Despite advances in surgery for acute type A aortic dissection (ATAAD) and its outcomes over the past four decades, the morbidity and mortality rate remain high, ranging from 10% to 26%, especially in patients with severe true lumen (TL) collapse and in cases of malperfusion syndrome.

The frozen elephant trunk (FET) technique for ATAAD, developed in the 1990s by Kato et al., has become increasingly popular due to its potential benefits, such as improving malperfusion syndrome through thrombosis of the false lumen (FL) and reducing the need for reoperations. It is believed that the FET technique allows adequate aortic remodeling by covering the initial portion of the descending aorta with a stent-graft. However, there is a possibility of causing spinal cord ischemia (SCI) when a stent longer than 150 mm is used or when the FET coverage extends below T8-10. On the other hand, insufficient coverage of the descending aorta is associated with a greater need for a second intervention. The optimal length of the FET stent and its distal position to achieve adequate aortic remodeling and improve distal malperfusion syndrome while avoiding SCI are still unclear in patients with ATAAD undergoing total arch repair. Recent studies, supported by good clinical outcomes, suggest a strategy of proximalizing the distal anastomosis as a promising approach.

At Akita University Hospital, Japan, since 2014, a strategy has been implemented that involves deploying the FET from the aortic zone 0 to the descending aorta, using stents of different lengths (60, 90, 120, or 150 mm). This approach has been characterized as easy and safe to apply and has yielded satisfactory postoperative outcomes. The purpose of this study was to investigate the optimal length of the stent and the distal location of the FET in patients with ATAAD undergoing total arch repair, where the FET is implanted from the aortic zone 0.

For this purpose, between October 2014 and April 2021, 191 patients (FET-150 group: 37 patients; stent length, 150 mm; age 66.3 years and FET-no-150 group: 154 patients; 60, 90, or 120 mm; age 64 years) underwent total arch repair with FET for ATAAD using the “zone 0 arch repair” strategy. In the FET-150 group, the proximal end of the stent was placed at the level of the origin of the innominate artery from the arch. In the FET-no-150 group, the distal end of the stent was positioned just proximal to the aortic valve level using transesophageal echocardiography (TEE). In both groups, the distal anastomosis was performed after trimming the unstented polyester prosthesis with four branches (for the individual reimplantation of the three supra-aortic trunks and another for the perfusion of the lower half of the body after the completion of the distal anastomosis) suturing it to the aortic wall 1 or 2 cm proximal to the origin of the innominate artery.

The distal ends of the stent were positioned as follows at the thoracic vertebral (T) levels: In T4-5 (0%), in T6-7 (32.4%), in T8-9 (67.6%), and in T10 (0%) in the FET-150 group, and in T4-5 (3.9%), in T6-7 (63.6%), in T8-9 (31.8%), and in T10 (0.7%) for the FET-no-150 group. No differences between groups in postoperative mortality were observed. The incidence of residual distal malperfusion syndrome and new-onset SCI in the FET-150 versus FET-no-150 groups was 2.7% versus 6.5% (p = 0.62) and 0% versus 1.9% (p = 1.00), respectively.

The authors conclude that positioning the FET with the distal end around T8 may reduce residual distal malperfusion syndrome when a FET with a 150 mm stent is implanted from zone 0 aorta in patients with ATAAD undergoing total arch repair.

COMMENTARY:

In most centers, the conservative approach with hemiarch surgery remains the preferred technique for treating ATAAD, as the priority is to save the patient’s life. However, this technique in most cases leaves a residual distal dissection. Therefore, under certain circumstances, a more aggressive strategy involving the complete replacement of the aortic arch is being promoted to prevent future complications, although this technique carries a significant risk, especially in centers with little experience. In this context, the use of the FET technique has become an alternative surgical option with indications for which there is consensus in most clinical guidelines, including: 1) rupture of the distal aortic arch and/or proximal descending thoracic aorta; 2) malperfusion syndrome; 3) patients under 70 years of age without significant comorbidities.

The FET improves blood flow to the TL by covering tears in the intima of the descending thoracic aorta and reduces pressure in the FL, improving malperfusion syndrome. In addition, it decreases the risk of dilatation in the distal FL, promoting adequate aortic remodeling and reducing late mortality and the need for additional surgeries to less than 20%. When a complete aortic arch replacement is considered to prevent long-term complications, the FET offers a more favorable postoperative recovery compared to the classic elephant trunk technique, in addition to reducing the need for subsequent procedures. Something very relevant, but rarely discussed, is that the distal anastomosis in the FET compared to the classic elephant trunk is safer and more hemostatic, as a consequence of the almost immediate thrombosis of the FL.

Takagi et al. present a highly relevant study that provides new valuable information, classifiable in four points of decreasing importance:

1. The deployment of the prosthesis using FET up to the level of T8 does not increase the risk of SCI and may improve distal malperfusion syndrome in patients with ATAAD. Until now, it had been suggested that positioning a FET approximately at the level of T8 might increase the likelihood of SCI. This is evidenced in the most current and robust meta-analysis, published by Preventza O et al. in 2020, which encompassed a total of 35 studies with more than 3,000 patients undergoing FET, and which revealed a significant association between FETs with stent-grafts longer than 15 cm or those reaching T8 or beyond, and SCI.
2. If a 15 cm prosthesis is released from the zone 0 at the level of the innominate artery, the distal part of the stent of the stent-graft is positioned at the level of T8-9, with very low probabilities of surpassing this level. This phenomenon seems to be valid in most patients of Japanese descent, who tend to have a not very tall stature. Therefore, it is very likely that this observation could be extrapolated to our own population. In other words, the 15 cm prosthesis released at the level of the innominate artery, even without precise measurements, is highly unlikely to be below T10, which would be dangerous.
3. Surgical technique novelties with the release of the prosthesis in the aortic arch and performing the distal anastomosis, always in zone 0. Unlike the more conventional technique in prostheses in our field, in which the release and anastomosis in zone 2 are recommended, between the left carotid artery and the left subclavian artery, which is generally ligated proximally in the aorta and reimplanted in isolation.
4. TEE presents itself as a valuable tool to avoid surpassing the limits that entail a greater risk of SCI, at least in the case of prostheses with lengths less than 15 cm. By releasing the prosthesis at the point where the distal position of the stent-graft does not exceed the proximal plane, right where the aortic valve is located, a positioning is ensured that rarely falls below T10. This practical and simple trick, although not completely accurate, can be very useful for adjusting the release zone of the prosthesis by a few centimeters.

The Frozenix® prosthesis (Japan Lifeline®) commercially unavailable in Spain, bears great similarity to the two devices most widely used in our country, the E-vita Open Plus® prosthesis (Jotec®) and Thoraflex Hybrid® (Vascutek Terumo®). The main difference lies in the absence of a collar for the anastomosis in the Frozenix® prosthesis, which implies the need to trim the prosthesis after releasing the stent of the stent-graft. In this way, after adjusting the length of the Dacron prosthesis, the distal anastomosis is performed. The rest of the steps of the procedure using the Frozenix® prosthesis are practically identical to those carried out in our hospitals.

It is crucial to highlight that SCI constitutes a devastating adverse event that, to some extent, could be prevented by using stent-grafts with a length less than 15 cm or by positioning them above T8. On the other hand, leaving a significant portion of dissected aorta uncovered, especially above T4-5, generally with short-length prostheses (60-90 mm), could be related to the lack of resolution of malperfusion syndrome. However, malperfusion does not only depend on the latter, but also on other factors, such as the time elapsed from the onset of the dissection until the restoration of blood flow in the affected organ. The release technique, which is always performed at the level of the innominate artery, and which has been developed in this hospital with 15 cm prostheses, seems to address both problems with a high degree of reliability, as it would cover a large part of the dissected aorta, resolving the malperfusion syndrome, without practically ever surpassing T8-9, thus avoiding SCI.

Although this study is extremely interesting from an educational and practical perspective, it is important to underline that it is a retrospective and single-center study. The lack of significance in the lower incidence of malperfusion syndrome observed with 15 cm prostheses (2.7% versus 6.5%) could be attributed to the limited statistical power of the research. However, it is risky to assert, as the authors do in their conclusions, that this prosthesis could be associated with a lower incidence of said syndrome based on a study of this nature. It is evident that more research is required before reaching a definitive conclusion on the ideal length of the stent-graft. To address this issue accurately, we must take into account the anatomical differences between patients, which can vary in terms of physical constitution, length, and shape of the aorta. Currently, with the widespread availability of highly precise imaging tests such as angioCT in our hospitals, it has become essential to perform an accurate and personalized calculation of the length and size of the aorta when considering the application of the FET technique. This ensures the selection of the most suitable prosthesis for each patient.

REFERENCE:

Takagi D, Yamamoto H, Kadohama T, Kiryu K, Wada T, Igarashi I. Optimal stent length and distal positioning of frozen elephant trunks deployed from the aortic zone 0 for type A acute aortic dissection. J Thorac Cardiovasc Surg. 2024 Jan;167(1):15-25.e2. doi: 10.1016/j.jtcvs.2022.03.007.