Thoracoabdominal Aneurysms

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Surgical Anatomy

  • Thoracoabdominal aortic aneurysms (TAAAs) are characterized by dilation of the aorta (to at least 1.5 times its normal diameter) at the diaphragmatic hiatus—the boundary that separates the descending thoracic and abdominal aortic segments—with varying degrees of extension into the chest and abdomen.

  • The normal diameter of the aorta varies by anatomic location and by the patient’s sex, age, and body size. Average normal aortic diameters for men and women, respectively, are 28 and 26 mm at the level of the mid-descending thoracic aorta, 23 and 20 mm at the celiac axis, and 19.5 and 16.5 mm at the infrarenal aorta. Body surface area is a better predictor of aortic size than is height or weight, particularly in patients younger than 50 years.

  • The Crawford classification of TAAA repairs ( Fig. 26.1 ) enables appropriate risk stratification and selection of specific treatment modalities based on the extent of the aortic replacement. Extent I aneurysms involve the descending thoracic aorta and upper abdominal aorta to the level of the renal arteries. Extent II aneurysms involve the descending thoracic aorta and infrarenal abdominal aorta to the level of the aortic bifurcation and can involve the iliac arteries as well. Extent III aneurysms involve the distal half of the descending thoracic aorta and varying portions of the abdominal aorta. Extent IV aneurysms start from the portion of the thoracoabdominal aorta where the visceral arteries (celiac and superior mesenteric arteries) arise and extend into most or all of the remaining abdominal aorta.

    Figure 26.1

  • Understanding the anatomy of the spinal cord circulation is necessary to prevent spinal cord ischemia. The arteria radicularis magna (artery of Adamkiewicz) is the largest of the radicular medullary arteries supplying the anterior spinal artery and, therefore, is often targeted for reimplantation during TAAA repair. This artery has a variable origin; it arises from a lower intercostal artery (T9–T12) in 60% of persons, from a lumbar artery (L1–L4) in approximately 25%, and from an upper intercostal artery (T5–T8) in about 15%. As a principle, we target large intercostal arteries with slow back-bleeding at the level of T7 to T10.

Preoperative Considerations

  • Nonoperative management, which consists of strict blood pressure control, cessation of smoking, and at least yearly surveillance with imaging studies, is appropriate for asymptomatic patients who have small aneurysms.

  • Indications for operation in asymptomatic patients include an aortic diameter exceeding 5 to 6 cm or a rate of dilation greater than 1 cm/year. In patients with Marfan syndrome or a related connective tissue disorder, the threshold for operation is lower for absolute size and rate of growth. In the case of TAAAs that cause symptoms, especially pain, or that are complicated by superimposed acute dissection, the risk of impending rupture warrants expeditious evaluation and urgent aneurysm repair, even when the above-mentioned threshold diameters have not been reached.

  • With the exception of patients who require emergency surgery, all patients undergo a thorough preoperative evaluation, with an emphasis on cardiac, pulmonary, and renal function. Patients who have asymptomatic aneurysms and severe coronary artery occlusive disease undergo myocardial revascularization before aneurysm repair. If clamping proximal to the left subclavian artery is anticipated in patients in whom the left internal thoracic artery has been used as coronary artery bypass graft, a left common carotid to subclavian artery bypass is performed to prevent cardiac ischemia when the aortic clamp is applied.

  • Preoperative renal insufficiency has been a major risk factor for early mortality throughout the history of TAAA repair. The main strategy used to reduce the risk of contrast-induced nephropathy from preoperative imaging studies is providing intravenous hydration. Periprocedural administration of acetylcysteine can be used as well. Ideally, surgery is delayed for 24 hours or longer after contrast administration. If renal insufficiency occurs or becomes worse after a patient receives contrast, the surgical procedure is postponed until renal function recovers or is satisfactorily stabilized.

  • Pulmonary complications are the most common form of postoperative morbidity in patients undergoing TAAA repairs. Patients with a forced expiratory volume in 1 second (FEV 1 ) greater than 1.0 L and a P co 2 less than 45 mm Hg are considered satisfactory surgical candidates. In suitable patients, borderline pulmonary function frequently is improved by smoking cessation, treatment of bronchitis, weight loss, and a general exercise program that the patient follows for a period of 1 to 3 months before operation.

  • An evolving aspect of selecting the appropriate treatment in patients with TAAAs is the choice between performing a traditional open graft replacement and using an endovascular approach. Purely endovascular TAAA repairs require the use of fenestrated or branched stent grafts. These stent grafts are currently custom-made based on the patient’s preoperative computed tomography (CT) scan and, therefore, are not immediately available. It can take 4 to 6 weeks for customization. Currently, no stent grafts have been approved by the US Food and Drug Administration (FDA) for TAAA repairs in the United States. Hybrid repairs entail the use of open visceral bypass grafting to secure organ perfusion before the entire aneurysm is covered with a stent graft. Complete endovascular TAAA repair and hybrid repair are used for patients who have limited physiologic reserve and are poor candidates for open repair. Both techniques are becoming increasingly popular.

Operative Steps

Intraoperative Management Strategy

  • A cell-saving device is used throughout the procedure to salvage shed blood from the operative field. The patient’s temperature is allowed to drift down to a nasopharyngeal temperature of 32° to 33°C (89.6°–91.4°F). To prevent acidosis, sodium bicarbonate solution is administered by continuous infusion at a rate of 2 to 3 mEq/kg/hr while the aorta is clamped.

  • Left heart bypass (LHB) and cerebral spinal fluid (CSF) drainage are used to optimize organ protection in patients undergoing Crawford extent I or II TAAA repair and in patients who are undergoing extent III or IV TAAA repair after a previous descending thoracic aortic replacement. Additionally, in patients with poor cardiac function, LHB is used to reduce cardiac strain and thus to improve the patient’s ability to tolerate aortic clamping. During aortic clamping, the patient’s blood pressure is controlled primarily by the anesthesia team with the help of LHB. The target mean pressure is approximately 80 mm Hg. If additional help is required to control hypertension, nicardipine or nitroglycerin is administered. Enough CSF is drained to keep the CSF pressure between 8 and 10 mm Hg during the operation.

  • Motor-evoked potential monitoring can provide useful information about spinal cord function and thereby anterior spinal perfusion during aortic repair. This method of spinal cord monitoring, which precludes complete neuromuscular blockade, requires the use of special anesthetic techniques.

Incisions and Aortic Exposure

  • The patient is turned to a right lateral decubitus position with the shoulders placed at 60 to 80 degrees and the hips flexed to 30 to 40 degrees from horizontal and stabilized with a beanbag ( Fig. 26.2 ). For extent I, II ( Fig. 26.3 ), and III TAAAs, the upper portion of the thoracoabdominal incision is generally made through the sixth intercostal space (see Fig. 26.2A ); the upper (more often) or lower ribs may be divided posteriorly to achieve additional proximal or distal exposure, respectively, as needed. For extent III aneurysms, sometimes we use the seventh intercostal space. The incision is gently curved as it crosses the costal margin to reduce the risk of tissue necrosis at the apex of the lower portion of the musculoskeletal tissue flap. In contrast, to approach extent IV aneurysms, a straight oblique incision is made through the eighth (most common), ninth, or even tenth interspace; the exact incision site is chosen based on the patient’s body habitus and specific anatomy (see Fig. 26.2B ). In most cases, the distal extent of the incision is at the level of the umbilicus. The incision is extended toward the pubis if there are iliac aneurysms to be repaired.

    Figure 26.2

    Figure 26.3

  • Fixed metal retractors attached to the operating table provide consistent static exposure. The diaphragm is divided in a semicurvilinear fashion to protect the phrenic nerve and preserve a 3- to 4-cm posterolateral rim of diaphragmatic tissue to facilitate closure when the operation is complete. The abdominal aortic segment is exposed via a transperitoneal approach; the retroperitoneum is entered lateral to the left colon, where the spleen, left kidney, and ureter are retracted anteriorly and to the right. The crus of the diaphragm is divided, and the left renal artery is identified but not circumferentially dissected or encircled with a tape. An open abdominal approach permits direct inspection of the bowel, abdominal viscera, and visceral blood supply after aortic reconstruction is completed.

  • When the aneurysm encroaches on the left subclavian artery, the distal aortic arch is mobilized gently by dividing the remnant of the ductus arteriosus. The vagus and recurrent laryngeal nerves are identified. Occasionally, the vagus nerve is divided below the recurrent nerve to provide additional mobility, thereby protecting the recurrent nerve from injury. If clamping proximal to the left subclavian artery is anticipated, this artery is separately and circumferentially mobilized to enable placement of a bulldog clamp. After achieving adequate exposure of the aorta, heparin (1 mg/kg) is administered before aortic clamping or the start of LHB.

Graft Replacement of the Aorta

  • Patients with extensive TAAAs (extents I, II, and often III) are at greatest risk of developing postoperative paraplegia or paraparesis, and LHB is used to provide distal aortic perfusion during the proximal portion of the aortic repair. This is achieved by using temporary bypass from the left atrium, via a cannula inserted through the inferior pulmonary vein, to the distal descending thoracic aorta with a closed circuit in-line centrifugal pump ( Fig. 26.4 ). Carefully examining CT scans or magnetic resonance images helps the surgeon select an appropriate site for direct aortic cannulation. Areas with intraluminal thrombus are avoided because cannulating them can lead to distal embolization ( Fig. 26.5 ).

    Figure 26.4

    Figure 26.5

  • A clamp is applied to the distal transverse arch, between the left common carotid and left subclavian arteries, or to the proximal descending thoracic aorta, just distal to the left subclavian artery. When LHB is used, a distal aortic clamp is placed between T4 and T7 (see Fig. 26.4 ). Bypass flows are adjusted to maintain normal proximal arterial and venous filling pressures. Flows between 1500 and 2500 mL/min are generally required. The aorta is opened, transected 2 to 3 cm beyond the proximal clamp, and dissected from the esophagus. Patent upper intercostal arteries are oversewn to avoid stealing blood supply from the spinal cord.

  • The proximal anastomosis is performed between the aorta and a 22-, 24-, or 26-mm Dacron graft with continuous 3-0 polypropylene sutures. In patients with fragile aortic tissues (e.g., those with a connective tissue disorder or complicated acute aortic dissection), 4-0 polypropylene sutures are often used. Interrupted polypropylene mattress sutures with felt pledgets are used to reinforce selected portions of the anastomoses. Surgical adhesives are avoided in these operations.

  • After the proximal anastomosis has been completed, LHB is stopped, the aortic cannula from the descending thoracic aorta is removed, and the entire remaining aneurysm is opened longitudinally. The origins of the visceral and renal branches are identified, and cold (4°C [39.2°F]) lactated Ringer’s solution or Custodiol solution is intermittently delivered to the renal arteries via balloon catheters ( Fig. 26.6 ). In patients receiving LHB, 9 F balloon-tipped Pruitt cannulas can be placed in the celiac and superior mesenteric arteries so that selective visceral perfusion can be delivered from the pump circuit. If the reconstruction is expected to be complicated and take more time than usual, we choose to perform selective visceral perfusion.

    Figure 26.6

  • For most extent I and II repairs, patent lower intercostal arteries are selected and reattached to an opening cut in the side of the graft (see Fig. 26.6 ); large arteries with little or no back-bleeding are considered particularly important. When none of these arteries is patent, endarterectomy of that aortic wall and removal of calcified intimal disease should be considered as a means of identifying arteries suitable for reattachment, after which the proximal clamp is often moved down the graft to restore intercostal perfusion ( Fig. 26.7 ).

    Figure 26.7

  • In extent I repairs, the reattachment of the visceral arteries is often incorporated into a beveled distal anastomosis. In extent II and III repairs, the visceral artery origins, usually those of the celiac, superior mesenteric, and right renal arteries, are reattached to one or more oval openings in the graft (see Fig. 26.7 ). Twenty-five percent of the patients have a stenosis at a visceral artery origin and require endarterectomy, stenting, or interposition bypass grafting. In the majority of cases, the left renal artery requires direct attachment to a separate opening in the graft (see Fig. 26.7 , inset) or is attached via interposition bypass grafting. Usually, for patients with a connective tissue disorder, a four-branched graft ( Fig. 26.8 ) is used to attach the celiac, superior mesenteric, and right and left renal arteries separately. A four-branched graft is also used if the origins of the visceral arteries are far apart from each other.

Jan 26, 2019 | Posted by in CARDIAC SURGERY | Comments Off on Thoracoabdominal Aneurysms
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