Vascular Surgery

Endovascular Grafting of Aortic Aneurysms

The New Age in Vascular Surgery

Sir Astley Cooper is credited with performing the first operation for abdominal aneurysms when he ligated the aortic bifurcation after rupture of an external iliac aneurysm in a 38-year-old man. The patient survived a further 40 hours before he died. From this time, people have been striving to improve the results and decrease the morbidity of operations for abdominal aneurysms. Endovascular procedures for the treatment of aneurysms in fact is not a new concept but dates back to early attempts to thrombose aneurysms by passing knitting needles into the aneurysm sac (Velpeau, 1831) or many meters of iron wire (Moore, 1865). These techniques were further refined with the passage of electric currents through wires placed in the aneurysm sac in an attempt to promote thrombosis (Corradi, 1879).

None of these techniques however proved successful and Matas is credited with the first long-term successful management of an aortic aneur ysm after performing an infra-renal ligation of the aorta in 1923. The patient survived a further 17 months before dying from TB. The modern era of aortic repair is as recent as 1951 when Dubost resected an abdominal aortic aneurysm (AAA), replacing it with an aortic homograft.

Problems with degeneration of these homografts led to the development of prosthetic grafts, initially using Vinyon-N (the material used for parachutes at the time) with the first report of successful use in humans by Voorhees in 1954. Since this time, developments in materials and techniques have led to the current standard of open surgical repair for AAAs using prosthetic grafts (eg Dacron) with very acceptable morbidity and mortality rates for elective repair in dedicated vascular surgical units. Open surgical repair however remains a high-risk surgical procedure and is not possible in some patients due to significant co-morbidities. This has lead to the continued search for less invasive ways of repairing AAAs, including the return to the endovascular route of repair.

What is endovascular grafting for AAAs?

This involves the treatment ofaneurysms via the passage of catheter based graft delivery systems from the groins (through the femoral arteries). The perceived advantage of such an approach involves avoiding the major morbidity of a large abdominal incision and transperitoneal / retroperitoneal dissection as well as the haemodynamic effects of aortic crossclamping.

This is replaced by two small incisions in the groins and no requirement for aortic clamping. The potential advantages for this type of repair include:

1. A “smaller” procedure with lower risk
   
2. Less demand (or no demand) for ICU or HDU beds
   
3. Short stay (or even day stay) treatment of AAAs
4. Better patient acceptance
   
5. Increased availability of repair (eg to higher risk groups)
   
6. Treatment of smaller aneurysms (ie at an earlier stage, reducing rupture risks)
   
7. Reduced costs (due to all above)

History Of Endovascular Grafting

Initial reports of endovascular grafting for aneurysms were in animal models (Balko, Journal of Surgical Research, 1986). However, the father of endovascular grafting is considered to be Juan Parodi who performed the first endovascular repair of an AAA in Buenos Aires, Argentina in 1991 (Figure 1). Since this time, there has been rapid development of the technique throughout the world with Australians playing a major role in leading the way.

The first generation endovascular grafts were all “home made” systems using standard materials hand sutured to available stents (usually balloon expandable).Since this time, commercial development has lead to improvements in the delivery systems (especially size and flexibility) as well as graft materials and stent designs. There are now more than ten endovascular graft systems available commercially around theworld (Figure 2) as well as many places still using home made devices.

The early systems were all tube graft designs. The requirement for a “neck” both proximally and distally in order to achieve attachment with stents (compared to the ability to hand sew a graft to the aortic bifurcation with no “neck”) meant that few aneurysms (~10%) were morphologically suitable for endovascular repair.

The development of modular bifurcated systems has greatly increased the number of aneurysms, which are suitable for endovascular grafting (up to 70%). The use of other endovascular interventions (eg coil embolisation of internal iliac arteries) allows even wider application of the technique on morphological grounds (Figure 3)

The main factors which now determinesuitability for endovascular grafting (based on morphological assessment) are the length (> 15mm) and diameter (< 30mm) of the proximal “neck”, and the suitability of the access vessels to allow passage of the delivery catheters (ie common femoral and iliac artery occlusive disease and/or tortuosity).

Endovascular Grafting: The Technique

The procedure of endovascular grafting for AAA begins with a careful pre-operative assessment, based both on patient characteristics and aneurysm morphology. If endovascular repair is considered, more intensive pre-operative imaging is required. While a routine abdominal CT is usually enough prior to open AAA repair, endovascular planning usually requires specific spiral CT angiography (with or without three-dimensional reconstruction, Figure 4) and formal aortography using a calibrated angiographic catheter.

Each endovascular graft must be carefully planned to ensure appropriate graft sizing (diameters at the proximal and distal ends as well as lengths) in order to achieve exclusion of the aneurysm sac.

The operation may be performed in an appropriately equipped radiology su ite (designed to operating theatre standards) using fixed fluoroscopic imaging or in anoperating theatre with high quality mobile imaging with vascular angiographic capabilities. General anaesthesia is still most commonly used, however the procedure can be performed under regional (eg spinal or epidural) blocks. It is also feasible for the procedure to be done under local anaesthetic. The common femoral arteries in both groins are usually exposed, and guide wires used to provide access to the abdominal aorta. The graft delivery systems are passed over these guide wires. The primary device is deployed from one groin with the second limb of the graft deployed via the contralateral groin (after a guide wire is passed through the open stump of the primary device). Completion angiography is performed to ensure patency of the renal arteries and that the aneurysm sac has been excluded from the circulation (Figure 5). The femoral arteries and groin wounds are then closed to complete the procedure.

Results Of Endovascular Grafting

With careful pre-operative assessment and measurement, successful endovascular graft deployment is possible in 95-98% of cases. Ultimately however, success must be based on successful exclusion of the aneurysm sac from arterial flow / pressure. This has proved to be the major concern for endovascular grafting.

Demonstration of flow outside the graft into the aneurysm sac (without rupture of the sac) after deployment of an endovascular graft has been defined as “endoleak” (to differentiate it from a “leaking” or ruptured aneurysm, Figure 6). The source of endoleaks is varied and can come from either the proximal or distal points of graft attachment or where the pieces of the graft join together (Type I endoleaks). Leaks may also occur through tears in the fabric of the graft (Type III endoleaks). Another potential source of flow into the sac are branch vessels of the aorta (ie Inferior Mesenteric and Lumber arteries).

At open surgery, these vessels are oversewn or ligated after the aneurysm sac is opened. This is clearly not possible with endovascular grafting, where thrombosis of these vessels is relied As with the pre-operative assessment, normal CT protocols are upon.

Failure of these vessels to thrombose and subsequent retrograde flow into the aneurysm sac is defined as a Type II endoleak.

The true incidence of endoleak and the significance (particularly of Type II endoleaks) is still not known. The reported incidence (from as low as 2% to as high as 30%) also relates to how carefully these patients are followed and on the techniques of follow-up. To exclude endoleaks. High quality three phase spiral CT scanning is important if CT is to be the method low-up.

More recently, interest has increased in duplex scanning as the main method of follow-up and may indeed have some advantages over CT however prospective trials comparing the two modalities are required.

Angiography is reserved for cases where leak has been demonstrated in order to plan “repair” of the leak. In a majority of cases, endoleak can be corrected using further minimally invasive techniques.

Conclusions

The long-term outcome for endovascular grafting of AAAs remains unknown. However, there is no doubt in the minds of those who have taken on this technique that endovascular repair is here to stay (at least until a new approach for aneurysm repair is devised). Open repair remains the gold standard, however careful follow-up studies have shown that the long term complications even for open repair are much higher than had previously been thought. Only with further experience will it become clear which aneurysms can be treated by endovascular grafting with a very high chance of long term success. For many patients even now, endovascular grafting provides a very good alternative and in some cases is clearly the best (or even only) option for repair of abdominal aneurysms.