An abdominal aortic aneurysm (AAA) represents a localized dilation of the largest blood vessel in the lower part of the body, which supplies blood to the legs. With the increasing age of the population, the incidence of AAA rises sharply. The incidence of AAA is approximately 50 to 55 per 100,000 per year in men aged 55 to 64 years and nearly ten times that number in men over 80 years.

Anatomy and Pathology

The aorta is the largest blood vessel in the body. It begins at the heart and passes backward to be located deep in the body just in front of the bony spine. When it passes through an opening in the diaphragm it becomes the abdominal aorta. As the aorta passes into the pelvis it splits into the common iliac arteries. The most important branches of the abdominal aorta feed the kidneys and bowel. (Figure 1).

Figure 1 - Anatomy showing the abdominal aorta and its branches. The abdominal aorta extends from the diaphragm to the iliac vessels. The branches feed the organs within the abdominal cavity.

The frequency of aneurysms to form in this location relates to abnormalities in blood flow as well as changes which occur to the blood vessel wall over time due to hardening of the arteries (atherosclerosis). The aneurysm wall may contain calcium deposits. And small blood clots (thrombi) may form on the wall. In general, aneurysms expand over time and ultimately burst, much like a balloon which continues to expand and finally bursts. When an aneurysm ruptures, there are often catastrophic consequences

• 50% of individuals do not survive long enough to receive medical attention
• Of those who make it to the hospital, 25-30% die of postoperative complications

These sobering facts point out the lethal nature of AAA and would seemingly strongly favor surgery for any AAA. However, It appears that small aneurysms (less than 4 cm. or 1.6 inches) seldom rupture at the same rate as do larger ones. When the aneurysm reaches around 5 cm (2 inches) there is a sharp increase in aneurysm rupture. Therefore most vascular surgeons do not recommend elective repair for aneurysms smaller than 4 cm.

History and Examination

• Predisposing factors to an AAA are
• Family history of AAA
• Male
• Smoking
• Vascular disease in the legs
• Increased age
• Diabetes mellitus
• High blood cholesterol
• High blood pressure
• An AAA that does not rupture, frequently causes no symptoms

Occasionally they produce some vague back pain or abdominal throbbing

• A large AAA may be felt as a pulsatile mass in the abdomen
• A piece of the clot on the wall of the AAA may break off (embolus) and block an artery in a leg
• These aneurysms are usually picked up by chance during an examination for complaints related to the bowel, kidney or back

In contrast, an AAA that has ruptured causes

• severe flank or abdominal pain
• hypotension (low blood pressure) that may progress to shock
• full, tense pulsatile abdominal mass

Rupture of an AAA is an urgent surgical emergency with mortality rates of 50 to 70%

Special Tests

Noninvasive tests

Abdominal Ultrasound. An ultrasonic sensor connected to a special computer and monitor is placed over the abdomen. This produces a picture of the abdominal contents and is an accurate method for measuring the size of an AAA. Ultrasound is commonly used to follow patients with aneurysms less than 4 cm in diameter. It provides an excellent method for assessing aneurysm growth over time. Once an aneurysm reaches the 5 cm, elective surgical repair is necessary (Figure 2).

Figure 2 - Abdominal ultrasound of a 6.5 cm. (2.6 inches) abdominal aortic aneurysm. The small arrows outline the aneurysm wall. Blood flow through the lumen is in color while clot lies between the wall and lumen (Courtesy S. Sadiq, M.D.)

• CT scan of the abdomen will show an AAA. In particular the CT scan will show calcification in the wall of the AAA as well as the relationship of the aneurysm to surrounding organs (Figure 3)

Figure 3 - CT scan through an 8.5 cm. (3.4 inches) abdominal aortic aneurysm. Note the clot that fills most of the aneurysm. The lumen is open and is where the blood flows. Courtesy S. Sadiq, M.D.

• MRI (magnetic resonance imaging) will also show an AAA and its relationship to surrounding organs

Invasive testing utilizes a special catheter placed in the artery under fluoroscopic control. A dye that shows up on X-ray is injected through the catheter. Serial X-ray films or aortogram are then obtained of the aorta. This produces the most accurate picture of the aneurysm. For routine AAA below the level of the arteries to the kidneys, a CT scan provides enough information to make arteriogram unnecessary (Figure 4).

Figure 4 - Aortogram of an abdominal aortic aneurysm. Note small linear calcifications in the wall of the aneurysm (multiple small arrows) and the clot (*) between wall and lumen. Courtesy S. Sadiq, M.D.

Surgical Treatment

Elective repair provides an excellent option for those individuals who have an AAA 5 cm or greater

• Under most circumstances the combined morbidity (chances of having something bad happen, i.e. heart attack, pneumonia, kidney failure) is approximately 3-5%
• Mortality (the chance of not surviving the operation) is approximately 2-3%

Therefore the overall risks associated with an elective repair is between 5-8% versus 60-75% risk of dying with emergent repair of a ruptured AAA. AAAs between 4 and 5 cm represent a group of patients whose management remains controversial. Some of the factors that increase the likelihood of AAA growth are:

• Hypertension
• Chronic pulmonary disease
• Aneurysm expansion of more than 0.5 cm in six months
• Development of symptoms 40% of patients with the above factors will undergo elective repair after three years. This suggests that good risk patients with these factors and 4-5 cm aneurysms probably should have elective surgery

Other considerations before carrying out surgery are

• Full evaluation of the heart and lungs
• Laboratory tests including
  1. Blood count
  2. Electolyte (salt) concentration in the blood
  3. Electrocardiogram (EKG)
  4. Chest X-ray
• Cleansing of the colon with a laxative or enemas to ensure that the colon is not distended

Surgical Procedure

• An antibiotic is given in a vein on the day of surgery
• The operation is performed under general anesthesia
  
• An incision is made from just below the breastbone to just below the belly button
  
• The bowel is moved out of the way
  
• The aneurysm along with a short segment of aorta above and below the aneurysm is exposed
  
• The aorta above and the common iliac arteries below the aneurysm are clamped off with special vessel clamps (Figure 5A-5E)
  
• The aneurysm is opened and any blood clot (thrombus) on the inside of the aneurysm is removed
  
• The wall of the aneurysm is usually not removed
  
• The any small artery openings in the aneurysm wall are oversewn
  
• The aorta above and below are cut across
  
• A prosthetic graft made of Dacron is sutured in place between the two ends. Dacron is a fabric that causes no reaction by the body
  
• The aneurysm wall is then wrapped around the Dacron graft
  
• The clamps are removed to allow blood to flow into the arteries to the legs
  
• The wound is then closed with sutures
  
• The operation usually takes between 4-6 hours to complete

  5A - AAA extending from just below the renal arteries to the iliac arteries.	5B - Vascular clamps are placed across the aorta below the renal arteries and across the common iliac arteries. The aneurysm is opened.
  5C - The clot that lines the inner wall is removed. The wall of the aneurysm is preserved.	5D - The graft is sutured in place between the upper and lower ends of the aorta.
  5E - The preserved wall of the aneurysm is wrapped around the graft and following which the clamps are removes to allow blood to flow through the graft.

Complications

• Kidney failure can occur but is mostly seen when the aneurysm is present above the level of the renal (kidney) arteries. This may be transient with only lab abnormalities to identify the problem, or it may require dialysis, which can be temporary or permanent. Fortunately the need to place clamps above the renal arteries is low occurring I in 25-50 surgeries
• If the patient has had previous abdominal surgery, adhesions will have formed. This may make access to the aorta hazardous with chance for bowel injury. If this occurs the procedure must be abandoned because the risk of implanting a graft in a potentially infected field is too high
• If a blood transfusion is necessary, the risk of transmissible infections (hepatitis, HIV etc.) is possible though very low
• Pneumonia. Individuals who smoke are particularly at risk for pneumonia. Smoking should be stopped at least 3-4 weeks preoperatively and not started
• Heart attack
• Clots in the leg veins that may go to the lung (pulmonary embolus)
• Infection
• Bowel or ureter injury

Post Operative Care

• After surgery the patient is placed in the intensive care unit for care and monitoring
  
• There are usually several catheters in place
  
  1. Urinary catheter to monitor how much urine is produced
     
  2. Arterial catheter to monitor the blood pressure
     
  3. Central venous catheter - a catheter in the heart placed through a vein to monitor pressure in the heart
     
  4. Epidural catheter - a catheter placed in the spine through which medication is given to control pain
     
  5. Nasogastric or NG tube - A tube passed through the nose into the stomach. This tube functions to decompress the bowel and is kept in place until bowel function returns to normal and the patient passes gas. After the NG tube is removed, a diet is started