MMCTS
(March 29, 2007). doi:10.1510/mmcts.2006.001990
Copyright © 2007 European Association for Cardio-thoracic Surgery
Procedure
The frozen elephant trunk technique
Maximilian A. Pichlmaier*,
Omke E. Teebken,
Hassina Baraki and
Axel Haverich
Department of Cardiac, Thoracic, Transplantation and Vascular Surgery, Hannover Medical School, Carl Neuberg Strasse 1, 30625 Hannover, Germany
* Corresponding author: * Corresponding author: Tel.: +49-511-532-3453; Fax: +49-511-532-8158 e-mail: pichlmaier.maximilian{at}mh-hannover.de
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Summary
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The so-called ‘frozen elephant trunk’ technique is adapted from the classical elephant trunk technique first described by H.G. Borst in 1983 and allows the repair of concomitant aortic arch and proximal descending aortic aneurysms in a single stage. A ‘hybrid’ vascular graft consisting of a conventional tube graft with an endovascular stented graft at the distal end is utilised to achieve a blood-tight seal in the descending aorta that cannot easily be accessed directly from an anterior approach. Thus, the concept of a traditional elephant trunk, otherwise completed with a secondary endovascular or surgical procedure, is achieved in one single step. First intra- and postoperative results of this technique in terms of successful exclusion of the proximal descending aortic aneurysm are good and following the learning curve, the prolongation of circulatory arrest and cerebral perfusion, as compared to the traditional elephant trunk procedure, is within minutes and thus acceptable. Currently all patients with thoracic aneurysms extending from the arch beyond the left subclavian artery are evaluated for this treatment at our institution. Furthermore, acute aortic dissections (type A and B) are an area of intensive clinical evaluation at present.
Key Words: Aortic aneurysm • Aortic dissection • Elephant trunk • ‘Hybrid prosthesis’ • Thoracic aneurysm • Thoracic stent grafting
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Introduction
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Patients with extensive aortic aneurysms involving the ascending aorta, the aortic arch as well as the descending aorta are still considered to be a challenge in terms of treatment strategy and intraoperative management [1]. Issues of discussion include on the one hand protection from ischaemia of the heart, brain, spinal cord and viscera as well as the avoidance of respiratory compromise due to prolonged circulatory arrest and on the other hand, the technical management of concomitant aortic valve pathology and that of the proximal descending aorta. Concerning the latter, Borst introduced in 1983 the elephant trunk procedure in order to allow and simplify staged surgery [2]. This well-known modification of arch surgery consists of the prosthetic replacement of the ascending aorta and the aortic arch with an additional elephant-trunk-like extension of the arch graft into the descending aorta. In a second stage, this elephant trunk facilitates replacements further down the descending aorta to the desired level through a lateral thoracic or thoraco-abdominal approach. Proximal clamping may thus be of the elephant trunk rather than the previously reconstructed distal arch, avoiding extensive dissection in scarred tissue and damage to recurrent laryngeal and phrenic nerves. However, despite avoiding circulatory arrest the second operation commonly performed anything between 4 weeks and 6 months later, again requires some form of distal bypass and results, besides the significant morbidity, in a cumulative risk for early mortality of up to 15%. Furthermore, a significant number of patients fail to return for the second operation as they meanwhile die due to aortic rupture or sometimes comorbidities do not allow for another large vascular procedure [3, 4].
Endoluminal stent grafting utilising the elephant trunk as proximal landing zone has changed the risk profile of the second stage operation for patients with suitable anatomy; mortality and morbidity are not, however, completely negligible [5] (Video 1).
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Video 1 The 3-D reconstructions show the thoracic aorta of a patient with combined ascending and descending pathology following operative replacement of the ascending aorta and arch. The procedure was completed by secondary implantation of three endoluminal stent grafts from the femoral artery using the elephant trunk as proximal landing zone.
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In some institutions stent grafting is performed at the end of the arch surgery retrograde via the femoral vessels, which may be done during the reperfusion period, thus avoiding the additional operating time. Rarely nowadays, a conventional endoluminal stent graft may be introduced antegrade either during circulatory arrest or through the arch graft during reperfusion. Both retrograde and antegrade methods require, besides the complex technical setup, a high degree of endovascular expertise both of the surgeons and the nurses. Especially for situations like acute type A dissections a simple, straightforward and self-explanatory concept and graft are required – such as the ‘hybrid prosthesis’.
Technical concept
In order to complete the surgical treatment of the above-described pathologies during one single operation through a median sternotomy, we introduced the ‘frozen elephant trunk’ procedure [6]. A vascular ‘hybrid prosthesis’ with a stented and a non-stented segment (Chavan-Haverich endograft, Curative Medical Devices GmbH, Dresden, Germany – MMCTSLink 142) is thus advanced with its stented end into the descending aorta in an antegrade fashion during circulatory arrest. Whereas this is analogous to the introduction of the traditional elephant trunk, the ‘hybrid prosthesis’ achieves haemodynamic seal and thus completes treatment for the descending aortic aneurysm without the need for a further operation. The aortic arch is then replaced with the non-stented segment of the graft in the usual fashion [6] (Photos 1, 2, Schematic 1).
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Photo 1 The ‘hybrid prosthesis’ (‘Chavan-Haverich’ endograft, Curative GmbH, Dresden, Germany – MMCTSLink 142) made of a woven vascular prosthesis with stainless steel stents affixed to the inner aspects at the distal end. The proximal portion of the ‘hybrid prosthesis’ is unstented and consists of a Dacron sleeve ready for conventional surgical handling. A gap of 5 mm between the stents provides flexibility of the graft. The diameter of the stents within the ‘hybrid prosthesis’ ranges from 30 to 46 mm and the stents have a length of 22 mm each. Two longitudinally oriented stainless steel wires maintain the distance between the stents. Depending on individual patient's anatomy, the number of stents may vary between three and four.
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Photo 2 The delivery system of the ‘hybrid prosthesis’ has a ‘sheath in sheath’ design, is flexible and can be advanced into the descending aorta directly or over a 0.035-inch super stiff guide wire (Back-up Meier; Boston Scientific Corporation, Boston, MA – MMCTSLink 143). It consists of a 39 French (F) outer sheath, a 34 F inner sheath and a central pusher. Withdrawal of the outer sheath while holding the inner sheath and the pusher steady releases the stented portion of the ‘hybrid prosthesis’.
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Schematic 1 (a) The stented end of the ‘hybrid prosthesis’ is deployed in the descending aorta distal of the aneurysmatic segment. Selective antegrade cerebral perfusion is performed during hypothermic circulatory arrest. (b) After suturing the ‘hybrid prosthesis’ circumferentially into the descending aorta directly distal to the origin of the left subclavian artery, the supraaortic branches are reimplanted into the graft as a single tissue patch. (c) The reconstruction may then be completed at any desired level of the ascending aorta while cardiopulmonary bypass is re-established.
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Patient selection
The typical patients are those requiring arch replacement in circulatory arrest but additionally have patho-logical changes in the proximal descending aorta. A situation that can also be considered suitable is the acute type A dissection where the tear extends into the arch or even the proximal descending aorta. In chronic dissection, the indication to use the ‘hybrid prosthesis’ is seen predominantly if aneurysmatic change has occurred. Unlike in acute dissection, it is, however, rare to be able to achieve readaptation of the aortic wall layers due to the rigidity of the dissection membrane (Photo 3).
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Photo 3 MRA showing a typical combination of ascending and descending aortic aneurysms with an ectatic arch connecting the two.
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Graft selection
The length of the stent graft is chosen so that at least 5 cm distal landing zone is achieved. The choice of diameter follows the conventional endoluminal practice and should be aimed at 15–20% oversizing as determined by angio-CT. The diameter of the proximal end of the hybrid graft is governed by the stent size but this rarely causes significant technical problems.
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Surgical technique
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In the following sections the standard surgical procedure concerning the ‘hybrid prosthesis’ is described and illustrated with a redo case of a 46-year-old man with a chronic type A aortic dissection with secondary aneurysmatic change to 12 cm. Eleven years previously, the aortic valve, ascending aorta and proximal part of the aortic arch had been replaced for the treatment of an acute type A aortic dissection.
The operation is performed via a median sternotomy, which gives adequate exposure of the ascending aorta and aortic arch. The latter is dissected as far as possible giving good control of the supraaortic vessels, especially the origin of the left subclavian artery. Snugger may be placed around the innominate artery and the left common carotid artery in order to later secure the cannulas for the antegrade cerebral perfusion. The aorto-pulmonary ligament is divided. Care should here be taken not to sever the left phrenic and recurrent laryngeal nerves (Video 2).
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Video 2 The ascending aorta encroached upon the sternum, therefore, the femoral artery and vein are cannulated for establishing the extracorporeal circulation. After resternotomy, the ascending aorta, aortic arch, brachiocephalic artery, left carotid artery and the left subclavian artery are dissected carefully. Special attention should be paid to left phrenic, vagus and the recurrent laryngeal nerves. They should be identified and protected.
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Following cannulation of the ascending aorta and of the right atrium, cardiopulmonary bypass is installed and the patient cooled to 26°C core temperature. In redo cases it may be favourable to cannulate the femoral vessels at the beginning of the operation. During cooling the vent catheter is introduced via the right superior pulmonary vein following the electrical introduction of ventricular fibrillation to avoid air embolism. The aorta is cross-clamped and incised. The type of incision depends on whether surgery is planned for the aortic valve. For simple aneurysm surgery, the incision may be longitudinal to start with, to allow a graft inclusion later on. Antegrade selective blood cardioplegia is applied via the coronary ostia. The necessary proximal reconstruction is then completed. The anastomoses are tested using cardioplegia given retrograde into the graft. Following this and as soon as the desired temperature is reached, the patient is tilted into a Trendelenburg position in order to prevent air embolisation, cardiopulmonary bypass is interrupted and the aortic arch opened. Antegrade selective cerebral perfusion at 14°C at a flow of 250–450 ml/min is installed using the cardioplegia line with special balloon catheters inserted into the innominate artery and the left common carotid artery. Perfusion pressure and transcranial oxygen saturation are continuously monitored to ensure adequate, symmetrical perfusion. Backflow from the left subclavian artery into the operating field may be controlled by a Fogarty catheter or the vessel may alternatively be clamped (Video 3).
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Video 3 The aneurysm of the ascending aorta is opened and the incision is extended into the aortic arch beneath the brachiocephalic arteries to the origin of the left subclavian artery. The old aortic graft is transsected distally and removed. Excess of aneurysmatic wall is removed.
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The arch is opened along its concavity to the level of the origin of the left subclavian artery and circumferentially detached here. The supraaortic vessels are isolated from the arch as single island taking care to leave enough suitable tissue at the proximal end of the descending aorta for the distal suture line. The super stiff guide wire that was introduced via one femoral artery retrograde into the descending aorta at an earlier stage to save time during circulatory arrest may now be advanced into the operating field (Video 4).
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Video 4 The delivery system is introduced into the descending aorta via the super stiff guide wire. The entire stent is inserted into the descending aorta. Withdrawal of the outer sheath while holding the inner sheath and the pusher steady releases the stented portion of the ‘hybrid prosthesis’. Under radiographic control the proximal Dacron tube is released by pulling back both sheaths simultaneously while holding the pusher steady.
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The ‘hybrid prosthesis’ is introduced on this wire antegrade into the descending aorta down to the desired level. Proper positioning may be determined fluoroscopically using a C-arm or simply by applying the preoperative measurements from the CT scan. The outer sheath of the delivery system is withdrawn while the inner sheath and the pusher are held steady. Thereby the stented portion of the ‘hybrid prosthesis’ is deployed in the descending aorta. The proximal Dacron tube is then released by pulling back both of the sheaths simultaneously while holding the pusher steady. Subsequently, the introducer set is then withdrawn completely. The stent may now, or better at the end of the operation under more physiological blood pressures, be balloon dilated to secure sealing (Video 5).
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Video 5 Balloon dilatation of the stent graft to improve the quality of the seal to the wall of the descending aorta.
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The steps to follow from here parallel those for the traditional elephant technique and are thus only briefly described.
Invaginating the unstented part of the graft into the distal stented portion allows for a comparatively comfortable placement of the distal suture line using 3/0 Prolene (Video 6).
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Video 6 The proximal unstented part of the prosthesis is sutured directly into the aortic orifice distal to the left subclavian artery by running sutures.
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Then the unstented part is pulled out again and the island carrying the supraaortic vessels reinserted as single patch into the graft (Video 7).
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Video 7 The graft is placed under tension and an oval segment corresponding to the size of the aortic cuff is excised in the prosthesis. The supraaortic vessels are sutured to the graft. Arterial cannulation is performed in the aortic arch prosthesis. Purse string sutures are set in the graft. Flow from the pump oxygenator is initiated to evacuate air from the distal aorta. Then, the proximal graft is cross-clamped and cardiopulmonary bypass is re-established.
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Cerebral perfusion is stopped as late as possible. Occasionally, the perfusion cannulas may be reinserted via the graft reducing the cerebral ischaemia time further. The main graft is then clamped, cannulated, cautiously de-aired and extracorporeal circulation is re-established (Video 8).
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Video 8 The remnant of the ascending aortic graft is stretched. The new aortic arch prosthesis is cut to the appropriate length adapted to the patient's new anatomy. Proximal graft-to-graft anastomosis is completed using 4/0 Prolene running sutures.
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During early warming the proximal anastomosis with the ascending aorta or the graft, replacing the latter is completed. Following careful de-airing the aortic cross clamp is released. Thus, the core part of the operation is accomplished (Video 9).
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Video 9 Finished procedure after decannulation and the postoperative chest X-ray.
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Postoperative surveillance
Patients are followed like those who received thoracic stent grafts. CT scans are performed at 3, 6 and 12 months and then at 1-year or 2-year intervals (Videos 10 and 11).
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Video 10 Cross sectional CT-reformation of a patient with thoracic aneurysm 26 months after implantation of the ‘hybrid prosthesis’.
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Video 11 Transversal CT-reformation of a patient with thoracic aneurysm 26 months after implantation of the ‘hybrid prosthesis’.
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Results
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From 9/2001 to 3/2006, 39 patients with combined pathologies of the ascending aorta/aortic arch and the descending aorta were operated on using the ‘hybrid prosthesis’. Mean patients age was 62 years. The majority of patients presented with aortic dissection: Four (10.3%) with an acute type A aortic dissection, 11 (28.2%) with a chronic type A dissection, three (7.7%) had an acute type B aortic dissection and 3 (7.7%) a chronic type B aortic dissection. The indication for intervention in patients with chronic dissections was aortic dilatation. The second most frequent pathology (n=18, 46.2%) was true aortic aneurysm proximal and distal of the left subclavian artery. The maximal aortic diameter in these patients was 59±8 mm (47–73 mm). Additional cardiac pathology included severe coronary artery disease in 11 (28.2%) and aortic valve disease in 11 (28.2%) cases.
There was no intraoperative mortality, however, five (12.8%) patients died within 30 days. Causes were in all but one patient related to their comorbidities. In the former one case rupture of the descending aorta occurred two days postoperatively.
The implantation of the prosthesis was successful in all but one patient. This patient had a marked kinking of the descending aorta distal of the aneurysmatic segment to be excluded. The implantation was tried over a stiff guide wire but the introducer system could not be advanced beyond the kinked segment. At this point perforation of the aortic wall occurred, which required surgical repair and additional transfemoral stent graft implantation to bridge the perforated and the aneurysmatic segments.
On follow-up the average diameter of the stented descending thoracic aorta decreased over time from 63±14 mm (47–113) to 54±11 mm (39–74) (Graphs 1 and 2).
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Graph 1 Actuarial survival in months after implantation of the ‘hybrid prosthesis’. Numbers in parentheses show the patients remaining at risk at each relevant time point.
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Graph 2 The maximal diameter of the descending thoracic aorta at the level of the tracheal bifurcation before and after the implantation of the ‘hybrid prosthesis’. CADA = chronic aortic dissection type A, CADB = chronic aortic dissection type B, AADA = acute aortic dissection type A.
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One patient with chronic aortic dissection exhibited a small proximal type 2 endoleak and in a second patient, the stented segment of the graft could not be anchored successfully in a previously implanted thoracoabdominal aortic stent graft giving rise to a distal endoleak. This was treated endovascularly. A distal type I endoleak was found at 6 months in a third patient. His thoracoabdominal aneurysm enlarged prompting us to perform an open repair after an interval of almost 4 years.
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Discussion
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In the conventional elephant trunk operation, the peri-graft space around the elephant trunk remains perfused, thereby allowing for further aneurysmatic dilatation of this aortic segment [3, 7, 8]. A second operation is inevitably required, either in the form of a traditional operative replacement or alternatively an endoluminal exclusion of the diseased descending aorta. In contrast, the ‘hybrid prosthesis’ is designed to definitively exclude the perigraft space in the descending aorta dealing with the complete set of pathologies. The results confirm this concept when compared to conventional stent grafting [9, 10] (Photo 4).
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Photo 4 Cross sectional CT-scan of a patient with aortic aneurysm following implantation of a ‘hybrid prosthesis’ 26 months postoperatively. The patient underwent a supracoronary replacement of the ascending aorta and the aortic arch and a coronary bypass graft with a left internal thoracic artery to the LAD. Perigraft space around the stented segment of the ‘hybrid prosthesis’ is thrombosed completely.
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The high incidence of concomitant procedures and the comorbidities of the initial patient group explain the perioperative mortality rate of 12.8% [11, 12, 13]. Other groups performing one-stage surgical replacement of the thoracic aorta also report mortality rates between 14 and 16% [12, 14, 15]. The incidence of stroke in 12.8% of cases remains a concern since it is higher than expected [12]. No patient developed paraplegia caused by spinal cord injury in our series. This rate is quite low compared to the significant paraplegia rates reported with similar open surgical approaches, but it correlates with reports of endovascular stent grafting of the descending aorta [16, 17, 18, 19].
The use of the ‘hybrid prosthesis’ has the main advantage over a conventional stent graft of a secure and blood-tight fixation proximally completely avoiding the problems of endoleak and migration at the expense of a marginally extended circulatory arrest time, when compared to the secondary antegrade implantation of a conventional stent graft [9, 10].
A technical aspect that has to be observed when using the ‘hybrid prosthesis’ is the risk to twist the unstented portion of the graft when reimplanting the supraaortic vessels as by expanding the stent, the prosthesis becomes fixed in the descending aorta in a rotational direction. A further detail that was learned is, that the liberal use of a stiff guide wire inserted prior to circulatory arrest from the femoral artery often helps achieving more rapid and precise positioning of the stented end of the prosthesis. It is furthermore advisable to do all sizing for the stent prior to surgery if possible, as during circulatory arrest the aorta collapses and shortens (reduced kinking) making intraoperative sizing unreliable.
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Footnotes
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 Statement: The authors declare that there are no conflicts of interest except that Prof. A. Haverich holds the patent for the device. 
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References
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- Borst HG, Walterbusch G, Schaps D. Extensive aortic replacement using "elephant trunk" prosthesis. Thorac Cardiovasc Surg 1983;31:37–40.[Medline]
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Summary
Introduction
