, with the upper body kept at 60 degrees and the hips kept at 30 degrees by using a suction-deflated beanbag [ Figure 4]. The left arm is positioned in a freestyle swimming position at an angle above the shoulders. The operating table is gently flexed slightly superior to the anterior superior iliac spine. This position allows for exposure of the TAAA and the femoral arteries. A sigmoid skin incision is made beginning a finger breadth below the left scapula and coursing along the seventh rib, across the costal margin, to the left of the umbilicus. The ribs are counted in the plane deep to the serratus muscle along the spine, and the 6th intercostal space is entered. A segment of the posterior rib may be removed to improve the exposure of the thoracic cavity. The diaphragm is divided circumferentially to preserve the neurovascular supply, leaving a 3 to 4-cm rim of tissue from the left costal margin to the aortic hiatus. Taking care to avoid injury to the visceral organs, we cautiously open the peritoneum. Left medial visceral rotation is carried out by entering the plane along the line of Toldt. The left renal artery is identified and used as an anatomical landmark. The esophagus is located by palpating the nasogastric tube, and the left ureter is retracted medially with the viscera. Meticulous hemostasis is maintained during this dissection to prevent blood loss, impaired oxygen delivery, and coagulopathy. The proximal aorta is circumferentially freed, while the left recurrent laryngeal nerve is protected.
Adhesions from a previous DeBakey type I aortic dissection repair can interfere with preparing the proximal aorta for a cross-clamp. Placing an elephant trunk graft extension during the previous proximal aortic repair simplifies subsequent TAAA repair [32] . If the proximal repair during the index DeBakey type I aortic dissection included only the ascending aorta and hemiarch, a redo sternotomy and total arch replacement with an elephant trunk extension may be performed before the TAAA repair because an aortic arch aneurysm would be difficult to replace through a left thoracoabdominal exposure. Epiaortic ultrasound is helpful for locating the free-floating elephant trunk graft within the proximal portion of the descending thoracic aorta.
Left heart bypass to provide isothermic, self-oxygenated blood perfusion is facilitated by inserting a drainage cannula in the left inferior pulmonary vein and a return cannula in the distal aorta or femoral artery [ Figure 5]. A careful review of preoperative imaging aids the selection of an ideal cannulation site, one that is free of thrombus and within the true lumen. However, multiple entry tears are common in the dissecting septum, and perfusion is typically maintained even if the false lumen is cannulated. Epiaortic ultrasound may also aid in identifying a distal aortic cannulation site in the true lumen. Heparin is administered (1.5 mg/kg) to achieve an activated clotting time > 280 s before LHB initiation at a flow rate of 3.5 L/min once the aortic cross-clamp is applied.

Proximal anastomosis
A straight, padded aortic cross-clamp is applied, distal to the left subclavian artery whenever possible; it is commonly placed proximal to the left subclavian artery in patients with chronic aortic dissection because the pattern of aortic dilatation often involves the distal arch. If a cross-clamp is placed proximal to the left subclavian artery, then a stiff bulldog clamp is applied across the left subclavian artery [ Figure 6]. If an elephant trunk graft is present, the distal aspect of the graft is identified by using epiaortic ultrasound, and a vascular clamp is applied across the aorta and elephant trunk. A Crafoord clamp is applied across the aorta at the distal aortic clamp site; most typically, this is at the T6-T7 vertebral level to preserve flow where the artery of Adamkiewicz commonly arises (T7-L2) from the LHB. The isolated aortic segment is checked to determine if it is depressurized by gently milking the blood. The LHB flow rate is increased to 3.5 L/min, and a MAP of 70-90 mmHg is maintained. To protect the brain, cerebral oximetry is monitored to ensure that blood flow to the left common carotid is not compromised. The aorta is opened longitudinally, and the aortic edges are retracted with silk stay sutures. The chronic dissection membrane is excised, recreating a single lumen. Briskly back-bleeding intercostal arteries are ligated with figure-of-eight sutures to improve visualization, minimize hemorrhage, and prevent shunting of blood from the spinal cord. In instances of hypothermic circulatory arrest, fenestration is carried out proximally. The aorta is trimmed for the proximal anastomosis, leaving a rim of approximately 3 cm to facilitate sewing. The back wall of the aorta is dissected away from the esophagus to avoid including esophageal tissue in the anastomosis, which could result in the late formation of an aortoesophageal fistula. The distance from the proximal anastomosis to the visceral branches is estimated by gently stretching the branched graft and trimming it to an approximate length. We typically use a 24 to 28-mm gelatinimpregnated Dacron graft that is soaked in rifampin.
We begin the anastomosis at the posterolateral corner, continuing medially while ensuring that all layers of the aorta are included. A 3-0 polypropylene suture is used in a continuous fashion; however, a finer suture is often used in patients with heritable thoracic aortic disorders. When an elephant trunk is present, a graft-tograft anastomosis is performed. In cases involving a previous frozen elephant trunk repair, one or more rows of the metal struts may need to be removed before the stented portion of the device is sewn to the Dacron graft [ Figure 7]. When the aortic arch is aneurysmal or an upstream aortic repair may be needed, a reverse elephant trunk may be used to simplify the subsequent proximal aortic repair. A reverse elephant trunk entails invaginating an 8-cm portion of the Dacron graft, with the folded edge of the invaginated graft anastomosed to the aorta end-to-end with a continuous polypropylene suture [33] . After the primary suture line is completed, the graft is carefully examined for gaps, overlapping sutures, or tears in the aorta. Felt-pledgeted mattress sutures are applied to ensure a secure anastomosis, given that blood pressure targets are higher in the postoperative period. The cross-clamp initially placed proximal to the left subclavian artery is then moved onto the graft to reestablish flow to the left subclavian and vertebral arteries.

Visceral and renal perfusion
After the patient is weaned from LHB, the distal cannula and distal aortic cross-clamp are removed. This portion of the aorta is then opened longitudinally [ Figure 8]. The dissecting septum and thrombus are removed, facilitating exposure of the entire aorta, including the intercostal, visceral, and lumbar branches. Briskly back-bleeding intercostal or lumbar arteries are oversewn to avoid steal. Selective visceral perfusion with isothermic, self-oxygenated blood is performed by using 9-French balloon-tipped catheters extending from a Y-branch of the LHB and inserted into the celiac axis and SMA at a flow rate of 300-400 mL/min. We protect the renal arteries with cold (4 °C) histidine-tryptophan-ketoglutarate perfusate (300 mL every 6-12 min) delivered by using 9-French balloon-tipped catheters extending from a separate cooling circuit; the patient's temperature is carefully monitored to avoid overcooling. Alternatively, the LHB circuit can be used to provide isothermic, self-oxygenated blood to all four branching arteries.

Reimplantation of intercostal or lumbar arteries
We identify 2 or 3 pairs of arteries feeding the spinal cord between T7 and L2 that are large and have minimal or no back-bleeding. These are to be used for reimplantation with an island patch or an 8-mm interposition Dacron graft. We typically use an island patch and a side-to-side anastomosis with 3-0 polypropylene sutures [ Figure 9]. To reduce the risk for subsequent pseudoaneurysm formation, we minimize the inclusion of the native aorta in the patch. Felt-pledgeted mattress sutures are liberally used for reinforcement if the aortic tissue is fragile. The proximal aortic cross-clamp is then moved distally to the island patch to perfuse the spinal cord through the intercostal or lumbar arterial patch.

Distal anastomosis
When a four-branched replacement graft is used, the distal anastomosis is performed before reattachment of the visceral and renal arteries to re-establish flow to the lower extremities while the visceral and renal organs continue to be perfused via the LHB and renal cooling circuit, respectively [ Figure 10]. The distal aortic anastomosis is usually sewn at the level of the aortic bifurcation but can be anastomosed to the iliac or femoral arteries, using additional grafts as needed. If the chronic dissection extends distally, the septum is fenestrated to ensure downstream perfusion into both lumens. The aortic graft is cut to a suitable length to prevent kinking or tension, and a 3-0 polypropylene suture is used in a continuous fashion to complete the anastomosis. As before, felt-pledgeted mattress sutures are used to reinforce the anastomosis.
The patient is then placed in the Trendelenburg position, and a 19-gauge needle is used to create deairing holes in the Dacron graft. Individual clamps are placed on each of the four branches of the prefabricated graft. The aorta is deaired through one of the branches of the graft. The aortic cross-clamp is slowly removed to reestablish blood flow to the lower body. Anesthesia facilitates patient resuscitation to maintain MAP after cross-clamp removal; typically, the use of bicarbonate and vasopressors is necessary.

Visceral and renal anastomosis with a four-branched graft
The ostia of the celiac, SMA, and renal arteries are isolated on buttons of aortic tissue and are carefully inspected for extension of dissection as well as for calcium or stenosis related to the atherosclerotic process. Endarterectomy, decalcification, or direct stenting (usually with a 7-mm × 15-mm balloon-expandable stent) may be required. If the dissection extends into the visceral arteries, fenestration may be necessary to ensure the perfusion of both lumens. The perfusion catheters are maintained to limit ischemic time. Because the right renal artery is most posterior, it is sewn first; we use a 4-0 or 5-0 polypropylene suture [ Figure 11]. The anastomosis is deaired, and the clamp is released to reestablish perfusion. Then, the SMA and celiac axis anastomoses are carried out. We typically leave the SMA clamped during the celiac axis anastomosis to prevent back-bleeding through collateral arteries, which affects visualization. The left renal artery anastomosis is typically sewn last. The inferior mesenteric artery is usually oversewn, but reimplantation may be necessary if bilateral hypogastric artery occlusions, previous colectomy, or a diseased SMA are present.

Hemostasis and closure
In preparation for closure, the four-branched graft is inspected for malorientation, kinks, or twists. The femoral arteries, renal arteries, celiac axis, and SMA [ Figure 12] are palpated for a distal pulse, and the kidneys are palpated for turgor. The bowel is visualized to confirm its perfusion. The spleen is inspected for any hematoma or laceration; any splenic injury that cannot be controlled with electrocautery or packing necessitates a splenectomy to avoid potential bleeding complications. The patient's temperature is passively rewarmed to > 34.5 °C. We confirm the position of the nasogastric tube before closure. An abdominal drain is placed in the retroperitoneal space. The left hemidiaphragm is reapproximated under tension by using a #1 polypropylene continuous suture that is locked every three bites. Two chest tubes are placed, anterosuperiorly and posteroinferiorly. Absorbable pericostal sutures are placed in and around the ribs in a figure-of-eight, interrupted fashion with #2 coated braided polyester sutures. Two #7 steel wires are placed to secure the thoracotomy closure. The abdominal fascia is closed and tied to the diaphragmatic suture. Soft tissue is closed, and single-lung ventilation is discontinued.

Postoperative care
Critical care monitoring and volume resuscitation form the cornerstone of postoperative care in patients with a TAAA. The MAP is maintained at 80-100 mmHg, and the patient's neurological status is checked every hour, particularly the patient's leg strength. The patient's hemoglobin level is kept above 10 g/dL to Figure 10. When a branched graft is used, the distal anastomosis is typically performed before the visceral arteries are reattached. Visceral perfusion is maintained while this anastomosis is performed. The figure is used with the permission of Baylor College of Medicine. Figure 11. The right renal artery is typically the first to be anastomosed when a branched aortic graft is used. The superior mesenteric artery, celiac axis, and left renal artery will continue to be perfused until they are anastomosed (insets). The figure is used with the permission of Baylor College of Medicine. Figure 12. The completed repair of a DeBakey type III aortic dissection. A four-branched graft was used to aid Crawford extent II thoracoabdominal aortic aneurysm repair. Alternates to this approach include a single visceral patch that incorporates the celiac axis, superior mesenteric artery, and both renal arteries (upper inset) or a three-vessel patch that incorporates the celiac axis, superior mesenteric artery, and right renal arteries, with the left renal artery reattached as a button (lower inset). The figure is used with the permission of Baylor College of Medicine. maximize oxygen delivery to end organs. The CSF drain is typically kept in place for 48-72 h postoperatively. The CSF drain is usually clamped for 8-12 h before removal to test for latent development of spinal ischemia. We typically extubate patients within 24 h and slowly start them on a solid diet on postoperative day 4. Typically, patients are discharged 7-10 days after open repair. Higher MAP goals are maintained for 4-6 weeks after surgery to prevent late neurological complications.
Open repair of thoracoabdominal aortic aneurysms precipitated by chronic dissection remains the gold standard, having exhibited favorable outcomes and durability. Nonetheless, patients with chronic dissection require a rigorous lifelong surveillance protocol, as there is a significant risk for subsequent intervention on any unrepaired aortic segments.