Thoracoabdominal Aneurysms

Step 1

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.

  

• ◆

  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.

Step 2

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.

Step 3

Operative Steps