“Proximalization is Advancement”—Zone 3 Frozen Elephant Trunk vs Zone 2 Frozen Elephant Trunk: A Literature Review

Abstract

Over the decades, the Frozen Elephant Trunk (FET) technique has gained immense popularity allowing simplified treatment of complex aortic pathologies. FET is frequently used to treat aortic conditions involving the distal aortic arch and the proximal descending aorta in a single stage. Surgical preference has recently changed from FET procedures being performed at Zone 3 to Zone 2. There are several advantages of Zone 2 FET over Zone 3 FET including reduction in spinal cord injury, visceral ischemia, neurological and cardiovascular sequelae. In addition, Zone 2 FET is a technically less complicated procedure. Literature on the comparison between Zone 3 and Zone 2 FET is scarce and primarily observational and anecdotal. Therefore, further research is warranted in this paradigm to substantiate current surgical treatment options for complex aortic pathologies. In this review, we explore literature surrounding FET and the reasons for the shift in surgical preference from Zone 3 to Zone 2.

Keywords

frozen elephant trunk zone 3 zone 2 aortic surgery aortic arch repair spinal cord ischemia

Introduction

It is a cardinal rule - you must take advantage of every second that you are in the zone - John Passaro

Aortic pathology involving the ascending aorta, aortic arch and descending thoracic aorta are surgically challenging even in the present era; it may require more than one intervention either surgically or percutaneously, requiring more than one hospital admission. The intervention depends upon the pathology of the disease, patient characteristics, the anatomy of the aorta or disease, and the surgeons’/center experience.¹ To simplify the management of this complex pathology, Borst et al introduced the two-staged concept of ‘elephant trunk’ in 1983 - presently called the ‘conventional elephant trunk’ (cET).² With the initial stage replacing the ascending and aortic arch via a median sternotomy, cET facilitated an easier second operation by reducing the dissection around the aortic arch and preparation for a proximal clamp in the thoracic aorta. Twenty years later, in 2003, Haverich et al,³ used a stent for the ‘elephant trunk’ segment, which is now termed as ‘Frozen elephant trunk’ (FET). This combines the concept of the cET and the endovascular stenting of the descending thoracic aorta (DTA).

The FET prosthesis is a hybrid vascular graft which consists of two segments (i) a dacron tube graft (branched or non-branched) and (ii) an endovascular stent. Allowing the combined advantages of the cET with modern stent technology, the FET technique has gained immense popularity over the years with more than 28,000 hybrid prosthesis implanted globally till December 2014.^1,4 Indications for FET include degenerative aortic aneurysm, chronic aneurysm of the distal aortic arch, chronic aortic dissection, and residual type A aortic dissection and. The FET technique can also be used to treat acute type A aortic dissection, especially in cases of localised aortic arch tear, distal aortic malperfusion and in younger patients.⁴ The advantages of FET technique are that it involves single-stage surgery and may decrease the chance of an additional aortic surgery being required. Having said that, FET is a technically demanding procedure even for the ‘master’ surgeon. Though the technique and technology of FET have evolved over years, the concerns of the procedure include spinal cord injury (SPI), laryngeal nerve palsy, bleeding apart from the availability and the cost of the device remain a matter of concern.^4-6

The technique of FET requires a distal anastomosis to either Zone 2 or Zone 3 of the aortic arch, and debate exists as to which is optimum. This review will outline the debate around Zone 2 and Zone 3 within FET.

Techniques and Variances within FET

The surgical technique for FET procedures has varied, both with time, between centres, and with ongoing advancements in devices and techniques. Key factors of the procedure include extracorporeal circuit management, cerebral and visceral protection and myocardial protection.

Depending upon the clinical scenario, cardiopulmonary bypass is established with a right axillary artery, innominate artery, carotid artery cannulation or femoral artery. Venous access and drainage can be achieved by right atrium cannulation or femoral vein cannulation in more complex scenarios.⁴ The extracorpeal circuit is altered to provide cerebral perfusion (preferably antegrade) and lower body after implantation of the hybrid prosthesis. Few centers have modified the extracorpeal circuit for continuous perfusion of the left subclavian artery to reduce the incidence of SCI.⁷ Hypothermia cardioplegia is the usual method of myocardial protection and can be achieved both antegrade and retrograde. However, Berger et al. state that normothermic selective myocardial perfusion may be just as effective in certain scenarios to reduce myocardial injury.⁹

The aortic arch is opened and the brain is protected usually by antegrade cerebral perfusion.¹⁰ Under deep hypothermic circulatory arrest of the lower body, the FET hybrid prosthesis is introduced in an antegrade manner into the descending aorta. A circumferential anastomosis between the collar of the device and the aorta is performed. Following this, systemic perfusion is restored to the lower body. In branched devices like Thoraflex and E-vita Neo, the lower body perfusion can be initiated via the side arm provided in the prosthesis. Whereas, in devices like E-vita open plus, that lack side branches, lower body perfusion is temporarily restored (usually for brief time) via the Foley’s catheter placed in the stent graft.⁷ After which the supra-aortic vessels are reimplanted either as a single patch or by using individual supra aortic branches. Proximal repair after LSA reimplantation in performed to reduce cardiac ischemia.

Since the introduction of the FET, a variety of devices have been manufactured, each one endeavouring to complete aortic aneurysm repairs in one singular procedure. A comparison of the variations in regard to the different devices and techniques is illustrated in Table 1.^3,8,11 It is reported that there is a negligible difference in outcomes between the various devices. However, a recent meta-analysis¹³ showed that the mortality rates are lower when the JOTEC-E-Vita hybrid prosthesis as compared to the Thoraflex device.¹¹

Table 1.

Comparisons of the Different Devices and Methods Utilised for the FET Technique.

Prothestic device	Design features	Benefits	Drawbacks
Chavan-Haverich Prosthesis (Curative GmbH, Dresden, Germany) – First used FET prosthesis.^3,7,11 Presently not available.	Proximal unstented Dacron tube, distal stented tube with stainless steel stents on the inner aspect, and may be up to 4 stents depending on anatomy.	Not available at present
E-Vita Open and E-Vita Open Plus® (Jotec® GmbH, Hechingen, Germany).^3,8,11	Stented portion with flexible nitinol wire, and a proximal non-stented woven graft.	First commercially available device in market	No Individual branch graft for supra aortic arch vessel No side branch for lower body anastmosis
E-Vita Neo¹²	Stented portion with flexible nitinol wire, and a proximal non-stented woven graft	Multiple design variants suited to be implanted in Zone 0/1 and Zone 2	Limited by its avilabilty
Thoraflex^TM Hybrid Prosthesis (Vascutek®, Terumo®, Inchinnan, Scotland, UK).^3,8,11	Proximal unstented Dacron tube, with four side branches for arch reconstruction and antegrade cerebral perfusion, distal stent with nitinol stents.	Branched grafts Easy anastomosis Side branch for lower body perfusion	Time consuming and technically demanding.
Cronus^TM (MicroPort Medical Co., Ltd, Shanghai, China).^3,8,11	Regular Dacron vascular tube with Z-shaped stents with a sewing cuff for hand-sewn anastomosis.	Condensed System, no use for guidewires.	Requires an aortic arch separate graft
Frozenix® J Graft open stent graft (Japan Lifeline, Tokyo, Japan).^3,8,11	Distal stented polyester tube and nitinol stents, proximal unstented graft.	Condensed system Stent placed inside the fabric	Requires an aortic arch separate graft Few report of graft kink

Zone 3 Versus Zone 2: “The Challenge” or “The Comfort”?

Anatomically, Zone 2 covers the part of the arch between the left common carotid artery (LCCA) and the left subclavian artery (LSA); and Zone 3 covers the proximal DTA distal to the LSA (Figure 1).¹⁴ The Zone 3 / zone 2 technique involves the integration of the FET hybrid graft into the region of the aortic arch referred to as Zone 3 / zone 2 respectively. (Figure 1). Traditionally, the distal anastomosis in cET was performed in Zone 3 and the same approach was followed when FET was started. In 2012, literature started to support the proximalization of the implantation,¹⁵ subsequently leading to a notable shift from Zone 3 to Zone 2 implantation over the past decade.^16,17 The Z-3-FET technique has grown out of favour in many centres in recent years due to various reasons.

Increasing Visceral Ischemic Time

The distal aortic arch and proximal DTA are deep inside the chest cavity and difficult to approach from midline sternotomy. Lower body circulatory arrest is required during the implantation of the FET hybrid prosthesis. After implantation, the distal anastomosis for Zone 3 is performed deep inside the thorax, which increases the time of the procedure, effectively increasing the aortic cross-clamp time as well as the visceral ischemia time, eventually increasing the cardiopulmonary bypass time (CPB). The proximalization of the distal suture line to Zone 2 allows for a technically simpler and easier operation with relatively improved access, by means of bringing the anastomosis forward in the surgical field closer to the surgeon.¹⁸ The procedure, in turn, provides easier access for the surgeon and simplifies haemostasis, through avoidance of the relatively deeper and time-consuming intrathoracic anastomosis with the Z-3-FET procedure, which in turn also contribute to surgical trauma.^19,20,21

Tsagakis et al. in their comparison of patients with Z-2-FET and Z-3-FET, reported significantly reduced times of cardiac arrest, visceral ischaemia and selective cerebral perfusion for those with anastomosis in Z-2-FET (P<0.001).²² A recent analysis from Bologna surgeons showed that there were significantly longer visceral ischaemic times in Z-3-FET compared to Z-2-FET (54 minutes versus 42 minutes, p=0.001).¹⁷ A similar difference was shown by the Essen’s team, where Z-3-FET carried a significantly longer visceral ischaemic time as compared to Z-2-FET (72 minutes vs 59 minutes, p<0.001).¹⁹

Spinal Cord Injury (SPI)

Neurological complications especially the SPI are a main concern with FET; the rate of stroke was measured by Preventza et al. in a large meta-analysis between 0 and 40.9%.⁵ This included all neurological deficits, including transient, post-operatively. In their analysis, longer stent lengths above 15 cm and acute dissection pathology increased the risk of spinal cord injury (SCI). A recent meta-analysis showed an overall prevalence of postoperative SPI of 4%.²³ The causes include increased circulatory arrest time, longer stent graft, and deeper deployment of the hybrid prosthesis. Further, it has also been speculated that the length of visceral ischaemic time could be a contributing factor to the development of postoperative spinal cord injury.¹⁷

There is a decrease in the rate of neurological complications, notably the rates of SPI and paraplegia in patients with Z-2-FET as compared to Z-3-FET. The Bologna group noted a postoperative SPI to be higher in those who underwent Z-3-FET versus Z-2-FET (7.5% versus 1.4%, respectively), however, this difference was not significant.²⁴ A similar outcome was reported by Tsagakis et al,²² where the rate of paraplegia was reduced to half in the Z-2-FET patients, but in their cohort the difference was also deemed not significant. The same team are also noted that the incidence of stroke resulting in paraplegia are 50% lower with Z-2-FET approach.²⁵ This decreased risk may be attributed to the shorter coverage of the DTA due to proximalization of anastomosis in Zone 2.^17,22,26 Whilst the relatively reduced complexity of the Z-2-FET certainly contributes to this significant reduction in ischaemic times, both the type of prosthesis employed, and variability in surgical technique are likely to be additional contributing factors.¹⁷

Whilst the rates of postoperative stroke have also been suggested to be lower in patients with Z-2-FET, the selection of patients as well as the level of surgical experience have been suggested to potentially impact this, warranting further exploration of this potential relationship.^24,21 It is important to note that whilst these complications appear to be reduced in the Z-2-FET in comparison with Z-3-FET, they are still present, and the potential consequences for the patient make them clinically significant. In attempts to further reduce the rate of these complications, there are on-going developments both in regards to the technical devices utilised, as well as potential adaptations of surgical technique, including further proximalization of the anastomosis to either Zone 1, located between the brachiocephalic artery and LCCA, or Zone 0, located in the ascending aorta, proximal to the brachiocephalic artery.^14,12 Little work has been done into assessing the association of SPI with zone integration. Henceforth, further research is warranted to investigate this and subsequently as to whether the incidence of SPI has reduced since the proximalization of the FET technique from Zone 3 to Zone 2.

Further, Angioscopy has also been introduced in an attempt to aid decision making for further intervention of the aorta as well as to control stent graft positioning.^7,27 Tsagakis detailed the use of angioscopy intraoperatively to identify pathology of the aorta within a patient, allowing them to make the decision of the precise location of the landing zone from Zone 2 at the level of a plaque rupture to exclude it from blood flow. This patient-specific and flexible mode of identifying an appropriate zone for the patient may highlight an important lesson in that there may be no “one size fits all”, and dynamic evaluation of each patient’s unique anatomy may be the answer.

LSA Re-Implantation

The stent graft in the DTA can block the intercostal arteries making the collateral circulation from the LSA vital to preserve the spinal cord.^28,29 Hence, any damage or complications in LSA re-implantation can be fatal for the spinal cord. Thus, re-implantation of the LSA is of vital importance, but a challenging task in FET procedure. The reasons are multifactorial which includes the fragile nature of the vessel, deep and posterior anatomy, more often dissected ostium in patients with aortic dissection. To overcome the difficulties, several methods to revascularize the LSA are described – including the the left common carotid artery - LSA bypass (LSCB), and the extra-anatomic aortic-axillary bypass.⁷

Z-3-FET warrants a deeper dissection of tissues making the reimplantation of LSA more difficult. This increases the operative times, and risk of complications or damage to both the artery itself and other nearby structures. In regards to the operative times however, whilst reimplantation of the Z-3-FET is thought to be a more technically difficult procedure, the intra-operative management of the LSA itself in the Z-3-FET procedure can be aided by the use of techniques such as a Carrel patch for the mobilisation of the head vessels, including the LSA, reducing time required for completion of the anastomosis by means of providing a small suture line.³⁰ Conversely, whilst the Z-2-FET procedure is thought to be simpler and easier operation overall, additional time may be required for the management of the LSA.

The approach utilised for the management of the LSA is often influenced by multiple factors, including pre-operative imaging, as well as the intra-operative establishment of the nature of the LSA in regards to both its position and the condition of the vessels involved.³¹ In cases where the LSA origin is established to be ventrally displaced, a median sternotomy is often utilised to obtain access prior to completing an intra-thoracic aortic-subclavian bypass; however, this approach is met with the difficult challenge of completing the anastomosis in the deep intrathoracic compartment, which may be further complicated when the LSA itself may have been involved in the disease process requiring careful handling.^32,31 When the origin of the LSA is located deeper within the intrathoracic compartment, an extra-anatomic aortoaxillary bypass may be performed, although the approach introduces the additional risk of injury to the brachial plexus.^31,33

The surgeons from Essen performed published their result of 78 patients who underwent rerouting of the LSA. The performed the anastmosis either Intra-thoracic aortic-subclavian bypass (n=40), aortic-axillary bypass (n=35), carotid-subclavian bypass (n=3). They concluded that LSA rerouting enabled total arch repair in less than 60 minutes (of selective cerebral perfusion (mean (56 minutes). They had no incidence of RNL palsy.³¹ Redaelli et al. describe a “debranch first” technique which requires left carotid to subclavian bypass (LCSB) a month prior to surgery, rendering it invalid in the acute setting. After a month, sternotomy was done and debranching of the left common carotid and innominate arteries were done.The endograft was deployed in Zone 0 with reimplantation of the debranched LCCA and innominate artery.³⁴ Recently, a novel technique of dividing the sternal head of sternocleidomastoid muscle and sandbag behind the left shoulder for better exposure of the LSA in the mediastinum has been described to improve access. Whilst this approach carries the disadvantage of a longer scar which may extend beyond the clavicle, it increases the exposure of the LSA and greatly decreases the difficulty of anastomosis whilst causing minimal functional disability of the SCM.³²

Recurrent Laryngeal Nerve (RNL) Injury

Another well recognised risk of Z-3-FET is recurrent laryngeal nerve injury, due to the close proximity of the nerve to Zone 3 of the aortic arch. Leone et al found the incidence of recurrent laryngeal nerve injury to be 11% in Z-3-FET, compared to 2% in Z-2-FET,¹⁷ whilst other studies suggest the risk to have been eliminated completely with proximalization of the procedure to Zone 2.¹⁹

Hemostasis

Post-procedural haemostasis and postoperative bleeding is a key challenge in FET surgery. Multiple vascular anastomoses coupled with deficient coagulation factor after a long CPB/circulatory arrest may at times cause profuse bleeding in these patients.³⁵ A deeper anastomosis in the left thorax makes post-procedural haemostasis and postoperative bleeding a challenge. This can be reduced when the anastomosis is more proximal, as surgeons can perform the anastomosis more efficiently when it is superficial and also helps in an easier post-procedural haemostasis. Moreover, as previously mentioned, the more proximally located anastomosis site allows improved haemostasis control, thus contributing to a reduced need for re-exploration for bleeding with the newer procedure.²² The Essen group in their recent study found the incidence of re-exploration for bleeding to be significantly higher in those who underwent Z-3-FET than those who underwent Z-2-FET (15% vs 2%, p=0.037).³⁶

Renal Complications

Renal failure is a recognised complication of the FET procedure, often requiring temporary dialysis post-procedure. In Tian et al’s systematic review on FET, the incidence of acute kidney injury post-procedure was as high as 15.5%.³⁷ Rates of acute kidney injury postoperatively following FET procedure and subsequent need for temporary dialysis have also been shown to be significantly higher in Z-3-FET versus Z-2-FET. The Essen group report the rate of postoperative temporary dialysis as 40% in Z-3-FET patients as compared to 20% in Z-2-FET patients (p=0.028).³⁶ This was also seen in the Bologna group, where the incidence of dialysis is more in Z-3-FET as compares to Z-2-FET (20.7% vs 14.5%).¹⁷ A proximally located anastomosis site coupled with reduced ischaemic times likely contributed to the significantly reduced rates of post-operative complications observed with procedures utilising Z-2-FET, including that of significantly lower rates of temporary dialysis and renal failure.³⁶

Respiratory Complications

Whilst the exact mechanism is unclear, Z-3-FET has been reported to have an increased risk of respiratory complications in multiple works comparing the Z-3-FET and Z-2-FET procedures. Mortality due to respiratory failure following FET was reported to occur in 16.7% of patients³⁸ and is a well-recognised cause of mortality in this patient population. The incidence of pulmonary complications has also been shown to be significantly higher (p<0.001) in Z-3-FET as compared to Z-2-FET (42% vs 19%).¹⁸ This was also seen in the Bologna group, where the incidence of respiratory failure is higher in Z-3-FET as compared to Z-2-FET (11.8% vs 7.1%).¹⁷

Mortality/Long Term Follow Up

It is well reported across literature that there is a significant benefit of performing zone 2 aortic repairs. Detter et al,²⁴ report that the 30-day mortality rate for Z-2-FET as compared to Z-3-FET was 3.3% vs 17.7% retrospectively. It was further shown that there is a 14.4% reduction in 30-day mortality when Z-2-FET repairs were performed as compared to Z-3-FET repairs.²⁴ In a six-year series, Jakob et al. detailed their use of Z-2-FET versus Z-3-FET implantation in 13% and 83% of their 77 patients, respectively.¹⁵ The Essen group,³⁶ indicated that the five-year survival was significantly improved in Z-2-FET. Additionally, there is a 15% increase in 5-year survival when comparing Z-2-FET to Z-3-FET repair.²⁵ Subsequent to this, Z-3-FET was identified as an independent risk factor for death following discharge.¹⁹

Limitations and Implications for Future Research

The comparison of Z-3-FET and Z-2-FET proves challenging for a number of reasons. Firstly, due to the complexity of the procedure itself. This is a niche and newly developed surgical procedure having been introduced in the last two decades and continually changing over the years, as have the indications for the procedures. Many variables and variations in technique exist and many outcome parameters tested are influenced by numerous confounders. For example, SPI, is hypothesised not only to be influenced by the vascularisation of the LSA as mentioned, but also by other factors not related to whether the FET was performed in Zone 3 or Zone 2. These include the landing zone of the distal end of the stent graft, cerebrospinal fluid drainage, perfusion of the spinal cord via LSA during the procedure and the devices utilised amongst others.³⁹ Secondly, many of the papers identified in our literature search did not specify the specific surgical techniques used, including the vital information of the location of where the FET was integrated and hence whether a Z-2-FET or Z-3-FET technique was used; often, the technique utilised was based on the surgeon’s choice, and likely influenced by patient-specific factors, including the duration of the dissection. Much of the literature available on this subject is limited by the relatively small sample size. Thirdly, most of the studies were retrospective cohort studies. There are currently no large-scale cohort studies or randomised control trials demonstrating neither a direct comparison between Z-2-FET and Z-3-FET, nor that of any long term follow up. Hence, further research is needed to truly distinguish the superior surgical technique.

Conclusion

Hopeful thinking can get you out of your fear zone and into your appreciation zone - Martha Beck

It is well reported across literature that there is a significant benefit of performing Zone 2 aortic repairs.^24,25,22 Clearly, further research and clarification of surgical methods are required in future novel FET literature to allow successful comparison of such techniques. Nonetheless, from the literature available, Z-2-FET has shown clear advantages over Z-3-FET in repairing aortic arch pathologies within a multitude of parameters, from mortality, to both intra-operative and post-operative complications, and with on-going advances in its techniques, holds the future to aortic arch repair.

Footnotes

Author Contribution

Rozina Yasmin Choudhury and Kamran Basharat are Authors made equal contribution.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Syeda Anum Zahra

Amer Harky

Mohammed Idhrees

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