Historical Background
The first surgeon to repair a thoracoabdominal aortic aneurysm (TAAA) was Etheredge in 1955 in a patient with a Type IV TAAA using a homograft. In 1956 DeBakey and associates reported the surgical treatment of four patients with TAAA by resection and homograft replacement. A Dacron graft was used as a shunt between the descending thoracic aorta and infrarenal abdominal aorta. Sequentially, the celiac axis, superior mesenteric artery, and both renal arteries were revascularized, limiting ischemic time for the liver, stomach, bowels, and kidneys to 10 to 15 minutes. Subsequent reports utilized Dacron grafts instead of homografts for primary reconstruction of the aorta with Dacron side arms to each visceral vessel, as the primary approach for repair from the 1950s to the late 1970s.
In 1965 Crawford ushered in the modern era of TAAA and descending thoracic aortic aneurysm (DTAA) repair. Three principles of repair were formulated: (1) the inclusion technique, as originally described by Matas, Javid, and Creech, in which the aneurysm wall is not excised, thus avoiding damage to surrounding structures; (2) the reattachment of the renal arteries, superior mesenteric artery, and the celiac axis into the larger graft, by either creating an orifice in the body of the graft or beveling the anastomosis, as described by Carrel ; and (3) reattachment of the intercostal arteries to prevent paraplegia, as initially described by Spencer in a canine model. From the 1970s until the 1990s, the “clamp and sew” technique was the primary surgical approach for treatment.
Beginning in the 1960s, multiple approaches to prevent paraplegia were introduced. As an early adjunct, cerebrospinal fluid drainage (CSFD) was described by Miyamoto and colleagues in 1960, as well as by Blaisdell and Cooley in 1962. Although the benefit of reduced cerebrospinal fluid (CSF) pressure during TAAA repair was demonstrated in a canine model, CSFD did not gain clinical popularity until the 1980s. The use of perfusion catheters from the descending aorta to both renal arteries, the celiac axis, and the superior mesenteric artery was first described by Korompai and Hayward in 1975.
Although Hollier and co-workers confirmed the benefits of CSFD, Crawford and associates initially reported that CSFD did not improve outcomes. Despite this early controversy, subsequent studies have demonstrated the benefit of CSFD during TAAA repair.
The use of distal aortic perfusion (DAP) or partial bypass was first reported in 1956 by DeBakey and associates and later by Connolly and colleagues to reduce distal ischemia and cardiac afterload. The benefit of DAP was confirmed in multiple studies conducted in the 1980s and 1990s.
Preoperative Preparation
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Cardiac evaluation. Echocardiography is obtained to determine ventricular function and the presence of valvular abnormalities. Normal ventricular function is a predictor of good outcome after TAAA repair. Severe valvular dysfunction should be treated before aortic repair if repair is not urgent. Cardiac evaluation should include a physical examination and determination of exercise tolerance. If coronary artery disease is identified, then the extent and location of disease and associated symptoms determine the order of repair. Drug-eluting stents should be avoided as TAAA repair mandates discontinuation of clopidogrel. If required, coronary artery bypass grafting should be performed at least 6 weeks before repair.
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Pulmonary function. Pulmonary function tests and arterial blood gases are obtained as a routine. Patients with severe chronic obstructive pulmonary disease, as suggested by a forced expiratory volume in 1 second (FEV1) of less than 0.8 L/min, should be evaluated by a pulmonologist and receive bronchodilators as well as pulmonary rehabilitation before repair.
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Nutritional and gastrointestinal status. Large aneurysms may cause “aortic dysphagia” as a result of intrinsic compression of the lower esophagus leading to a nutritionally depleted state that may benefit from preoperative enteral alimentation.
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Renal function. Preoperative renal function is a strong predictor of postoperative mortality after TAAA repair, and glomerular filtration rate (GFR) is more sensitive than serum creatinine in predicting postoperative outcome. Patients with renal dysfunction may benefit from admission before surgery for intravenous hydration. Advanced, irreversible renal failure constitutes a relative contraindication to surgery.
Operative Strategy
A classification for thoracoabdominal aortic aneurysm describes five anatomic types: Type I extends from the left subclavian artery to just above the renal arteries; Type II from the left subclavian to the aortic bifurcation; Type III from the sixth intercostal space to the aortic bifurcation; Type IV from the twelfth intercostal space to the aortic bifurcation; and Type V from the sixth intercostal space to just above the renal arteries ( Fig. 18-1 ). This classification has been used in the prediction of complications, especially the risk of spinal cord ischemia, which is highest for Type II TAAA.
A separate classification scheme has been devised for those aneurysms confined to the descending thoracic aorta. Type A extends distal to the left subclavian artery to the sixth intercostal space; Type B arises between the sixth and twelfth intercostal spaces, above the diaphragm; and Type C extends distal to the left subclavian artery to the twelfth intercostal space.
Avoiding Spinal Cord Ischemia
Spinal cord protection may be achieved through distal aortic perfusion by cannulating both the left atrium or the left lower pulmonary vein and the femoral artery or distal aorta, either directly or through use of a Dacron graft, as a sleeve for the cannula, sutured end to side to the left common femoral artery. In addition, CSFD should be employed to maintain the CSF pressure less than 10 mm Hg, both intraoperatively and extending 3 days postoperatively. Although disputed by some, patent intercostal arteries in the T8-T12 distribution should be reattached at the time of surgery. Somatosensory-evoked potentials (SSEPs) and motor-evoked potentials (MEPs) may be helpful in pursuing a selective approach to intercostal revascularization.
Avoiding Visceral Ischemia
Avoiding visceral ischemia relies on sequential aortic clamping, DAP with retrograde perfusion, and direct visceral and renal artery perfusion using balloon perfusion catheters. DAP with sequential aortic clamping allows for retrograde flow to the abdominal aorta, avoiding ischemia to the visceral and renal arteries. Visceral and renal artery perfusion with balloon-tip catheters requires a centrifugal pump with two perfusion heads to allow infusion of blood to the celiac axis and superior mesenteric artery, and cold crystalloid into the renal arteries, maintaining renal temperature below 68°F (20°C).
Coagulopathy
Patients should be evaluated for history of bleeding and easy bruising. Although aspirin discontinuation is not mandatory, clopidogrel and warfarin should be discontinued. Intraoperatively, meticulous attention to hemostasis is mandatory and reduces coagulopathy. In cases of persistent coagulopathy, vacuum-assisted closure of the abdomen may be instituted. Infusion of platelet-rich plasma after repair may also be considered.
Avoiding Embolization
Transesophageal echocardiography and computed tomography (CT) or magnetic resonance imaging (MRI) may be used to determine the degree of atheromatous plaque in the proximal descending thoracic aorta. If atheromatous disease is severe, profound hypothermic circulatory arrest and reconstruction without clamping may be considered.
Avoiding Diaphragmatic Paralysis
Radial division of the diaphragm to the aorta has traditionally provided good exposure but rendered most of the diaphragm paralyzed, adversely affecting respiratory function. As an alternative, partial division of the anterior muscular portion of the diaphragm can be performed, avoiding injury to the phrenic nerve, followed by division of the crus of the diaphragm to create an aortic hiatus for the graft ( Fig. 18-2 ). This approach has reduced the incidence of diaphragmatic paralysis and has aided expeditious extubation.
Avoiding Injuries to the Vagus Nerve
The vagus nerve enters the chest cavity, lies in front of the transverse aortic arch near the left subclavian artery and the recurrent laryngeal nerve, and then runs parallel to the descending thoracic aorta, as well as the esophagus. With dissection initiated at the level of the hilum of the lung and progressing cephalad, the vagus nerve is dissected away from the descending thoracic aorta until the concave portion of the transverse arch is reached, and the atretic ductus arteriosus is divided where the recurrent laryngeal nerve curves around the transverse arch and ascends to the neck.
Avoiding Injuries to the Esophagus
The esophagus is located directly behind the thoracic aorta. When performing the proximal anastomosis in patients with DTAA and TAAA, the aorta is circumferentially divided and lifted off the esophagus to prevent an inadvertent esophagograft fistula ( Fig. 18-3 ).
Somatosensory- and Motor-Evoked Potential Monitoring
Neuromonitoring is led by a neurologist or neurophysiologist, in conjunction with anesthesia. SSEPs are recorded bilaterally at three levels. A baseline SSEP tracing is obtained before the start of the operation. All subsequent tracings are compared with baseline. An abnormal response is defined as a 10% change in latency or 50% change in amplitude. The evaluation of three channels allows one to distinguish spinal cord injury from peripheral nerve ischemia or cerebral injury.
For MEP monitoring, electrodes are placed at C3 and C4 and myogenic responses are recorded bilaterally with electrodes placed in the abductor digiti minimi, tibialis anterior, and abductor hallucis muscles. Compound muscle action potentials are checked throughout the operation as present or absent.
Intraoperative Corrective Measures
If there are signs of potential spinal cord dysfunction based on an abnormal SSEP or MEP finding, a series of corrective measures may be instituted, including increasing the mean blood pressure to at least 80 mm Hg and distal aortic pressure to at least 60 mm Hg. CSF pressure may be reduced by gravity drainage and hemoglobin increased by transfusion. Furthermore, additional patent intercostal arteries should be reimplanted, especially those between T4 to T7 and L1, as necessary.
Operative Considerations
Positioning
The patient should be positioned with the shoulder blades at a right angle to the edge of the operating table, stabilized by a bean bag, with both axillas well padded. The hips are tilted 60 degrees so that both femoral arteries are accessible. The left knee should be flexed and a pillow placed between both lower extremities ( Fig. 18-4 ). The patient is secured to the table with the bean bag vacuumed into a supportive shape. Tape can be used where feasible but should not compromise exposure. The table should then be flexed and the kidney rest elevated at the flexion point, just above the dependent iliac crest, for greater lateral flexion and to improve access to the retroperitoneal space.
Incision
An incision, which extends from just medial to the tip of the scapula to the costal cartilage along the sixth rib, can be applied to Type A DTAAs, as well as Type I or II TAAAs. For most thoracoabdominal aneurysms, the incision begins, as described previously, at or below the level of the scapula depending upon the proximal extent, which is then carried down along the anterolateral margin of the abdominal wall, lateral to the rectus sheath, to the level of the umbilicus or pubis as dictated by the distal extent of the aneurysm. This incision may be used for Type III, IV, and V TAAAs. The superior portion of the skin incision and the selected thoracic interspace to enter the thoracic cavity is dictated by the proximal extent of the aneurysm. The thoracic interspaces are defined by counting up from the tip of the twelfth rib. The proximal portion of the incision is made in the eighth intercostal space (top of the ninth rib) for a Type IV TAAA, the sixth intercostal space (top of the seventh rib) for a Type III/V TAAA, and the fifth intercostal space (top of the sixth rib) for a Type I/II TAAA, where the fourth or fifth rib can be shingled or divided posteriorly, if more proximal exposure is required in the thoracic cavity.
Retroperitoneal or Transperitoneal Thoracoabdominal Exposure
The left kidney and the viscera are mobilized medially, exposing the aorta from the aortic hiatus to the iliac bifurcation. The retroperitoneal approach is preferred because it prevents the viscera, and especially the small bowel, from obscuring the operative field, facilitates closure, and decreases water and heat loss.
Distal Aortic Perfusion
The left inferior pulmonary vein is used for aortic outflow. On exposure of the left inferior pulmonary vein, the pericardium is incised and the pericardial cavity entered. A pledgeted 3-0 polypropylene pursestring suture is placed in the left inferior pulmonary vein, followed by a transverse incision. The vein is dilated, and the cannula is inserted into the left atrium and connected to the centrifugal pump and inline heat exchanger ( Fig. 18-5 ). Although the femoral artery can be directly cannulated, it may be preferable to suture an 8-mm graft onto the femoral artery, as a sleeve for the cannula, to prevent warm ischemia to the left leg. As an alternative to avoid accessing the femoral artery, the distal aortic anastomosis can be performed initially using a premanufactured, single-arm branched aortic graft. The single arm is then connected to the inflow portion of the distal perfusion circuit.
