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EndoVascular Aneurysm Repair - Technical aspects

EVAR involves the use of a stent-graft to exclude aneurysms of the abdominal aorta. A stent-graft is a self-expandable stent, similar to that used when treating occlusive vessel disease, with an outer (endo-skeleton) or inner (exo-skeleton) lining of fabric. This fabric is either Dacron or polytetrafluorethylene (PTFE), very similar to the graft used when performing open aneurysm repair. The stent-graft is positioned between normal artery proximal to the aneurysm and normal artery distal to the aneurysm, such that blood flows from normal artery through the stent-graft and back into normal artery. The remaining blood in the aneurysm sac will thus clot off and the aneurismal sac should remain static or shrink with time. The area where the stent graft is in contact with normal artery is known as the sealing zone. The outward radial force of the stent is what causes a seal in this area preventing the flow of blood outside the stent-graft and into the aneurismal sac. The proximal sealing zone is the non-aneurysmal infra-renal aortic neck. Since almost all abdominal aortic aneurysms extend to the aortic bifurcation or beyond, the distal sealing zone is both common iliac arteries for purely aortic aneurysms and the external iliac arteries for aorto-iliac aneurysms. All current devices for elective cases are therefore of a bifurcated (trouser) configuration.

When considering the endovascular management of infra-renal abdominal aortic aneurysms a proximal and distal landing zone of 1.5 cm is recommended. A shorter landing zone will result in the radial force of the sealing stent being exerted over a lesser area and therefore a greater risk that with time the stent migrates distally with loss of the seal between normal artery and stent. In an attempt to prevent distal migration most current stent-grafts have a bare suprarenal component ie the most proximal stent is not covered by graft so that it can be positioned above the renal arteries, thereby increasing the surface area over which the stents radial force is applied. There is now good evidence that having bare metal stents across the renal orifices does not effect renal perfusion and function. The stent-graft most commonly used in our unit also has hooks/barbs on these supra-renal stents to further prevent migration. Using this stent-graft we have treated patients with infra-renal necks shorter than the recommended 1.5cm, thereby increasing the number of patients who benefit from endovascular repair. Another factor affecting patients’ suitability for EVAR is the angle between the non-aneurysmal infra-renal neck and the aneurysm. An angle of up to 60 degrees is normally recommended, however more flexible stent-grafts are making such recommendations obsolete.

Stent-grafts come in various sizes. The proximal diameter of the main body is chosen to be 20% larger than the outer diameter of the neck of the aneurysm. The distal diameter of the iliac limbs is chosen to be 10% larger than the outer diameter of the iliac vessels. The length of the main body and limbs depends on the type of stent-graft used, but in most cases is dependent on the distance from the lowermost renal artery to the aortic bifurcation and the internal iliac artery origins.

Stent-grafts are packed (compressed) in an outer sheath (tube) with a pusher rod at their distal end. When the sheath is withdrawn backwards over the pusher rod, the stent graft is exposed and opens as a result of the radial force of the self-expanding stent. Most current stent-grafts are tri-modular (main body for the aorta and 2 limbs for each iliac artery) or bi-modular (main body and limb for one iliac as one component and contra-lateral iliac limb as the second component). The sheath containing the stent grafts measures 18 to 22 Fr for the main body and 14 to 16 Fr for the iliac limbs. The stent-grafts are usually introduced through the common femoral arteries. Most surgeons will perform a surgical cut-down onto the vessels due to the size of the sheaths. Closure devices such as Perclose and Sutura are used in some centers where a totally percutaneous procedure is performed. The risk of groin complications is however significant with these closure devices and we personally perform a cut-down in both groins using transverse skin crease incisions. We prefer these to longitudinal incisions since they affect less dermatomes and therefore are associated with less post-operative discomfort. We only use longitudinal incisions if additional procedures such as a femoral crossover graft or an endarterectomy may be required. There are occasions where the iliac arteries may be too small to safely advance the sheathed stent-graft without damaging these vessels (the main device requires an iliac diameter of 7 to 8 mm). In such a situation one may expose the iliac artery and use a dacron conduit anastamosed to this vessel to introduce the stent-graft.

Once the common femoral arteries are exposed a J-wire (similar to that used to introduce a central venous line, but much longer) is introduced through an arterial puncture needle. Under fluoroscopic guidance the J- wire is advanced proximally to the descending thoracic aorta. In the case of very tortuous or diseased arteries the J-wire is swapped for a hydrophilic (Teruma) wire. Various shaped catheters are then used to guide this wire proximally. On the side through which the main body is to be introduced the J-wire is exchanged (through a catheter such as a RIM or COBRA) for a very stiff wire (Lunderquist or Mayer wire). Such a stiff wire will straighten the vessels making it easier to advance the device and will also not kink. On the contra-lateral side a PIG-TAIL catheter is advanced over the J-wire and then connected to a power injector such that contrast (we use Visipaque which is less nephrotoxic than Omnipaque) can be injected very rapidly and at high pressures, opacifying the abdominal aorta and its visceral branches. If the patient is under a general anesthetic the anesthetist will be asked to suspend respiration whenever digital subtraction angiography is performed. This is because if there is even the slightest movement, the x-ray image with contrast will be slightly different from that without contrast, and when the latter image is subtracted from the former, a very blurred picture would result. The pig is positioned just above the level of the renal orifices as may be determined from the CT-angiograms. The stent-graft is then introduced over the stiff wire and positioned such that the junction between covered and uncovered stent is just below the lowermost renal orifice. This area usually has gold-markers stitched to it so that it is easily identifiable on fluoroscopy. A digital subtraction angiogram is obtained to confirm correct positioning of the stent-graft in relation to the renal orifices. The stent graft is moved up and down as required and further digital subtraction angiograms obtained until one is definitely satisfied with the position. When this is the case, the stent-graft is deployed under fluoroscopic control by withdrawing the outer sheath distally whilst fixing the position of the pusher rod. The next step is to cannulate the contra-lateral limb of the main body from the contra-lateral common femoral artery. This is achieved by using a hydrophilic wire and catheters of varying shape. This part of the procedure may sometimes be very fiddly and is the main factor effecting the duration of the procedure. Once the contra-lateral limb is cannulated, the hydrophilic wire is swapped for a stiff wire and the contra-lateral limb stent graft is inserted and deployed such that its distal edge is just above the origin of the internal iliac artery. Depending on the type of stent-graft being used an ipsi-lateral limb stent-graft may also be required. Some stent-graft manufacturers recommend that a molding (semi-compliant) balloon is blown up at all the sealing zones (including the aortic neck) to further fix the stents in place. One of the stiff wires is then swapped for a pigtail and a final digital subtraction angiogram obtained to confirm that the stent-graft has been deployed in the correct position with preservation of both renal and internal iliac arteries and that there is no filling of the aneurysm sac. If this is satisfactory, all sheaths, catheters and wires are withdrawn. Vascular clamps are applied proximal and distal to the arteriotomy resulting from insertion of the stent-grafts. The arteriotomies are closed with a fine prolene suture.

As may be seen from above, the main limitations to performing EVAR is the absence of a proximal landing zone (short or wide neck), and poor access ie. the iliac vessels are smaller than 8mm in diameter (to allow a 22 fr device to be advanced safely). The absence of a distal landing zone means the presence of common iliac aneurysms. This is usually resolved by embolising (putting metallic coils) in the internal iliac arteries to prevent back bleeding from this vessel into the aneurysm sac and using the external iliac artery as the distal landing (sealing) zone. Embolising the internal iliac carries a small risk of buttock claudication, impotence and pelvic ischemia. In order to prevent such complications, iliac limbs with side branches to perfuse the internal iliac artery are used in our unit. The use of such devices requires advanced endovascular skills, significantly lengthens the procedure and may require higher doses of contrast. The absence of a proximal landing zone may be resolved by using custom made fenestrated devices. These devices are designed specifically for the individual patient such that small holes (fenestrations) are pre-cut into the graft based on the CT-reconstructions to correspond with the openings of the appropriate relevant vessels. The gap between these fenestrations and their target vessels is bridged using covered stents similar to the bare stents used for treating renal artery stenosis. Such procedures are very complex and lengthy. Depending on the number of fenestrations, they mat take several hours. High doses of contrast may be required. Since the visceral vessel origins are usually around 6 mm in diameter there is no room for error and if the stent graft is not deployed in the correct position the patient could loose a kidney or end up with ischemic bowel.

 

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