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EDITORIAL |
Hong Kong China
In the past decade, we have witnessed rapid growth in the use of an endovascular aortic stent graft to treat abdominal aortic aneurysms. In many large vascular surgery centers in Asia, the endovascular repair rate for intact aneurysm has already exceeded 50%. Endovascular repair has been widely accepted by patients and surgeons, due to the procedure’s low morbidity and mortality, while randomized studies have confirmed similar midterm efficacy compared to open repair.1 Long-term durability seems to be less important as many patients are perceived to be at high risk for open surgery, and compromises in anatomical guidelines are often considered acceptable as an alternative to no treatment.
In the thoracic aorta, the advantage of an endograft over open repair is even more apparent, as complex surgery of arch and thoracoabdominal aneurysms often incur large incisions and extracorporeal bypass, with prohibitive risks of hemorrhage, stroke, renal impairment, and paraplegia. Although the long-term durability of thoracic endografting remains relatively unknown, there is evidence from the literature that the early results are at least comparable to those of surgery.2 However, for a number of reasons, the development of thoracic stent grafts has lagged behind their abdominal counterparts.
The main constraint in thoracic aortic endografting is the anatomy of the aortic arch. The proximity of the pathology close to the great vessel origins in the arch mandates accurate placement of the graft, yet the considerable hemodynamic forces compounded by the tortuosity of the thoracic aorta and a remote insertion site pose many challenges to deployment. The substantial diameter of the arch and ascending aorta also requires larger devices with big delivery systems, which are often less flexible. The concern of a need for access conduits and iliac artery injuries in Asians with a small stature is without doubt valid, particularly in women. The initial enthusiasm for endografts in the thoracic aorta was soon met with a number of practical concerns as experiences grew around the world. Device-related complications such as bare stent erosion, stent graft collapse, and retrograde aortic dissection have been reported, notably with excessive over-sizing and heedless ballooning. It is now widely regarded that thoracic endografting is a more technically demanding and thus more risky procedure than its abdominal counterpart.
In the United States, only the Gore TAG device (Gore, Flagstaff, AZ, USA) is currently approved by the FDA for treating thoracic aneurysms. The Medtronic Talent/Valiant (Medtronic, Santa Rosa, CA, USA) and the Cook Zenith TX2 grafts (Cook Medical, Bloomington, IN, USA) are close to completing their pivotal trials, and their usage is currently limited to trial centers. In Asia, thoracic endografting is less restricted by regulations in many countries, and surgeons have access to a wider availability of devices. China and Japan have developed innovative locally manufactured thoracic endografts. Although their usage is exclusively within the parent institution or country, anecdotal results are promising.3 Given its large population and high incidence of acute aortic dissections, China has probably the largest and fastest growing experience worldwide in thoracic endografting, with more than 1,000 procedures performed to date, mostly on acute dissections, and about half using locally manufactured devices from companies based in Shanghai, Beijing, and Guangzhou. In Japan, thoracic endografts have yet to be licensed for general use, and only a few specialized centers have reported experience in the hundreds using a variety of trial and homemade devices. Hong Kong and India carry out 30–40 procedures each year, followed closely by Singapore, Taiwan, and Thailand where a thoracic program has recently started. In Korea, approximately 20 thoracic endografts are performed each year, and there is already a locally manufactured product.
The main application of thoracic endografts is in the treatment of thoracic aneurysms and dissections. The incidence of thoracic aneurysm in Asians remains largely unknown, but with more liberal use of computed tomography for other pathologies, it is likely to increase with time. Initial results of endovascular treatment for aneurysms from non-randomized registries are promising. The Gore TAG Pivotal (phase II) multicenter trial confirmed a high success rate of 98%, with 30-day mortality of 2%, a 3% rate of paraplegia, and 4% incidence of stroke, although much lower than the surgical controls.4 All causes of mortality throughout 3 years of endografting did not differ from open repair, and no ruptures have been reported. Freedom from aneurysm-related mortality at 3 years was higher in the TAG group (97% vs 90%), as was freedom from major adverse events (48% vs 20%). Alarmingly, there was a 17% late expansion rate, attributed probably to the porosity of the early device. The multicenter VALOR trial using the Medtronic Talent graft completed enrollment in June 2005. Preliminary unpublished data show a procedure success rate of 98% and a 30-day all-cause mortality rate of 8.4%, a 5.5% incidence of paraplegia, and 8% incidence of stroke. At 12 months, there was a 5% risk of type I endoleak and a 2% rupture rate. The Cook Zenith STARZ-TX2 trial is near completion but no result has yet been made available.
More complex endovascular techniques are required when the arch vessels are involved in the pathology. The usual approach is one of combined surgical "debranching" using a variety of bypasses, such as carotid–subclavian, carotid–carotid or ascending aorta–innominate/carotid bypass, followed by a hybrid endovascular stent graft insertion. Other more technically demanding but ingenious ways of applying custom-made fenestrated or branched grafts are largely performed only in certain specialized centers.5 The ascending aorta with its wide diameter, high flow, and proximity to the heart valves and coronary arteries remains the ultimate challenge. Techniques of deploying a branched stent graft from the right carotid artery have been reported by Chuter and colleagues6 and popularized by a few others in the treatment of ascending and arch aorta disease.
Endovascular treatment for aortic dissection is usually indicated during the acute stage in a symptomatic patient, or for complications. The general principle is to cover the primary tear, realign true lumen flow, and allow thrombosis of the false lumen. Some surgeons would prefer a more aggressive approach by adding a long distal bare metal stent to stabilize the flap and expand the true lumen, as well as stenting branch vessels and covering secondary tears. This holistic approach requires meticulous planning and endovascular skills. There is still controversy as to when to treat a symptomatic patient with Stanford type B dissection. Endovascular repair to close the tear during the acute phase may incur a higher incidence of rupture and tearing of the flap, and performing the procedure after a delay of 2–3 weeks seems to be the more accepted option.
The European INSTEAD trial randomized patients with uncomplicated type B dissections of 2 weeks to 1 year duration to endografting or antihypertensive treatment alone. The Medtronic Talent graft was used, and there was an 11% crossover between groups. Preliminary 1-year data show a slightly higher but not significant all-cause mortality in the stent-graft group (10%) compared to the medical treatment arm (3%). The 30-day complication rate also seemed to be higher in the stent-graft group. This suggests that uncomplicated type B dissections should only be treated selectively, if at all, and stable chronic type B dissections are best managed conservatively.
Treatment of type A dissections is also evolving. There have been attempts to cover the entry tears in the ascending aorta by retrograde insertion of a stent graft from the right common carotid artery. As subsequent surgery for arch and proximal descending aorta aneurysms after standard repair of a type A dissection is often difficult, one may contemplate replacing the ascending aorta with a longer prosthetic graft, and effect an immediate bypass to the carotid arteries to pave an easier approach for future endovascular repair of any arch aneurysmal dilatations. Diethrich and colleagues7 have also described one-stage repair of type A aortic dissection by a hybrid procedure, with prosthetic replacement of the ascending aorta with a simultaneous "debranching" and descending thoracic aorta endograft placement to effect a complete repair of the dissection.
The design of thoracic stent grafts is currently evolving. With modern delivery systems, negotiating tortuous anatomy to reach the arch has become less of a challenge. There is still a large variation in design regarding deployment and fixation methods. There has yet to be an optimal stent graft for the thoracic aorta, and indiscriminate usage with poor patient selection will lead to major potentially life-threatening complications. Our experience to date with 62 cases has been favorable, with a technical success rate of 97%, and a midterm primary success rate of 93%, with only one case of stroke, one incidence of paraplegia, and no operative mortality. In the future, we will continue to expect evolution of better-designed stent grafts for the thoracic aorta; in particular, a more flexible device with a smaller profile and better delivery control for more accurate deployment. The cardiothoracic surgery community should be enthusiastic in exploring this new frontier and acquiring endovascular skills to offer a less-invasive option for treating their patients.
REFERENCES
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