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19 May 2010

Indications and Patient Selection for Abdominal Aortic Aneurysm Surgery

Mr R Chalmers, Consultant Vascular Surgeon (Royal Infirmary of Edinburgh).

The purpose of elective abdominal aortic aneurysm (AAA) repair, be it open or endovascular, is to prevent premature death from AAA rupture.  In all patients the critical issue in deciding whether to proceed with treatment is the balance between the risk of aneurysm rupture versus the risk of the intervention in the individual patient’s case.  The risk of AAA rupture is currently best estimated in relation to the maximum diameter of the aneurysm.

The UK Small Aneurysm (UKSAT) trial and the US Aneurysm Detection and Management (ADAM) trial both indicated that there was no survival advantage for open surgical treatment of small (4-5.5cm) AAA compared to continued ultrasound surveillance.  The situation as regards endovascular AAA repair (EVAR) and small AAA is still the subject of ongoing randomised trials (CAESAR and PIVOTAL trials) and the results will prove interesting. Standard current practice is to manage asymptomatic AAA of diameter < 5.5 cm conservatively with regular monitoring with ultrasound surveillance. AAA of diameter > 5.5cm are considered for intervention.

The decision regarding the type of intervention (open or EVAR) is made on a number of grounds. The EVAR 1 trial showed a significant advantage for endovascular repair over open surgery in terms of operative mortality when the two interventions were compared in a randomised controlled trial of AAA patients deemed fit for open surgery.  However, in patients not thought fit, EVAR did not confer any survival advantage. Therefore, patient selection for intervention is dictated by a thorough assessment of fitness for intervention.  Nowadays, all patients thought fit for intervention undergo CT scan evaluation as to their suitability for EVAR as this intervention has a lower associated operative mortality.  The decision about suitability for EVAR is dictated by issues related to AAA morphology, including length and angulation of the infrarenal neck, status of iliac arteries including co-existing iliac aneurysm disease, iliac tortuosity and the calibre of the lumen of the external iliacs (they need to be at least 7mm to allow passage of the stent-graft via the femoral arteries).  The proportion of patients suitable for EVAR in an institution with a normal referral pattern (ie not a tertiary referral centre) is likely to be at least 50% nowadays and will undoubtedly increase over time as technology advances further.

Centres practising AAA intervention subject patients to a thorough, multidisciplinary assessment of overall medical fitness.  Included in this should be formal assessment of cardiac status, respiratory function, renal function and the presence of other diseases known to represent increased risk for major non-cardiac vascular surgery (eg diabetes, peripheral vascular disease, stroke, hypertension).  In patients with no cardiac history usually resting echocardiography and assessment of exercise tolerance (eg the stair test) will be an adequate baseline test of cardiac status. In patients who have known ischaemic heart disease treated either medically or post intervention, some form of stress testing is mandatory.  Treadmill test (Bruce protocol) is reasonable but often not feasible in the presence of co-existing peripheral arterial disease. A number of centres are investigating the use of bicycle ergometers for cardio-pulmonary exercise testing. This may come to replace other forms of testing if the evidence supports its use. Currently many centres will use dobutamine stress echocardiography in this setting.  This is a very sensitive indicator of the existence of significant coronary artery disease.  A negative stress echocardiogram is very reassuring.  Equally positive cardiac stress tests will indicate a strong possibility of significant coronary artery disease (and increased operative risk) and might well indicate the need for coronary intervention prior to AAA treatment.

All AAA patients should be discussed in a multidisciplinary environment that involves vascular surgeons, radiologists, and anaesthetists.  The decision about intervention and the preferred treatment modality (EVAR vs open repair) should be reached of the basis of standard, nationally agreed criteria.


References

The UK Small Aneurysm Study Participants. Mortality results for the randomised controlled trial of early elective surgery or ultrasonographic surveillance for small abdominal aortic aneurysms. Lancet 1999;352:1649-55

Brown LC, Thompson SG, Greenhalgh RM, Powell JT; UK Small Aneurysm Trial Participants. Fit patients with small abdominal aortic aneurysms (AAAs) do not benefit from early intervention. J Vasc Surg. 2008 Dec;48(6):1375-81.

EVAR trial participants.
Endovascular aneurysm repair versus open repair in patients with abdominal aortic aneurysm (EVAR trial 1): randomised controlled trial. Lancet. 2005 Jun 25-Jul 1;365(9478):2179-86.

EVAR trial participants.
Endovascular aneurysm repair and outcome in patients unfit for open repair of abdominal aortic aneurysm (EVAR trial 2): randomised controlled trial. Lancet. 2005 Jun 25-Jul 1;365(9478):2187-92.

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