Abdominal Aortic Aneurysms and Aortic Dissections

Abdominal Aortic Aneurysms (AAAs)

1. Etiology
• abdominal aorta > 3.0 cm in maximal diameter is considered aneurysmal
• majority of AAAs are fusiform in shape and arise below the level of the renal arteries
• major risk factors include age, male sex, and tobacco use
• definite familial tendency exists
• pathologic examination usually shows inflammation and degeneration of the arterial wall
• loss of elastic tissue is a critical factor
• an imbalance may exist between 2 enzymes important in the metabolism of elastin, elastase (degradation) and α₁-antitrypsin (synthesis)
• inherited connective tissue disorders (Marfan’s syndrome and type IV Ehlers-Danlos syndrome) are associated with a high incidence of arterial aneurysms


2. Risk of Rupture
• AAAs < 4 cm have a very low risk of rupture
• AAAs between 5 and 6 cm have an estimated yearly rupture risk between 3% - 15%
• AAAs > 8 cm have a 30% - 50% risk of rupture/year
• rate of growth > 5 mm in 6 months or 1 cm per year is also a risk factor for rupture
• women also have a higher rate of rupture
• symptomatic AAAs have a higher rate of rupture than asymptomatic AAAs


3. Clinical Manifestations
• most are AAAs are asymptomatic and are discovered as a pulsatile periumbilical mass on routine physical exam or are found incidentally during abdominal imaging studies for other reasons
• abdominal symptoms range from vague abdominal pain to excruciating flank or back pain
• unusual presentations include aortocaval fistula, ureteral obstruction, thrombosis, or embolization (limb ischemia)


4. Diagnostic Studies
1. Plain Films
• calcification of the aneurysm wall is often incidentally observed, especially on the lateral view



2. Ultrasound
• accurate, safe, low cost study
• study may be limited in obese patients or if excessive bowel gas is present
• the proximal and distal extent of disease may be impossible to discern
• most valuable as a screening tool or following the size of known aneurysms
• misses 50% of ruptures, so not very useful in symptomatic patients


3. CT and CTA
• accurately studies the entire aorta
• detects vessel calcification, thrombus, concurrent arterial occlusive disease
• useful for showing the relationship between the renal arteries and the proximal cuff
• extremely valuable in evaluating patients with symptoms
• drawbacks include high radiation exposure and the use of iodinated contrast agents




4. MRI and MRA
• typically used if there a contraindication to CT (kidney disease)
• does not detect vessel calcification, which can be important in operative planning


5. Medical Treatment
• no drug is currently indicated for reducing AAA enlargement
• medical management is primarily indicated to reduce the risk of a future MI or stroke
• beta blockers are often used to control blood pressure and to reduce aortic wall stress
• statins may be associated with reduced rates of AAA enlargement
• smoking cessation offers the most benefits in reducing the rate of aneurysm expansion and in improving perioperative morbidity and mortality


6. Elective Surgical Treatment
• intervention is recommended when the risk of rupture is greater than the risk of the procedure
• endovascular approaches (EVAR) comprise the majority of procedures performed (80%)


1. Indications for Repair
• aneurysms > 5.5 cm in maximal diameter for males, 5.0 cm in females
• > 5 mm of growth in 6 months or > 1 cm of growth in 12 months
• saccular aneurysms
• symptomatic aneurysms
• coexistent aneurysm or peripheral artery disease requiring repair


2. Preoperative Evaluation
• important medical comorbidities such as CAD, CRF, diabetes, COPD, peripheral vascular disease must be identified and optimized


1. Coronary Artery Disease
• CAD is the primary cause of morbidity and mortality after open or endovascular AAA repair
• at least 2/3 of AAA patients have clinically significant CAD
• patients with symptomatic CAD with require a full cardiac evaluation, and may require cardiac angiography and immediate revascularization
• asymptomatic patients with significant CAD risk factors should also undergo echocardiography and stress testing – evidence of ischemia would mandate angiography
• asymptomatic patients with a functional capacity < 4 METS will also require a full cardiac workup
• asymptomatic, low-risk patients with a normal EKG do not require any further workup
• AAA repair can follow cardiac stenting procedures almost immediately, and 4 – 6 weeks after CABG


2. Chronic Renal Insufficiency
• coexistent renal artery occlusive disease is present in 20% to 38% of patients with AAA
• open repair or endovascular repair can also worsen preexisting renal disease
• correction of renal artery occlusive disease should be performed at the time of open AAA repair or EVAR
• to avoid worsening kidney function during the procedure, adequate hydration, avoidance of hypotension, and discontinuation of ACE inhibitors and angiotensin receptor blockers is recommended


3. COPD
• increased risk of prolonged ventilator support after surgery
• ABG, pulmonary function tests are recommended in patients with poor pulmonary function
• some patients will benefit from bronchodilators
• smoking cessation for more than 2 weeks before the procedure is likely beneficial


4. Preoperative Imaging
• necessary for determining the best approach – open or EVAR
• anatomic variants that influence approach and technique will be detected – retroaortic left renal vein, horseshoe kidney, variant inferior vena cava
• demonstrating vascular calcifications allows the surgeon to assess the feasibility of clamping the aorta and iliacs at various levels


3. Open Repair
1. Indications
• short aneurysm neck (< 1.5 cm)
• significant angulation (> 60 degrees) of the neck
• suprarenal aneurysm
• young patients – because of the uncertainty about long-term endograft durability and the risk of late rupture
• endovascular capabilities are not available


2. Patient Preparation
• bowel preparation is prudent
• epidural catheters reduce the amount of anesthesia required, allow for prompt extubation, and control pain in the perioperative period
• Swan-Ganz catheters or intraoperative transesophageal echocardiography are used in patients with a significant cardiac history
• cell savers reduce the need for banked blood
• perioperative antibiotics effective against S. aureus should be given


3. Surgical Approaches
• AAA repair may be performed through a transabdominal or retroperitoneal approach


1. Transperitoneal Approach
• necessary when access to the right renal artery is required, when exposure of the right internal and external iliac arteries is required, and when concomitant abdominal procedures must be performed
• disadvantages include a longer ileus, more insensible fluid losses, and more difficult proximal control for juxta renal and suprarenal AAAs


2. Retroperitoneal Approach
• useful when there are extensive intraperitoneal adhesions, or when gastrointestinal or urinary stomas are present
• also valuable when extensive suprarenal exposure is required, and when the left kidney must be revascularized
• disadvantages include poor access to the right iliac and right renal arteries, and inability to fully explore the abdomen
• may also be associated with more chronic pain, wound problems, and incisional hernias
• patient is positioned in the right lateral decubitus position


4. Operative Conduct
• patient is heparinized (100 U/kg) after the aorta has been exposed
• iliac arteries are clamped first to prevent embolization, followed by the proximal aorta
• aneurysm is opened longitudinally, and thrombus removed
• lumbar artery backbleeding is controlled with suture ligation
• inferior mesenteric artery is also usually suture-ligated because it is often chronically occluded
• if there is poor back-bleeding from the IMA, it should be reimplanted into the graft
• a straight tube graft is used unless there is involvement of the iliac arteries, in which case a bifurcation graft is chosen



• the aorta is not transected, and the posterior wall of the graft is sewn from within the aneurysm sac
• right before the completion of the anastomosis, the aortic clamp should be removed temporarily to flush out any debris that might embolize distally
• after the anastomoses are complete, the wall of the aneurysm is sutured over the graft to prevent intestinal adherence to the graft


5. Complications
1. Cardiac Complications
• open repair has a 30-day operative mortality rate of 4% to 5%, with most deaths caused by MIs, CHF, or arrythmias
• prevention starts with an adequate preoperative risk assessment
• in the perioperative period, meticulous control of heart rate, blood pressure, and cardiac filling pressures are necessary


2. Ischemic Colitis
• most commonly involves the sigmoid colon
• much more common after surgery for a ruptured AAA (20% - 40%)
• usually occurs within the first 3 days postop
• symptoms include left lower quadrant pain and tenderness, bloody diarrhea, fever, tachycardia
• initial diagnostic test is proctoscopy or flexible sigmoidoscopy



• initial management is with IV antibiotics, fluid resuscitation and maximization of cardiac output, bowel rest
• patients with peritonitis, or those who do not improve with conservative management, will require a sigmoid colectomy
• a primary anastomosis is not recommended because a leak may contaminate the aortic graft


3. Lower Extremity Ischemia
• usually due to embolization of debris from the aneurysm sac
• prevention is the best treatment: careful dissection of the aorta, careful clamping of the iliacs, flushing of the vessels, and sequential clamp removal
• thrombosis is a less common cause of ischemia, and management may require thrombectomy, anticoagulation, and revision of the distal limb


4. Other Complications
• ureteral injury occurs in < 1% of cases
• sexual dysfunction (impotence, retrograde ejaculation) occurs in 10% to 20%
• paraplegia occurs in < 1% of cases, but the incidence is much higher (1.4%) after a ruptured AAA
• graft infection and aortoenteric fistula are late complications and occur in less than 1% of cases


4. Endovascular Repair (EVAR)



1. Overview
• 75% to 80% of elective AAA repairs are performed with endovascular techniques
• EVAR is associated with a shorter length of stay and a decreased 30-day mortality, when compared to open AAA repair
• EVAR does not improve quality of life beyond 3 months or survival beyond 2 years
• EVAR is associated with a higher rate of re-intervention (which is usually endovascular) than the open procedure
• EVAR is also associated with late AAA rupture


2. Patient Selection
• success of EVAR depends heavily on appropriate patient selection
• requirements for EVAR include an appropriate aortic neck, a distal sealing zone, and a suitable path for the endograft to be placed through
• CTA with reformatted 3-dimensional reconstructions is required to assess the suitability of the anatomy for EVAR


1. Aortic Neck Anatomy
• aortic neck is the aorta above the aneurysm and below the renal arteries
• this is the site of proximal fixation of the device
• a circumferential seal must be obtained in this area to prevent blood leakage into the aneurysm sac
• the aortic neck should be at least 15 mm in length, and its diameter should be less than 32 mm
• angulation of the neck in the vertical plane also needs to be less than 60 degrees to prevent endoleaks, kinking, and stent migration
• additional factors that can prevent an effective seal and fixation include severe aortic wall calcification and circumferential thrombus


2. Iliofemoral Anatomy
• presence of thrombus, calcifications, and tortuosity can prevent obtaining a distal seal
• most endograft systems require a 15 mm segment of suitable iliac artery
• largest iliac diameter that can be treated is 25 mm


3. Complications
1. Access Complications
• occur in 3% of cases
• hematoma, pseudoaneurysm, arterial occlusion or dissection
• iliac artery rupture or transection


2. Endoleaks
• defined as the persistence of blood flow outside of the endograft within the aneurysm sac
• systemic circulation in the aneurysm sac results in aneurysm sac expansion and potential rupture
• endoleaks are the most common cause of secondary interventions and aneurysm-related morbidity following EVAR


1. Type I
• results from an inadequate seal at the proximal aortic (Ia) or distal iliac (Ib) attachment sites
• can result from a size mismatch between the graft and vessel, severe aortic neck angulation, or a heavily calcified aortic wall
• usually detected by angiography immediately after deployment
• requires immediate repair since these leaks result in growth and potential rupture of the aneurysm sac


2. Type II
• most common type of endoleak (10% - 20%)
• occurs from retrograde filling of the aneurysm sac from a patent lumbar artery or IMA
• management consists of close observation, since many will resolve spontaneously and the risk of rupture is low


3. Type III
• caused by defects in the fabric of the graft at the junction of modular components
• treatment consists of placing covered stents to exclude the aneurysm sac from systemic pressure


4. Type IV
• caused by leaking of blood between the interstices of the graft fabric
• often resolves after anticoagulation is reversed
• if the leak persists, the endograft may need to be relined


5. Type V (Endotension)
• persistent growth of the aneurysm sac in the absence of a detectable leak
• exact etiology is unknown, but may result from an undetected endoleak from one of the other types
• must be followed closely because of the risk of rupture


3. Iliac Limb Thrombosis
• occurs in 11% of cases within 6 months of EVAR
• ipsilateral buttock and/or limb claudication are typical symptoms
• symptomatic patients require an attempt at thrombolysis or a femoral-femoral bypass


7. Ruptured AAAs
• overall mortality approaches 90%
• mortality rate of patients who reach the hospital alive is > 50%


1. Diagnosis
• classic triad consists of sudden, severe abdominal, back, or flank pain, hypotension, and a palpable periumbilical mass
• if the hematoma is contained, the initial symptoms may be surprisingly mild
• differential diagnosis includes: perforated viscus, biliary disease, pancreatitis, kidney stones, acute myocardial infarction
• if the patient is stable, then a CT scan will confirm the diagnosis and help with the operative planning and approach



2. Resuscitation
• patient should be approached like a trauma patient
• large-bore IVs should be inserted, and blood transfused as necessary
• preventing hypothermia is critical: all blood and fluids should be warmed
• overresuscitation should be avoided
• resuscitation goal is a conscious patient with systolic pressures in the 80 to 100 mm Hg range (permissive hypotension)
• operation should not be delayed by resuscitation: once the diagnosis is made, the patient should be brought to the OR


3. Aortic Control
• percutaneous placement of an occlusive balloon in the suprarenal aorta will gain proximal control in most patients
• aortic control via a left thoracotomy should be performed if an occlusive balloon is not available and the abdomen is expected to be hostile


4. Open Repair
• patient should be prepped and draped before the induction of general anesthesia
• open or retroperitoneal approaches are possible
• after the abdomen is opened, if an occlusion balloon has not been placed, the supraceliac aorta can be compressed manually with a retractor, allowing the anesthesiologist time for resuscitation
• if infrarenal control appears to be difficult, a clamp can be placed on the suprarenal aorta
• the aneurysm should be opened only after proximal and distal control have been achieved
• once the proximal anastomosis is complete, the proximal clamp should be moved to the graft
• must take great care to avoid injury to the left renal vein, vena cava, iliac and lumbar veins


5. Endovascular Repair
• some studies suggest that there is less morbidity and mortality for EVAR
• operative planning will require a CTA or angiogram
• the contraindications for EVAR for ruptured AAA are the same as for elective AAA repair
• procedure may be done under local anesthesia with sedation, and is essentially performed just like an elective EVAR