Ann Thorac Surg 2009;87:597-601. doi:10.1016/j.athoracsur.2008.09.004
© 2009 The Society of Thoracic Surgeons
New Technology
Off-Pump Pulmonary Valve Implantation of a Valved Stent With an Anchoring Mechanism
Guo-Wei Meng, MDa,b,
Jian-Ye Zhou, PhDa,
Yue Tang, MD, PhDa,
Zan-Kai Ye, MDa,
Yan Zhang, MDa,
Guang-Mao Liu, MDa,
Sheng-Shou Hu, MDa,*
a Key Laboratory of the Cardiovascular Regenerative Medicine, Ministry of Health, Fu Wai Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, Peoples Republic of China
b Department of Cardiovascular Surgery, Second Hospital of Shandong University, Shandong University, Jinan, Peoples Republic of China
Accepted for publication September 4, 2008.
* Address correspondence to Dr Hu, Key Laboratory of the Cardiovascular Regenerative Medicine, Ministry of Health, Fu Wai Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, 100037, Peoples Republic of China (Email: shengshouhu{at}yahoo.com).
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Abstract
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Purpose: The purpose of this study was to evaluate the longitudinal performance and anti-migration effect of a bovine valved stent equipped with an anchoring mechanism implanted off-pump in the pulmonary position.
Description: Through a delivery system, the bell-shaped pulmonary valved stents were implanted off-pump in the pulmonary valve position into six sheep by the transventricular approach. Hemodynamic, angiographic, and echocardiographic evaluations were carried out before, immediately after, and 2 months after implantation. Macroscopic and radiographic examination were performed for evaluation.
Evaluation: The valved stents were all successfully implanted off-pump in the pulmonary position on six sheep. Early and late angiographic, echocardiographic, hemodynamic, and macroscopic studies confirmed firm anchoring and good positions of the stents. All valved stents were potent, except one mild stenosis with a 24 mm Hg transvalvular pressure gradient that developed and one mild insufficiency that were discovered at the end of the study.
Conclusions: Transventricular implantation of the bell-shaped pulmonary valved stents was evaluated during a 2-month period in the sheep in the present study and showed a good structural and functional outcome with no migration.
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Technology
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Percutaneous or transventricular implantation of a pulmonary valve has made considerable progress since Bonhoeffer and colleagues [1] published a landmark report of percutaneous pulmonary valve implantation in a lamb. However, some drawbacks (eg, migration of the valved stent) related to the design of the valved stents and device remain.
To overcome this problem, we refined an existing tubular pulmonary valved stent and designed a bell-shaped pulmonary valved stent. We inserted the modified stent into the pulmonary position of the sheep from the right ventricle without cardiopulmonary bypass. The aim of this study was to evaluate the functional improvement in experimental sheep with the implantation of this newly modified valved stent implanted into the pulmonary position during a 2-month, follow-up period using echocardiographic, hemodynamic, angiographic, and macroscopic measurements.
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Technique
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Stent Design and Preparation of Valved Stent
The bell-shaped pulmonary valved stents were constructed by suturing a bovine jugular vein into three rings of the nitinol "Z" stent (Grikin Advanced Materials Co Ltd, Beijing, China). The ring in the distal extremity of the valved stent was bent outward (bell portion) to prevent stent migration (Figs 1A and 1B). The diameter of the bell portion is 20% larger than that of the proximal tubular portion. The valved jugular vein was cross linked with a buffered saline solution containing 0.6% glutaraldehyde for 36 hours at 4°C. After fixation, it was transferred to a 60% ethanol solution for storage. Before implantation, the valve assembly was immersed three times and was carefully soaked in a physiological saline solution to remove the ethanol [1].
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Fig 1. (A, B) View of valved venous segment in vascular nitinol stent. (C) Custom-made delivery system. (D) Valved stent was implanted off-pump in pulmonary valve position by transventricular approach.
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Clinical Experience
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Implantation Procedure
Six sheep weighing 35 to 45 kg underwent off-pump transventricular pulmonary valve implantation. All animals were cared for in compliance with "Regulation to the Care and Use of Experimental Animals" (1996, revised 2004) of the Beijing Council on Animal Care. This study was approved by the Institutional Committee on Animal Research at our institution.
General anesthesia was induced with 25 mg/kg of ketamine and 2 mg/kg diazepam and was maintained with ketamine, fentanyl, and suxamethonium chloride with the use of mechanical ventilation. After general anesthesia, tracheal intubation and mechanical ventilation, with continuous monitoring of electrocardiogram, arterial and central venous pressure, and oxygen saturation, the chest was opened through a left anterior thoracotomy. Heparin was intravenously administered (200 U/kg). The size of the valved stent was selected based on the size of the pulmonary annulus measured by epicardial echocardiography. After a short incision (4 mm) on the right ventricular outflow tract, controlled by a pursestring on 4-0 polypropylene suture through the custom-made delivery system, the valved stent was implanted off-pump in the pulmonary valve position by a transventricular approach (Figs 1C and 1D). The correct positioning of the valved stent was evaluated and confirmed before definitive deployment by digital palpation and epicardial echocardiography. After the procedure, the ventriculotomy was closed with the suture of the purse. The chest was closed in a routine fashion.
After the completion of the operation, sheep were returned to the controlled animal facility where their general conditions were monitored daily. Anticoagulant therapy was continued with low-dose heparin for 24 hours postoperatively before it was substituted by low-dose aspirin (100 mg/day) for 2 months. Penicillin was administered for 5 postoperative days.
Evaluation
Epicardial echocardiography and angiography was performed immediately after and 2 months later after the procedure for assessment of the position, sealing, and function of the implanted valve.
All grafts were explanted 2 months after implantation. Before harvesting, heparin (300 IU/kg) was given intravenously. All explanted valved stents were inspected macroscopically and were subjected to radiographic examination under mammographic conditions.
Statistics
The paired t test was used, and all values are presented as mean ± standard deviation. The two-tailed p values less than 0.05 were considered statistically significant.
Results
The mean diameter of the pulmonary annuli was 19.2 mm (range, 18.3 to 23.2 mm) measured with epicardial echocardiography. The valved stents implanted were 30 to 35 mm in length, 22 to 26 mm (bell portion), and 18 to 22 mm (tubular portion) in the internal diameter of 25 to 28 mm (bell portion) and 21 to 25 mm (tubular portion) in the external diameter when fully expanded.
All six devices were successfully implanted correctly at the target site over the native pulmonary valve. All the sheep survived the entire 2-month observation period. No complications were noted during the procedure and follow-up.
All six valves were echocardiographically competent immediately after the procedure. The implanted valves were competent echocardiographically, except that one mild transvalvular insufficiency and one mild stenosis was detected at the end of the protocol (Fig 2). Angiography revealed mild transvalvular regurgitation, which occurred in one sheep 2 months after implantation, whereas the other five sheep showed competent valved stents (Fig 3). No paravalvular leak and migration were observed at the end of the study.
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Fig 2. (A, B) Epicardial echocardiography at the 2-month follow-up demonstrating a competent implanted valve in the pulmonary position.
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Fig 3. (A) Angiography of right ventricle before implantation. (B) Angiography of pulmonary artery at 2-month follow-up after implantation demonstrating a competent implanted valve in the pulmonary position.
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The hemodynamic data before and immediately after implantation and before retrieval are shown in Table 1. At the end of the follow-up period, one valved stent was mildly stenotic, with a 24-mm pressure gradient across the valved stent. However, no significant changes were found in pulmonary and right ventricular pressure, as well as transvalvular pressure gradient before and immediately after the implantation and at the end of the follow-up period.
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Table 1 Comparison Among Hemodynamic Data Obtained Before and After Valved Stent Implantation and at 2-Month Follow-Up
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Postmortem examination after 2 months confirmed the correct position of the valved stents in all sheep (Figs 4A and 4B). The stents were firmly adhered to the pulmonary artery wall and were covered with a neointima, which was continuous with the intima of the pulmonary artery. There was no sign of erosion and perforation of the stent across the pulmonary artery wall (Figs 4B and 4C).
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Fig 4. A postmortem examination of bell-shaped pulmonary valved stents at the end of the protocol (A) leaflets were thin and perfectly mobile and the valve was competent. (B) Correct position of the valved stent. (C) Valved stent was firmly adhered to the pulmonary wall and was covered with a neointima, which was continuous with the intima of the pulmonary artery.
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Two implanted valves were covered with a thin fibrous sheath on both the inflow and outflow side. The postmortem examination revealed slight stiffening of the leaflet with a slightly narrowed orifice in one case and with a slightly incomplete coaptation in the other. Whereas the other four neovalves were free of any fibrous tissue formation, the leaflets were pliable and thin and perfectly mobile (Fig 4A). All six valves were free of calcification and thrombus formation at postmortem examination and x-radiography. In addition, no paravalvular defects were observed in all implanted valves.
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Comment
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Significant pulmonary regurgitation or stenoses after operative correction of the right ventricular outflow tract and the pulmonary trunk in patients with Fallot syndrome or variant of this disease often requires pulmonary replacement with cardiopulmonary bypass. Despite the fact that surgical repair can be performed with low mortality, the likelihood of reoperations and surgical replacement of these valves, particularly when they are replaced at a young age are quite high [2–4]. Therefore, minimizing procedural invasiveness and associated risks is mandatory.
Percutaneous or transventricular implantation of the pulmonary valve, which was recently introduced into clinical practice, has emerged as a promising alternative or additional option for this situation. However, some drawbacks related to the design of the ever-reported tubular valved stents and devises remain, such as migration of the valved stent [5–7].
The ever-reported pulmonary valved stents are all tube-like in shape. However, generally the diameter of the pulmonary trunk is 2 to 
3 mm larger than that of the pulmonary annulus; this is one of the causes of stent migration. Different from the existing tubular stents, the bell portion of the modified stent is 20% larger than the tubular portion in diameter, so that when the stent fully expands, the continuously exerted radial force of the expanded valved stent, especially the bell-shaped portion may prevent the valved stent migration.
Our study demonstrated that the bell-shaped design assured the secure anchoring of the valved stent in the correct position. From Attmann and colleagues' [5] experience, an oversizing of approximately 20% in nitinol stent proved to be sufficient for secure anchoring. In fact, the exerted radial force of the nitinol stent on the pulmonary wall is not only related to the diameter of the stent, but also to the thickness of the nitinol wire, the structure, and the profile of the stent. Regarding the valved stent we designed, the external diameter of the tubular portion is 3 mm larger than the size of the pulmonary annulus, and the bell portion is 4 to 5 mm or 25% to 30% larger than the size of the pulmonary trunk, and it is verified to be sufficient for secure anchoring, whereas a larger than 30% in diameter bell portion is not necessary and may have the potential risk of erosion and perforation across the pulmonary wall by the protruding stents.
Hemodynamic performance of the valved stent is related to the design of the stent. Compared with balloon-expandable stents, the continuously exerted radial force of self-expanding stents and their high flexibility assure a geometric adaptation to the surrounding structures and tissue property changes, and the mechanical forces to the contracting tissue are equally distributed. Our experiment demonstrated that the fully expanding nitinol stent and good attachment of the stent to the vessel wall could provide a favorable effective orifice area and allow adequate hemodynamics without paravalvular leak.
There was no macroscopic calcification and thrombus formation that appeared in any of the six valved stents at the end of the protocol, except two implanted valves that were covered with a thin fibrous tissue. It demonstrates fibrous formation of the implanted valves that plays a major role in deterioration of the impanted valves in short-term follow-up; this is similar to previous studies [1, 8]. Glutaraldehyde treatment of the valves, surrounding reaction to the foreign body or tissue, and somatic growth of the experimental animals may contribute to the impairment of the neovalves.
However, compared with other bioprosthetic conduits (contegra bovine jugular vein graft) [9, 10], the pathologic changes of the bell-shaped pulmonary valved stent wall was quite slight. This may be due to the removal of the outer layer of the bovine jugular vein, and only very thin intima is maintained to reduce surrounding reaction. Nevertheless, in terms of long-term durability, a further long time follow-up is required to prove longevity of this newly-designed pulmonary valved stent.
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Limitations
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First, only animals with normal right ventricular outflow tracts were evaluated. Because the tortuous or more complicated pulmonary anatomy as seen in Fallot patients appears late after surgical repair, a similar setting was impossible to achieve in animals. Second, the 2-month observation period may be too short to provide valid information on the durability of the biological valve in the stent, the reaction of the surrounding tissue, or the risks of secondary erosion of the valved stents. The long-term performance of this newly designed bell-shaped valved stent requires further long-term experimental studies before clinical application.
In conclusion, transventricular implantation of the bell-shaped nitinol valved stent was evaluated during a 2-month period in sheep in the present study showing a good structural and functional outcome with no migration.
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Disclosures and Freedom of Investigation
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This work was supported by the Beijing Municipal Science & Technology Commission (Y0405003000091). The nitinol stent discussed in this report was designed by the researchers, and its manufacture was commissioned at Grikin Advanced Materials Co Ltd, Beijing, China. The authors do not have any other material or financial relationships with this commercial enterprise. The authors had full control of the design of the study, methods used, outcome measurements, analysis of data, and production of the written report.
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Footnotes
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Disclaimer The Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
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References
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- Bonhoeffer P, Boudjemline Y, Saliba Z, et al. Transcatheter implantation of a bovine valve in pulmonary position: a lamb study Circulation 2000;102:813-816.[Abstract/Free Full Text]
- Dore A, Glancy DL, Stone S, Menashe VD, Somerville J. Cardiac surgery for grown-up congenital heart patients: survey of 307 consecutive operations from 1991 to 1994 Am J Cardiol 1997;80:906-913.[Medline]
- Bove EL, Kavey RE, Byrum CJ, Sondheimer HM, Blackman MS, Thomas FD. Improved right ventricular function following late pulmonary valve replacement for residual pulmonary insufficiency or stenosis J Thorac Cardiovasc Surg 1985;90:50-55.[Abstract]
- Connelly MS, Webb GD, Somerville J, et al. Canadian Consensus Conference on Adult Congenital Heart Disease 1996 Can J Cardiol 1998;14:395-452.[Medline]
- Attmann T, Jahnke T, Quaden R, et al. Advances in experimental percutaneous pulmonary valve replacement Ann Thorac Surg 2005;80:969-975.[Abstract/Free Full Text]
- Coats L, Tsang V, Khambadkone S, et al. The potential impact of percutaneous pulmonary valve stent implantation on right ventricular outflow tract re-intervention Eur J Cardiothorac Surg 2005;27:536-543.[Abstract/Free Full Text]
- Berdat PA, Carrel T. Off-pump pulmonary valve replacement with the new Shelhigh injectable stented pulmonic valve J Thorac Cardiovasc Surg 2006;131:1192-1193.[Free Full Text]
- Attmann T, Quaden R, Jahnke T, et al. Percutaneous pulmonary valve replacement: 3-month evaluation of self-expanding valved stents Ann Thorac Surg 2006;82:708-714.[Abstract/Free Full Text]
- Delmo-Walter EM, Alexi-Meskishvili V, Abdul-Khaliq H, Meyer R, Hetzer R. Aneurysmal dilatation of the Contegra bovine jugular vein conduit after reconstruction of the right ventricular outflow tract Ann Thorac Surg 2007;83:682-684.[Abstract/Free Full Text]
- Shebani SO, McGuirk S, Baghai M, et al. Right ventricular outflow tract reconstruction using Contegra valved conduit: natural history and conduit performance under pressure Eur J Cardiothorac Surg 2006;29:397-405.[Abstract/Free Full Text]
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Ann. Thorac. Surg. 2009 87: 601-602.
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