CHAPTER 25 Abdominal Aortic Aneurysm Repair
Case Study
A 70-year-old female with a medical history of coronary artery disease, hypertension, and tobacco use presents to the emergency department complaining of 10 hours of severe abdominal and back pain. Physical examination shows a pulsatile, tender epigastric mass. A computed tomography (CT) scan is performed without contrast and shows a 7-cm abdominal aortic aneurysm (AAA) (Fig. 25-1).
INDICATIONS FOR ELECTIVE SURGERY
Indications for elective AAA repair include:
I. Asymptomatic AAAs: Asymptomatic aneurysms 5 to 5.5 cm or greater in diameter and those that are expanding by 1 cm or more in diameter per year are repaired because of the relatively high associated risk of rupture.
PREOPERATIVE EVALUATION
I. Ultrasound is an excellent imaging modality for the diagnosis and surveillance of aortoiliac aneurysms; however, it does not provide sufficient objective anatomic information to enable operative planning and cannot reliably identify aneurysm rupture. Moreover, the presence of bowel gas and an obese body habitus may decrease the sensitivity of a sonographic evaluation.
II. Magnetic resonance imaging (MRI) and magnetic resonance arteriography (MRA) have been used frequently as diagnostic modalities and in operative planning, especially in the evaluation of patients with chronic renal insufficiency. The incidence of nephrogenic systemic fibrosis in patients with impaired renal function after gadolinium exposure and the frequency of other contraindications to MRI (e.g., pacemakers and defibrillators), however, limit the utility of this modality.
III. Arteriography is not sensitive in detecting AAAs or in measuring aneurysm extent or diameter because of the frequent presence of thrombus in the aneurysm sac, which narrows the lumen through which blood can flow. Angiography, however, is useful for delineating the anatomy and patency of aortic branches, including the celiac, superior and inferior mesenteric, renal, and hypogastric arteries.
IV. Computed tomography and, in particular, computed tomography arteriography (CTA) are extremely useful for both diagnosis and operative planning. They also are highly sensitive for the detection of rupture. Most surgeons use CT or CTA as their imaging modality of choice.
V. Three-dimensional modeling of aortic aneurysms after cross-sectional imaging (MRI or CT scan) has become more popular with the increased application of stent grafting for the treatment of AAAs. Such modeling allows for the detailed evaluation and measurement of segments of the aorta and aortic branches. In addition, center-line lengths along the course of the aorta and iliac arteries can be measured, obviating the geometric corrections necessary when grafts are sized or designed with axial imaging. Some software even allows insertion of a virtual stent graft and visualization of the projected completed repair.
COMPONENTS OF THE PROCEDURE AND APPLIED ANATOMY
Preoperative Considerations
I. Open AAA Repair
A. Patients undergo open AAA repair under general endotracheal anesthesia, almost exclusively. Frequently, this is supplemented with epidural anesthesia to ensure intraoperative and postoperative analgesia. Some studies suggest a decrease in cardiac complication rates with the use of epidural anesthesia.
B. Open aneurysmorrhaphy requires aortic cross-clamping. The associated hemodynamic variability necessitates continuous arterial line blood pressure monitoring with a radial artery catheter. Furthermore, obligate blood loss (from lumbar arteries arising from the aortic sac) calls for large-bore intravenous access for resuscitation.
C. Any repair that will require a suprarenal (or more proximal) aortic clamp entails a period of renal, and possibly visceral, ischemia, frequently resulting in oliguria at some stage of a patient’s intraoperative or early postoperative course. A pulmonary artery catheter may facilitate monitoring of intravascular volume and resuscitation.
II. Endovascular AAA Repair
A. An EVAR procedure can be undertaken with general anesthesia, epidural or spinal anesthesia, or even sedation with local analgesia.
B. Because EVAR does not require aortic cross-clamping and because blood loss in uncomplicated EVAR is frequently minimal, the requirements for intra- and postoperative monitoring are fewer. Foley catheter placement, large-bore intravenous access, and continuous arterial pressure monitoring are recommended. Because arterial access is integral to this approach to AAA repair, sheaths for delivery and manipulation of grafts can be transduced intraoperatively and sometimes postoperatively in place of radial arterial catheterization.
Operative Approach
I. Anterior Approach: The anterior or transperitoneal approach to open aneurysmorrhaphy provides excellent exposure for the repair of infrarenal AAAs. Advantages of this approach include easy access to the bilateral iliac and, if necessary, femoral arteries. An aortic clamp site proximal to the renal arteries, however, is generally more difficult to obtain compared with the retroperitoneal approach. In addition, previous abdominal surgery and the presence of a stoma are relative contraindications to this approach. Because it is most useful for infrarenal aortic aneurysms, the transperitoneal approach is undertaken more infrequently in the era of EVAR.
B. The abdomen is entered and explored, and the transverse colon is retracted superiorly. The ligament of Treitz is divided to allow retraction of the small intestine to the right side of the abdomen.
C. The parietal peritoneum overlying the aneurysm is incised and opened from the crossing of the left renal vein superiorly down to the aortic bifurcation and then down each iliac artery. Care is taken to protect the ureters as they cross anterior to the iliac bifurcations.
D. If access to the left iliac bifurcation is required (e.g., in the case of aneurysmal degeneration throughout the extent of the common iliac artery), it is most easily accessed by reflecting the sigmoid mesocolon medially to allow a separate retroperitoneal counterincision. The left limb of a bifurcated graft can subsequently be tunneled within the unopened portion of the left common iliac artery directly beneath the mesentery (Fig. 25-2).
E. Clamp sites are identified immediately distal to the renal arteries and on the bilateral iliac arteries. The superior clamp is placed as close to the renal arteries as possible, so that the graft can be sewn to exclude as much proximal aorta as possible, thereby preventing subsequent aneurysmal degeneration of the juxtarenal segment.
F. Most surgeons systemically heparinize the patient (50–150 units/kg) before clamping the aorta and iliac arteries to prevent thrombosis of the arteries of the legs.
G. Some surgeons clamp the bilateral iliac arteries first, then the aortic neck, to prevent embolization of thrombus at the proximal clamp site to the legs and pelvis. Others clamp the proximal aorta and then open the aneurysm, allowing the iliac arteries to back-bleed to flush out any thrombus, before clamping or occluding the iliac arteries with balloons (Fig. 25-3). In either case, the anesthesia team must be kept informed of the sequence and timing of clamping, and blood pressure should be pharmacologically lowered to avoid severe hypertension with proximal aortic clamping.
H. The aneurysm is opened longitudinally, with care taken to avoid injuring the origin of the inferior mesenteric artery (IMA), so that it can be reimplanted if necessary. Thrombus is removed manually to expose back-bleeding lumbars and the IMA. The lumbars are suture ligated from within the sac.
I. If the IMA is chronically occluded, it need not be identified or interrogated. However, if it is patent, it should be locally heparinized and clamped just outside the sac, to allow interrogation after re-establishment of in-line aortic flow.
J. Dacron or polytetrafluoroethylene (PTFE) grafts are used for aneurysmorrhaphy. The surgeon must decide at this juncture whether a bifurcated graft, sewn to the bilateral iliac arteries, or a tube graft, sewn to the distal aorta, will be required.
K. The proximal anastomosis is prepared by dividing the aneurysm from the healthy proximal aorta transversely around the anterior 180 degrees of its circumference. The back wall of the aorta is not disturbed to prevent disruption of the veins lying posterior to the aorta. The anastomosis incorporates a double layer of the posterior aortic wall. If a bifurcated graft is to be used, the main body is kept short, and the limbs long, to allow flexibility in completing the repair and to prevent kinking of either limb.
M. The graft is clamped, and the proximal clamp is released to test the proximal anastomosis; if intact, the aortic clamp is left in place, but loosened to allow for rapid reclamping, if necessary.
N. If a tube graft is indicated, the distal anastomotic ring is fashioned in a similar manner to that used for the proximal anastomosis, leaving the posterior wall of the aorta and iliac arteries intact and encompassing both iliac orifices in the sewing ring. Alternatively, in the case of a bifurcated graft, one anastomosis is performed to the distal common iliac artery initially. This limb is opened to allow back-bleeding through the still open contralateral limb. Aortic flow can be re-established to one leg while the contralateral anastomosis is performed.
O. The anesthesia team must be informed again about the unclamping of each iliac artery. Hypotension can result from reperfusion of the pelvis and legs. Usually, this hypotension is easily treated with volume resuscitation, but reclamping one iliac limb, or partially clamping the aortic graft in the case of a tube graft, may be transiently necessary to allow the anesthesiologist to “catch up.”
P. At the conclusion of the distal anastomosis, the IMA should be assessed. If pulsatile back-bleeding is encountered, the IMA may be sacrificed. However, if a patent IMA is not bleeding or is bleeding slowly, if the patency of one or both internal iliac arteries is in question, or if one internal iliac has been sacrificed, it should be reimplanted (Fig. 25-4).
Q. Perfusion to the bilateral legs is assessed next, by palpation of the external iliac artery pulses at their egress from the pelvis, by palpation of the femoral pulses in the groins, and if necessary, by visual inspection of the feet. If possible, thrombosis of, or embolism to, a leg should be recognized at this juncture, before reversal of heparinization and closure of the abdomen.
R. Protamine is administered to reverse the effects of heparin, the sigmoid colon is inspected to ensure adequate perfusion, and the sac and peritoneum are sutured over the graft to isolate it from the serosa of the intestines and avoid the entirely preventable late complication of graft-enteric fistula.
II. Retroperitoneal Approach: Because the liver and vena cava obstruct access to the aorta from the right retroperitoneum, the left retroperitoneal approach is commonly used in patients who have had previous abdominal surgery, patients with stomas, and those in whom suprarenal or higher proximal aortic clamping is necessary. The right iliac artery is difficult to access or even visualize from the left retroperitoneum, so this approach is less attractive in cases of AAA with associated right iliac artery aneurysm.
A. The patient is positioned with the table flexed between the iliac crest and the costal margin. Some surgeons attempt to elevate the left shoulder to a 45- degree to 60-degree position, while maintaining the pelvis relatively flat (i.e., parallel to the floor). However, few patients are both thin enough and flexible enough to allow sterile access to the right groin with the thorax positioned vertically enough to allow easy access to the visceral aortic segment. We therefore commonly tilt the entire patient 60 to 90 degrees from the horizontal position and forgo operative access to the right groin. In all cases, a “bean bag” is extremely useful because it allows fixation and support of the patient.
B. The incision is begun at the lateral border of the left rectus muscle and carried cephalad and laterally in a gentle curve to the 10th intercostal space. The external and internal abdominal oblique and transversus abdominis muscles are divided, allowing access to the retroperitoneal space. This potential space is developed bluntly with the surgeon’s hand. It is most easily freed posteriorly. The parietal peritoneum is then swept off of the inside of the transversus abdominis muscle. At the cephalad extent of the incision, the dissection is carried into the 10th interspace, at which level access to the supraceliac aorta is easily accomplished. In opening the intercostal muscles at this level, the thoracic cavity is frequently entered. Care is taken to protect the lung parenchyma.
C. Once the incision is entirely opened, dissection is carried posteriorly and deeply in a blunt manner until the lumbar vertebrae are identified. Then it is continued more anteriorly until the pulsatile aorta is identified.
D. The left kidney is generally brought anteriorly with the abdominal viscera; if left posteriorly in the incision, the left renal vein will generally impede access to the aorta. The left ureter is dissected off the aorta and the left iliac bifurcation and is protected behind self-retaining retractors. There is usually a lumbar branch of the left renal vein that overlies the juxtarenal aorta when the left kidney rests anteriorly in the wound; this vein should be ligated and divided before clamping.
E. Access to the visceral segment of the aorta and to the supraceliac aorta is facilitated by division of the left crus of the diaphragm overlying the left side of the aorta. The left renal, celiac, and superior mesenteric arteries are easily identified with this exposure (Fig. 25-5); the ostium of the right renal artery will be evident once the aorta is clamped and opened.
COMPLICATIONS
I. After aortic repair, bleeding is suggested by hypotension, oliguria or anuria, an uncorrected metabolic acidosis, or a persistent transfusion requirement, and a return to the operating room is frequently indicated.
II. Strict attention must be paid to the femoral and more distal pulses at the conclusion of the procedure and in the SICU. Both distal embolization and anastomotic thrombosis can result in limb-threatening ischemia and may require femoral exploration and thromboembolectomy, or operative revision of an iliac anastomosis.
III. Unexplained leukocytosis, fever, left lower quadrant abdominal pain, or bloody diarrhea suggests colonic ischemia and justifies an evaluation with flexible sigmoidoscopy. Pale mucosa with patchy sloughing, suggesting reversible ischemia, can most often be treated conservatively with parenteral antibiotic therapy, bowel rest, and observation. Full-thickness necrosis, peritonitis, or shock mandates operative exploration and possible bowel resection.
AAA STENT GRAFTING
Endovascular aneurysm repair is dependent on sealing of the proximal and distal (iliac) attachment sites. The proximal seal zone must be of small enough diameter to accommodate the chosen endograft, must not be excessively calcified or thrombus-lined, and must be approximately 15 mm long. The common iliac attachment sites must not be aneurysmal themselves, unless the surgeon plans to extend the limbs of the graft into one or both external iliac arteries, covering the internal iliac arteries. Finally, the external and common iliac arteries and the aortic bifurcation must be of sufficiently large caliber to permit introduction of the graft and its delivery system. Preoperative planning and device design are aided by CTA, by three-dimensional modeling of radiographic studies (Fig. 25-6), and by software allowing interactive endograft design.
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Schermerhorn ML, Cronenwett JL. Abdominal aortic and iliac aneurysms. In Cronenwett JL, editor: Rutherford Vascular Surgery, 6th ed, Philadelphia: Elsevier, 2005.
Solis MM, Harvey RL, Hodgson KJ. Abdominal aortic aneurysm: Endovascular repair. In Cameron JL, editor: Current Surgical Therapy, 8th ed, Philadelphia: Elsevier, 2004.
