The development of endovascular stent grafts in the descending aorta has significantly reduced the number of open surgical repairs being performed. However, the ascending aorta may be considered one of the last frontiers of endovascular aortic treatment, and encompasses the most challenging anatomy and contraindications. Surgeons, researchers and innovators are increasingly looking towards new surgical approaches and devices to treat this difficult area. Vascular News spoke to several of those at the forefront of this challenging and exciting field.
Advancements and limitations with endovascular treatment of ascending aortic lesions
Ali Khoynezhad, Cedars-Sinai Heart Institute, Los Angeles, USA
The use of image-guided therapeutics and thoracic endovascular aneurysm repair (TEVAR) has become the standard of care for most patients with descending thoracic aortic pathologies. In fact, many patients with distal aortic arch aneurysm or thoracoabdominal aortic aneurysm are currently treated with endografts, especially if they are deemed to be poor surgical candidates. This has to do with morbidity and mortality and protracted recovery associated with open repair of distal arch and thoracoabdominal aorta.
Concerning the ascending aorta, TEVAR has been used successfully employing mostly off-label descending thoracic stent grafts in high-risk subgroups, such as type A aortic dissection and surgical turn-downs. In addition, zone 0 debranching and branched thoracic stent grafts have increased the overall operator experience. Finally, the ever-expanding transcatheter aortic valve implantation (TAVI) experience and development of newer catheter and wires has helped make ascending TEVAR a more reproducible and reliable procedure.
There are many limitations to the current practice of ascending stent-grafting. Results with off-label stent grafts are less than desirable with significant stroke and mortality risks, while the learning curve remains steep for new operators. Pre-existing or iatrogenic aortic valve regurgitation is poorly tolerated and currently is not treatable in most centres by percutaneous means. The latter may occur with interaction of the stent graft with the aortic valve in the proximal ascending aorta. Co-axiality of the stent graft to aortic centreline at the sinotubular junction remains a challenge, and this is also the case with the dedicated ascending stent grafts that were made for this location. Obviously, newer devices and newer delivery systems are needed.
Rodney White and I applied for US Food and Drug Administration-approved physician-sponsored investigational device exemption (PS-IDE) for ascending endografting approximately four years ago. While 48 patients with ascending aortic pathologies have been treated, 14 of them have been entered into this PS-IDE, which is currently in the process of expanding to additional sites. Compared to the off-label procedures, the PS-IDE has many advantages: it guarantees procedural/device reimbursement, has significant legal ramifications in case of a malpractice suit, and may assist with future regulatory approval of the ascending indication as the PS-IDE results are closely followed by the FDA.
The future of this technology is very exciting and has significant implications for future treatment of ascending aortic pathologies, comprising 60–70% of aortic pathologies. The addition of TAVI to ascending TEVAR allows the aortic specialist to treat a significant number of patients with valvular aortic (stenosis, regurgitation or mixed lesions) and ascending aortic pathologies, such as type A aortic dissection with aortic regurgitation. Figure 1 is an example of an “Endo-Wheat” procedure, a name taken from a cardiac surgical operation in which the aortic valve and the ascending aorta are replaced by conventional open-heart means, leaving the native sinuses of Valsalvae and the coronaries intact. This patient developed a type A dissection following a TAVI and was treated with the Valiant PS-IDE stent graft (Medtronic). Further advancements of technology will allow for an “Endo-Bentall” procedure, a name taken from another cardiac surgery, where the aortic valve, aortic root and the ascending aorta are all replaced by conventional open-heart means. While we are still far away from this iteration, many aortic specialists believe this will be feasible within the next decade.
Promises and current limitations of ascending aortic endografts
Ralf Kolvenbach, Augusta-Krankenhaus, Düsseldorf, Germany
The ascending aorta is one of the last frontiers of endovascular aortic treatment. There are still a substantial number of contraindications such as aortic valve incompetence, congestive heart failure and end-stage coronary artery disease requiring a coronary artery bypass graft procedure, which prohibit a safe endovascular approach.
Deployment of an ascending stent graft requires an identical infrastructure and logistics like a transfemoral aortic valve repair such as transoesophageal ultrasound and transvalvular catheter manipulations.
A distal landing zone is, in most cases, the most essential factor for successful ascending aortic endografting. Ideal patients are those who have undergone a previous surgical ascending prosthetic aortic repair. Here we find a cylindrical landing zone permitting safe deployment of a stent graft. True aneurysms with a reversed tapered supracoronary aortic segment are still the real challenge for any endovascular solution and therefore in most cases should rather be operated on. Access can be problematic, particularly in cases where there is no safe steerability possible once the aortic valve is passed with the tip of the stent graft because of the kinks of the descending aorta. To avoid kinking of the stent graft, a transapical approach or a transcarotid access should be considered in some of these cases, which permits a more direct approach with a reduced risk of compromising graft function by mal-deployment.
Appropriate diameter and length of the stent graft can be an issue, as there are only a few dedicated ascending stent grafts available today. Many off-the-shelf thoracic stent grafts are too long, considering that in most cases the distance from the sinutubular junction to the origin of the innominate artery does not exceed 8–9cm.
Straightforward cases are often those with embolising wall adherent thrombus, pseudoaneurysms or penetrating ulcers. Here, only a short segment of the ascending aorta needs to be covered. In many of these cases a thoracic cuff, either as a standalone procedure or in combination with an occluding device, can be used.
More localised type A dissections, which exclude the sinutubular junction and the coronary arteries as well as the aortic valve, can be treated with a dedicated ascending stent graft. If necessary the whole arch can be covered using chimney grafts or off-the-shelf branched or fenestrated devices. Flow modulating bare metal stents can be another easy to deploy option, though in all of these cases there have been no larger cohorts of patients with intermediate-term follow-up treated so far. Endovascular treatment of type A dissections can be offered to the high-risk patient either as a bridging procedure or as a definitive solution.
Most large true ascending aortic aneurysms must still be excluded from any endovascular approach. Unless in the future we have the technical means to perform a total endovascular Bentall procedure, ascending aneurysms, because of the lack of a distal landing zone, will still primarily require an operation with cardiopulmonary bypass and deep hypothermia. Active fixation of a stent graft with an endostapler can be one option worth pursuing.
Alternatively, a supra-aortic landing zone can be shaped using a surgical wrapping procedure in aneurysms smaller than 6cm in diameter. Through a mini-sternotomy the ascending aorta is exposed. After circumferential dissection of the ascending aorta a wrapping procedure preferentially with a polypropylene mesh or a Dacron graft is performed. This permits creation of a sufficiently long distal supra coronary landing zone. This technique can be used to create a sufficiently long landing zone for an ascending graft or in cases of a total endovascular arch repair. Though performed through a mini sternotomy only and as an off-pump procedure, it is still, though less invasive, a hybrid surgical technique.
The near future will show whether alternate techniques, such as attaching a stent graft to the struts of a prosthetic aortic valve implanted through a transfemoral access, will broaden the indication for an endovascular ascending aortic repair.
Early feasibility trial for the Gore thoracic branched graft in treating type A dissection
Michael J Reardon and Jean Bismuth, Houston Methodist DeBakey Heart & Vascular Center, Houston, USA
The introduction of stent grafting as an alternative to open surgery in the treatment of aortic disease has shifted much of what were once large open surgical procedures to less invasive stent graft procedures. The ascending aorta, because of its challenging anatomy and physiology, represents the pinnacle of this process. Type A dissection remains a disease of very high mortality without treatment, and that treatment has continued to be open surgical repair requiring cardiopulmonary bypass with varying degrees of hypothermia and circulatory arrest. Open surgical repair can be life-saving but still carries appreciable morbidity and mortality. Surgery for type A dissection is usually urgent or emergent, is technically demanding, and requires a high level of operating room, anaesthesia, and intensive care support that many institutions do not have. Further, we do not cure the dissection, but rather treat the portion in the ascending aorta that is an immediate threat to life and convert this into a type B dissection requiring medical treatment and follow-up. A less invasive approach to type A dissection would be a welcome addition to our treatment armamentarium.
The early feasibility study for evaluation of the Gore TAG thoracic branch endoprosthesis (Gore) in the treatment of DeBakey type I/II aortic dissection is an industry-sponsored clinical trial being run by Gore. The early feasibility study pathway is a recent regulatory addition by the FDA to allow for early clinical evaluation of devices to provide proof of principle and initial clinical safety data.
This study is currently approved for enrolment of up to 10 eligible dissection patients. Early results are encouraging, showing a good ability to cover the tear and occlude the false lumen in the ascending aorta. Appropriate patient selection and procedural best practices are being worked out in this feasibility stage. The technical challenges of using grafts designed for other portions of the thoracic aorta are being addressed with grafts designed specifically for the ascending aorta and we hope to have these in use for the investigational device exemption trial.
The national principal investigator is Michael J Reardon, from the Houston Methodist DeBakey Heart & Vascular Center working with Jean Bismuth. There are three actively enrolling clinical sites: The Houston Methodist DeBakey Heart & Vascular Center, Baylor/St Lukes/CHI and University of Texas at Houston Health Science Center. Expansion of the study to an additional three sites is planned and underway.
Current and future progress with ascending aortic endografts
Carlos Donayre, University of California—Irvine, Orange, USA
The success of transcatheter aortic valve implantation (TAVI) continues to encourage the application of endografts to treat ascending aortic pathologies. As a result endografts designed to treat the abdominal aorta such as cuff extensions are being deployed in the ascending aorta. Due to the short length of delivery sheaths, designed to house such endografts, deployment from femoral access is not always possible. Thus, right carotid artery and transapical access have been utilised for the off-label deployment of aortic endograft extensions. Mid-term results have documented remodeling of the diseased ascending aorta and in cases of type A dissections enlargement of the true lumen and thrombosis and regression of the false lumen.
Hybrid techniques utilising open creative debranching techniques followed by deployment of thoracic endografts just above the origin of the coronary arteries have also been pursued in high-risk patients with operative mortalities in the 6–10% range. However, complications are still significant, mostly strokes due to catheter-related manipulations and higher incidence of retrograde type A dissection following endograft deployment in zone 0. In the patients not able to tolerate a hybrid approach or when emergent treatment is required thoracic endografts are deployed with smaller parallel grafts (chimney procedures) to maintain aortic and arch vessels perfusion. This has lead to persistant type Ia endoleaks, device kinking, occlusion, and concerns about a compromised sealing zone.
Branched endografts specifically designed to deal with the ascending aorta and aortic arch have been developed but are being used mostly in Europe and Japan. As opposed to branch endografts designed for the abdominal aorta, the main body with large apertures and internal tunnels is usually deployed first. This is followed by cannulation from the the target vessels and deployment of a bridging endograft that seals in the internal tunnel. Proper aligment of the main body is required to avoid coverage of the arch vessels with a resultant compromised blood flow and stroke. The Relay NBS plus endograft (Bolton Medical), relies on a covered proximal stent desing and a self-aligning main body to minimise cannulation difficulties and potential retrograde dissections due to zone 0 deployment. The Nexus arch and ascending aorta endograft designed by Mario Lachat has a main body with single branch that is deployed in the innominate with a through and through wire technique to allow for proper orientation, positioning, and anatomical fixation. A more proximal endograft can also be added to achieve a longer seal and fixation zone.
The aortic arch curvature and tortuosity combined with constant aortic pulsation are formidable challenges to achieve initial and long-term success. Longer seal zones in the proximal aorta should be pursued to allow for regression of aortic pathologies. Arrival of branched endografts in the USA is expected around the end of this year and will permit further assessment of this therapy.
