25/02/2012

ABDOMINAL AND THORACIC AORTIC ANEURYSMS

DR N. GOVENDER

Commentators:Dr Z. Moolla & Dr T. Mbebe Moderator: Ms A. Ramnarain

In 1948, Dr. Rudolph Nissen, performed an exploratory laparotomy for abdominal pain on Albert Einstein at the Brooklyn Jewish Hospital. A "grapefruit-sized" aortic aneurysm was discovered. During that era, ligation of the abdominal aorta had already proved to be ineffective, and replacing the aorta with a graft was still a decade away. The only available treatment was to attempt to reinforce the aortic wall and delay the inevitable rupture. Dr. Nissen wrapped the visible anterior portion of the aneurysm with cellophane (a tissue irritant), in the hopes of stimulating an intense fibrous tissue reaction that would strengthen the bulging aortic wall. Einstein recovered from the operation after a 3-week hospital stay.On April 12, 1955, Einstein developed worsening abdominal pain. Einstein refused surgery, saying: “I want to go when I want. It is tasteless to prolong life artificially. I have done my share, it is time to go. I will do it elegantly”He demised on April 18, 1955.

DEFINITIONS1

Aneurys :focal permanent dilatation of an artery more than 1.5 times the normal diameter

The normal infrarenal aortic diameters in patients older than 50 years are 1.5

cm inwomen and ±1.7 cm in men.

By convention, an infrarenal aorta ≥ 3-5 cm in diameter is considered aneurismal.

Arteriomegaly:diffuse arterial enlargement with an increase in the diameter more than 50%

of normal.

Ectasia : permanent dilation of an artery of less than 50% of the normal diameter.

Pseudo aneurysm/False aneurysm: is the contained extravasated blood from an arterial wall

disruption and is contained by the adjacent structures. It does not contain all 3 layers of

the vessel wall.

ABDOMINAL AORTIC ANEURYSMS(AAA’S)AETIOLOGY2

Degenerative / Atherosclerotic, Fibromuscular dysplasia
Infective/Mycotic / Bacterial, fungal, syphilis, viral
Pseudoaneurysm / Trauma and anastamotic disruption
Congenital /
  • Idiopathic
  • Tuberous sclerosis
  • Turner’s syndrome
  • Menke’s syndrome
  • Persistant sciatic artery

Inflammatory /
  • Takayasu’s disease
  • Behçet’s disease
  • Kawasaki’s Disease
  • Polyarteritis Nodosa
  • Giant cell arteritis
  • Periarterial(pancreatitis)

Connective tissue disorders /
  • Marfan’s syndrome
  • Ehler’s Danlos syndrome
  • Cystic medial necrosis
  • Berry(cerebral)

Post dissection / Idiopathic, Cystic Medial necrosis, Trauma
Post Stenotic / Thoracic outlet syndrome, Coarctation
Miscellaneous / Pregnancy induced, Inflammatory

PATHOPHYSIOLOGY3,4

  • Loss of elastin is the initiating event in abdominal aortic aneurysm (AAA) formation, whereas loss of collagen is required for continued expansion
  • The primary event in the development of an AAA involves proteolytic degradation of the extracellular matrix proteins elastin and collagen by various proteolytic enzymes, including matrix metalloproteinases
  • There is a reduced amount of vasa vasorum in the abdominal aorta (compared to the thoracic aorta); consequently, the tunica media relies mostly on diffusion for nutrition which makes it increasingly susceptible to damage.
  • Haemodynamics affect the development of AAAwhich has a predilection for the infrarenal aorta as thediameter decreases from the root to the bifurcation, and the wall of the abdominal aorta also contains a lesser proportion of elastin. The mechanical tension in abdominal aortic wall is therefore higher than in the thoracic aortic wall. The elasticity and distensibility also decline with age, the half life (t½) of elastin being 71years, which can result in gradual dilatation of the segment.
  • Higher intraluminal pressure in patients with arterial hypertension contributes to the progression of the pathological process.

SYMPTOMS3

  1. Local compression
  • Compression of the duodenum usually present with early satiety, nausea and vomiting
  • Compression of ureters occurs less frequently and presents with obstructive uropathy and recurrent urinary tract infections
  • Erosion of vertebral bodies presents with back pain

2.Rupture/imminent rupture

  • The classical triad of acute abdominal pain, haemodynamic instability and pulsatile abdominal mass is only present in 50%
  • Retroperitoneal rupture is usually secondary to a small posterolateral tear in the aortic wall. The bleeding is initially contained within the retroperitoneum, but is followed at unpredictable interval by frank intraperitoneal rupture
  • Free intraperitoneal rupture is catastrophic and is heralded by acute abdomen and cardiovascular collapse
  • Expansion presents similar to rupture without haemodynamic instability. The pain associated with expansion is poorly understood but may be related to stretching of layers of the aortic wall or pressure on adjacent somatic nerves

3.Embolism

  • Distal macroembolism presents with acutely threatened limb,
  • Distal microembolism presents with blue toe syndrome

4.Fistulation

  • Aortoduodenal fistulas are less frequent, present classically with intermittent unexplained haematemesis and maelena or massive uncontrollable haematemesis.
  • Aortocaval fistula is also less common, and presents with abdominal bruit, widened pulse pressure, venous hypertension and high output cardiac failure

INVESTIGATIONS 4

CT ANGIOGRAM

  • Gold standard
  • Characteristic findings of AAA on CT are dilation of the calcified wall of the aorta with varying degrees of surrounding thrombus
  • Accurate for defining anatomy and anomalies including:

Aneurysm rupture(disruption of calcifications with obliteration of surrounding soft tissue

InflammatoryAAA (thickening of the aortic wall and adherence to surrounding structures)

Mycotic Aneurysm(eccentric thickening of aortic wall with intramural air)

  • Enables assessment of graft function as well as complications following open repair
  • Vital in selection of candidatesfor endograft repair (reconstructed images allows for precise length measurements within aorta and tortuous iliac arteries, crucial to proper graft sizing and success of the endovascular repair)
  • Disadvantageous as it is expensive and exposes the patient to both contrast and ionising radiation

ANGIOGRAM

  • Shows visceral and occlusive disease(can clearly identify mesenteric and/or renal artery stenosis, accessory renal arteries, inferior mesenteric artery occlusion)
  • Can determine the exact relation of aneurysm to major aortic branches.
  • Useful in evaluating distal runoff, in patients with symptoms of claudication
  • Reserved for patients with abnormal anatomy, juxtarenal,supracoeliac, and complex aneurysms
  • Expensive and exposes patient to contrast load
  • Invasive(local complications)

 Haematoma, AV fistula and/or a pseudoaneurysm at arterial puncture site

 Intimal tear, resulting in arterial rupture, thrombosis, pseudoaneurysm formation;

 Distal embolization related to atheroemboli or pericatheter thrombosis.

  • Incorrect assessmentof aneurysm size in the presence of thrombus

MRA

  • Avoids radiation and invasive angiography but is expensivewith limited availability
  • Motion artefact in breathing patient
  • Patient Contraindications (Cardiac pacemakers etc)

SCREENING5

Screening for AAA would most benefit those who have a reasonably high probability of having an AAA that is large enough or will become large enough to benefit from surgery. U.S. Preventive Services Task Force recommends one-time screening for abdominal aortic aneurysm (AAA) by ultrasonography in men aged 65 to 75 who have smoked, as this was proven tolead to decreased AAA-specific mortality and showed a benefit for all-cause mortality

Because of the low prevalence of large AAA’sin men who have never smoked and in women, the number of AAA-related deaths that can be prevented by screening this population is small. There is good evidence that screening and early treatment result in important harms, including an increased number of surgeries with associated morbidity and mortality, and psychological harms.

MANAGEMENT:1,5,6,7

Intervention should be considered when the risk of rupture outweighs the operative risk. As such, the diameter is considered the most important factor in predicting the risk of rupture. The law of Laplacedecrees that the tensile strength of the arterial wall is a function of the pressure multiplied by the radius. Consequently larger vessels are more likely to rupture.

Greatest Diameter(cm) / Annual Rupture Risk(%)
3.0 – 5.5 / 0.6
5.6 – 5.9 / 5 – 10
6.0 – 6.9 / 10 – 20
7.0 – 7.9 / 20 – 30
>8.0 / 30 – 50

Other factors such as rate of expansion and risk factors associated with rupture are taken into account and the risk can be categorised into low, average or high.

Low / Average / High

Diameter

/ <5.0cm / 5.0 – 6.0 cm / > 6.0 cm
Expansion / < 0.3 cm / year / 0.3 – 0.6 cm / year / > 0.6 cm / year
Smoking / COPD / Mild / Moderate / Severe / steroids
Family History / No relatives / One relative / More than one
Hypertension / Normal BP / Controlled / Poorly controlled
Shape / Fusiform / Saccular / Very eccentric
Wall stress / Low ( 35 N / cm2) / Med ( 40 N / cm2 ) / High ( 45 N / cm2 )
Gender / Male / Female
Anticipated mortality / 1-3% / 3-7% / 5-10% add 3-5% for each co-morbid condition

Guidelines for the Management of AAA’s

Joint Council of the American association for Vascular Surgery and Society for Vascular Surgery)

The arbitrary setting of a single threshold diameter for elective AAA repair that is applicable to all patients is not appropriate ,and the decision for repair must be individualized in each case

Randomised control trials, including the ADAMS trial, and UKSAT have shown that the risk of rupture of small (<5 cm) AAA’s is quite low and that a policy of careful surveillance upto a diameter of 5.5 cm is safe ,UNLESS rapid expansion(>1cm/yr) or symptoms develop.(However early surgery is comparable to surveillance with later surgery , so patient preference is important, especially for AAA’s 4,5 to 5,5cm in diameter)

A diameter of > 5.5cm is an appropriate threshold for repair in an average patient

  • Younger , low risk patients with longer life expectancy may prefer earlier repair
  • If surgeons documented operative mortality rate is low, repair may be indicated at smaller sizes (4.5 to 5.5cm) if that is the patients preference.

For women or AAA’s with greater-than-average risk for rupture , 4.5 – 5.0 cm is an appropriate threshold for elective repair.

For high risk patients , delay in repair until larger diameter is warranted , especially if endovascular repair is not possible.

In view of its uncertain long term durability and effectiveness , as well as increased surveillance burden,EVAR is most appropriate for patients at increased risk from open aneurysm repair

EVAR may be the preferred treatment method if anatomy is appropriate for older, high risk patients , those with ‘hostile abdomens’,or other clinical circumstances likely to increase the risk of conventional open repair

Use of EVAR in patients with unfavourable anatomy markedly increases the risk of adverse outcomes, need for conversion to open, or AAA rupture

There does not appear to be any justification that EVAR should change the accepted size thresholds for intervention in most patients.

BEST MEDICAL MANAGEMENT6

The therapeutic goal is to prevent small aneurysms from reaching a size at which the rupture risk is higher than the operative riskor decrease the expansion rate for larger aneurysms.

Medical management should focus on reducing co morbid conditions and monitoring the aneurysm for enlargement

  • Ultrasound surveillance is recommended at

3 monthly for 4.5 to 5.5cm

6 monthly for 3 to 4.5cm

  • The approaches proposed to prevent progression of aneurysmal diseaseinclude haemodynamic management, inhibition of inflammation, and protease inhibition.
  • Beta blockade has been postulated to decrease the rate of AAA expansion. Two randomized trials failed to show any reduction in growth rate with beta blockade.A randomized trial from Toronto showed that patients taking beta blockers had worse quality of life and did not tolerate the drug well.
  • Doxycycline (150 mg daily) shown to slow the rate of AAA expansion in one small randomized trial .This antibiotic has activity against C. Pneumonia (present in many AAAs.(Level B evidence)
  • A number of studies agree that tobacco use is associated with an increased rate of aneurysm expansion. Level B and C evidence is available to suggest that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) may inhibit aneurysm expansion.
  • There are animal data but no human data demonstrating that angiotensin-converting enzyme inhibitors or losartan, an angiotensin receptor blocker, will decrease the rate of AAA expansion.

PREOPERATIVE OPTIMISATION9

  • Cessation of smoking and structured exercise programme may improve cardiopulmonary reserve
  • Antiplatelet medication to protect against thromboembolic complications, if aspirin is not tolerated clopidogrel should be given
  • Statins - improve both the short- and long-term outcomes following non- cardiac surgery due to plaque stabilisation. Statins should be given irrespective of serum cholesterol concentrations
  • Β-blockers for inducible ischemia
  • Hypertension should be controlled preferably with ACE inhibitor

OPERATIVE MANAGEMENT10,11,12

The relative indication for OPEN repair versus Endovascular repair(EVAR) is in a constant state of evolution,and currently the choice is left to the surgeon.

Outcome of Trials in the decision for Open versus Endovascular management of AAA’s

  • The endovascular aneurysm repair (EVAR) 1 and 2 and Dutch Randomized Endovascular Aneurysm Management (DREAM) trials address management of abdominal aortic aneurysms (AAAs) larger than 5.5 cm in diameter.
  • The DREAM and EVAR 1 trials randomized patients appropriate for open repair between endovascular repair (EVAR) and open repair (OR), and the EVAR 2 trial randomized patients unfit for OR between EVAR and conservative non-operative management (No Rx).
  • The EVAR 1 trial showed a 3% lower initial mortality for EVAR, with a persistent reduction in aneurysm-related death at 4 years. Improvement in overall late survival was not demonstrated. Similarly, the DREAM trial observed an initial mortality advantage for EVAR, but overall 1-year survival was equivalent in both groups. Both trials found significantly higher complication and intervention rates and higher hospital costs with EVAR, and by 1 year a quality of life (QOL) benefit was not evident.
  • The EVAR 2 trial did not demonstrate a survival advantage of EVAR with respect to non-operative management; noted that EVAR was associated with greater likelihood of treatment complications, subsequent interventions, and threefold higher costs.
  • Both EVAR trials were limited by long delays between randomization and treatment. Moreover, 27% of patients in EVAR 2 crossed over from non -operative to endovascular repair, and these patients had a lower procedure mortality from EVAR than those originally assigned to it (2% v 9%)
  • Outcomes of the EVAR 2 trial have not settled the choice between EVAR and no treatment in this scenario to everyone's satisfaction. In patients with large AAAs who are fit for OR, EVAR offers an initial mortality advantage over OR, with a persistent reduction in AAA-related death at 4 years. However, EVAR offers no overall survival benefit, is more costly, and requires more interventions and indefinite surveillance with only a brief QOL benefit. It may or may not offer a mortality benefit over nonoperative management in patients with large AAAs who are unfit for open repair, but the statistical significance of this comparison is inconclusive

PREOPERATIVE ASSESSMENT OF THE ELECTIVE AAA REPAIR9,10

Patients presenting for abdominal vascular surgery have a high incidence of co morbidities

  • Coronary artery disease, often with left ventricular dysfunction which is responsible for 50-60% of all peri-operative and late deaths

The ability to exercise is an excellent indicator of cardiovascular and respiratory fitness. Patients who cannot climb a flight of stairs frequently have adverse outcomes

Other major cardiac risk factors include:

  • recent myocardial function (MI) < 1 month
  • unstable or severe angina
  • decompensate heart failure
  • significant arrhythmias
  • severe valvular heart disease
  • coronary artery bypass graft or percutaneous transluminal coronary angioplasty < six weeks

The risk of surgery in elective repair can be stratified on the basis of the presence of the following preoperative conditions and co-morbidities

Risk Stratification for Open AAA repair

Risk Factor / Low Risk / Intermediate Risk / High Risk
Age / Before 8th Decade / 8th Decade / 9th Decade or later
Functional Status / Active regular physical exercise / Sedentary but otherwise independant / Minimally able to accomplish daily activites
Cardiac / No or minor ACC clinical predictors / Intermediate ACC clinical predictors / Severe ACC clinical predictors
Pulmonary / No Clinical Disease / Mild COPD, FEV1> 1L/sec / O2 dependant , FEV1 <1L/sec
Renal / Normal Renal function / Creatinine 2.0 – 3.0 / Creatinine >3.0
Other / Non-inflammatory infrarenal AAA / Juxtarenal,suprarenal or Inflammatory AAA / Pugh-Child’s class B or C liver failure

Low risk patients: no clinical predictors, not associated with postoperative cardiac events, no need for further cardiac testing

Intermediate risk patients: at least two major cardiac risk factors – non invasive evaluation of cardiac status with dobutamine stress echocardiogram. Patient with positive stress test should have coronary angiogram

High risk patients: multiple major risk factors should undergo coronary angiogram. If necessary coronary revascularisation should be performed prior to aneurysm repair

Local practise:CPX (Cardiopulmonary exercise testing) has been used to identify high-risk surgical patients and to predict long-term outcome in patients with heart failure This test measures four variables associated with survival: anaerobic threshold, ˙V O2 peak,˙VE/˙VO2 and ˙V E/˙VCO2. CPET is performed on a cycle ergometer with respiratory gas analysis and simultaneous ECG recording

Patients who underwent preoperative CPX testing were found to have improved 30-day and mid-term survival if the ˙V E/˙VCO2was less than 43

An anaerobic threshold of < 11ml/kg/min, particularly if associated with ECG evidence of ischaemia, is associated with high operative mortality. Low risk patients are considered for open repair, the high risk patients are subjected to endovascular repair

Patients identified as unfit by a preoperative test may benefit from the opportunity to improve their chances of survival through interventions such as drug therapy (statins), an exercise programme and dietary advice.

Alternatively a transthoracic echocardiogram is performed to assess left ventriclar ejection fraction, and lung functions to evaluate pulmonary reserves

All patients undergo objective test to assess the quality of renal function (Creatinine Clearance)

OPEN REPAIR:11,12

Exposing the aorta for repair can be achieved via a transperitoneal /transabdominal(TPA) or Retroperitoneal (RPA) approach

COMPLICATIONS

Risk factors for postoperative complications are age, emergency operation, duration of clamp time and operation, the amount of transfusion during clamp and operation, the amount of crystalloid during clamp, and blood loss.

Cardiac / Usually occur in first 48hrs post-op
Cross clamping of the infrarenal aorta causes increased vascular resistance → arterial hypertension
A diseased coronary system may be unable to respond to increases in cardiac load, resulting in cardiac failure and myocardial ischaemia.
Haemorrhage / Problematic haemorrhage intra-operatively is due to bleeding from the proximal suture line, or inadvertent iatrogenic vein injury
Iatrogenic / Urethral
Splenic
Pancreatitis(secondary to handling of mesenteric base)
Renal Failure / Related to Ischaemic clamp time
Premorbid renal impairment important risk factor
GI Complications / Paralytic Ileus(Retroperitoneal dissection)
Colonic Ischaemiamay occur due to inappropriate inferior mesenteric artery ligation, inadvertent collateral vessel damage or internal iliac artery thromboembolic event
Limb Ischaemia / Due to either distal embolization of debris from aneurysm sac or more rarely graft thrombosis.
Impaired sexual function / May result from damage to autonomic pathways found on the left side of the infrarenal aorta
Late Complications
  • Less than 10% of patients experience late complications of AAA repair during their lifetime. Most of these are severe, however, and often fatal.
  • Anastomotic disruptionresulting in a pseudoaneurysm
  • Aortoenteric fistula
  • Thrombosis of an aortoiliac graft after
  • In long-term follow-up after AAA repair, approximately 5% of patients develop complications secondary to other aneurysms. If these secondary aneurysms rupture, less than 5% of patients survive.It is important to detect these aneurysms before rupture occurs.Hypertension significantly increases the risk of secondary aneurysm development.

ELECTIVE ENDOVASCULAR AAA REPAIR (EVAR)11,15