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Ann Thorac Surg 2006;82:708-713
© 2006 The Society of Thoracic Surgeons

New technology

Percutaneous Pulmonary Valve Replacement: 3-Month Evaluation of Self-Expanding Valved Stents

Tim Attmann, MDa, René Quaden, MDa, Thomas Jahnke, MD, PhDb, Stefan Muller-Hulsbeck, MD, PhDb, Andreas Boening, MD, PhDa, Jochen Cremer, MD, PhDa, Georg Lutter, MD, PhDa,*

a Department of Cardiovascular Surgery, Christian-Albrechts-University of Kiel, School of Medicine, Kiel, Germany
b Department of Radiology, Christian-Albrechts-University of Kiel, School of Medicine, Kiel, Germany

Accepted for publication January 26, 2006.

* Address correspondence to Dr Lutter, Department of Cardiovascular Surgery, Christian-Albrechts-University of Kiel, School of Medicine, Arnold-Heller-Str. 7, Kiel, 24105 Germany (Email: lutter{at}kielheart.uni-kiel.de).


   Abstract
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PURPOSE: In a recent study our group established an acute animal model of percutaneous pulmonary valve replacement using self-expanding nitinol stents. The present study was performed to evaluate these valved stents over a 3-month period.

DESCRIPTION: Bovine jugular xenografts were sutured into nitinol stents. Transfemoral implantation in the pulmonary position using a modified commercially available application device (with a 22-French outer diameter) was evaluated in 9 sheep.

EVALUATION: Two sheep died shortly after successful valved stent implantation due to internal venous hemorrhage. Another 1 sheep died 2.5 months after the procedure due to vegetations on the neovalve leading to subtotal stenosis. All other animals survived the 3-month study time (n = 6). An orthotopic pulmonary valved stent position was achieved in 4 animals and a supravalvular position in 1. During the deployment procedure, rhythm disturbances occurred in all animals, and mean arterial blood pressure dropped from 83.9 ± 26.0 mm Hg to 68.3 ± 22.3 mm Hg (p = 0.006) (n = 5). The peak-to-peak transvalvular gradient was 5.1 ± 4.0 mm Hg initially (n = 5), and 3.6 ± 1.6 mm Hg at follow-up (n = 5). Three-month angiographic and echocardiographic follow-up confirmed competent neovalves without paravalvular leakages.

CONCLUSIONS: After 3 months of implantation, percutaneously implanted memory nitinol valved stents demonstrated good function in the sheep.


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Percutaneous pulmonary and aortic valve implantations have been clinically introduced by Bonhoeffer and colleagues [1] in 2000 and Cribier and colleagues [2] in 2002. This appealing new approach to valve disease has made remarkable progress. Recently, Bonhoeffer and colleagues [1] emphasized the potential impact of percutaneous pulmonary valve stent implantation on right ventricular outflow tract (RVOT) reintervention. This method proved to be a promising additional and complimentary approach to a successful surgical program [3]. Nevertheless the anatomic spectrum of pulmonary regurgitation after surgery for congenital RVOT disease is broad. With currently available valved stents and devices it is not possible to treat all the concerned patients. So far, wide or severely calcified and kinked RVOTs are not suitable for percutaneous therapy. To overcome these problems new stent designs and operative and interventional hybrid approaches are under investigation [4, 5].

Our group demonstrated the feasibility of totally percutaneous transfemoral pulmonary valve implantation in an ovine model using self-expanding nitinol stents [6]. The continuously exerted radial force of self-expanding stents and their high flexibility assure a geometric adaptation to anatomical and tissue-property changes. Therefore the use of this kind of stent may be beneficial for the percutaneous treatment of patients with the previously mentioned anatomical anomalies.

The aim of this study was to evaluate the function of percutaneously implanted self-expanding stents carrying a biological valve into the pulmonary position of juvenile sheep during a 3-month period using angiographic and echocardiographic, hemodynamic, and macroscopic measurements.


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Valved segments of bovine jugular veins were sutured into nitinol stents with a length of 28 mm and a diameter of 24 mm as described previously [6]. The maximal radial force when expanded to 20 mm at 37°C is approximately 0.1 Newton.


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Animal Model and Implantation Procedure
Nine studies in sheep weighing 25 to 32 kg are reported. Animals received humane care in accordance with the "Principles of Laboratory Animal Care" and the "Guide for the Care and Use of Laboratory Animals" (National Institutes of Health, Publication 85-23, revised 1985). A committee on animal research at Kiel University approved the protocol.

The anesthesia and implanting procedure were carried out as previously described [6]. Seven French sheaths were introduced into the left femoral artery and vein for hemodynamic measurements. Through a 24-French sheath positioned in the right groin, the 22-French application device (Fig 1) was inserted and the valved stents were deployed directly over the native pulmonary valve under fluoroscopic control. The 24-French sheath was removed and after manual compression for 20 minutes, the skin in the right groin was closed with a single stitch. The animals were studied by analyzing various measurements described as follows.


Figure 1
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  		Fig 1. Pre-procedural loading of the application device with valved nitinol stent.

 
Afterward the sheep returned to the controlled animal facility where their general health was checked daily. Animals were not treated for anticoagulation or inhibition of platelet aggregation. After the occurrence of an endocarditis in 1 animal, prophylactic i.m. antibiotic therapy (i.e., penicillin and streptomycin) was administered weekly.

Three months later the animals were reanalyzed (described as follows) and were sacrificed.

Angiography
An angiography unit (Multistar Top, Siemens, Erlangen, Germany) was used for fluoroscopic assessment of the position and function of the neovalves.

Echocardiography
Transthoracic echocardiography (TTE) was performed at the 3-month follow-up with the Ultrasound System Five (Vingmed Sound, Horten, Norway) using a multiplane 2.5-MHz transducer (n = 4). The echocardiograpic probe was applied at the right hemithorax between the fourth and sixth intercostal spaces. The measurements were recorded and stored on a magneto-optical disk and analyzed offline by an experienced investigator (RQ).

Hemodynamic Measurements
Arterial pressure, right ventricular pressure, left ventricular pressure, and pulmonary artery pressure (Micro-Tip Millar Catheter, Millar Instruments, Inc, Houston, TX) were recorded using Haemodyn-Software (Hugo Sachs Electronics, Hugstetten, Germany). The software calculates the maximum of the first derivative of the ventricular pressures (dp/dtmax) as an indicator for the contractility and its minimum (dp/dtmin) indicating the relaxation behavior.

Macroscopic Examination
Valved stents were grossly inspected and photographs were taken. Special attention was given to the retraction of the cusps or any deformed or indurated parts of the valve. The atrial and ventricular chambers and the pulmonary artery were exposed to look for catheter-induced damage and to look for penetration of stent struts.

Roentgenogram Assessment
Roentgenogram examination of the explanted valved stents (n = 4) was performed under mammography conditions to demonstrate and localize macroscopic calcification.

Statistics
Values are presented as mean ± standard deviation. Data were analyzed based on the Wilcoxon test to compare related data for non-normally distributed data using the SPSS 10.1 software (SPSS, Inc, Chicago, IL). The p values less than 0.05 were considered statistically significant.


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Results
Six of the total 9 sheep from the study survived the entire 3-month observation period. Two sheep died 2 and 4 hours after successful valved stent implantation due to internal venous hemorrhage from the right common iliac vein and the caudal vena cava. The vessels were damaged by the 24-French sheath during its insertion. Another animal died 2.5 months after the percutaneous procedure due to subtotal pulmonary stenosis caused by endocarditic vegetations.

The mean diameter of the pulmonary annuli was 20.0 ± 1.4 mm (range, 18.0–21.8 mm) as revealed by angiography. Valved stents with a maximal outer diameter from 20.6 to 23.7 mm were used.

Mean duration of the procedure from insertion of the application device through the sheath to deployment was 52.9 seconds (range, 9.0 to 124.5 seconds). The fluoroscopy time of the entire procedure ranged from 7.0 to 26.7 minutes (mean, 11.9 minutes).

Angiography
Angiography revealed no insufficiencies after 3 months and showed competent valved stents in all 6 surviving animals (Fig 2).


Figure 2
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  		Fig 2. Representative angiography at the 3-month follow-up demonstrating a competent valved stent in the pulmonary position. Chronological sequence from a to d. (a) Early phase; (b) complete contrast in the pulmonary arteries; (c) still complete contrast in the pulmonary arteries without regurgitation; (d) late phase.

 
Echocardiography
The TTE demonstrated a laminar blood flow across the valved stents in all four analyzed cases. One mild central insufficiency and no paravalvular leakage were detected. Left ventricular and right ventricular functions and dimensions seemed normal.

The mean systolic internal diameter of the valved nitinol stents was measured at 15.6 ± 1.1 mm, and the mean internal diastolic diameter was measured at 20.1 ± 0.9 mm (Fig 3).


Figure 3
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  		Fig 3. Representative transthoracic echocardiographic image after 3 months of implantation. Note marked difference in (1) systolic valved stent diameter and (2) diastolic valved stent diameter.

 
Hemodynamic Measurements
During the deployment procedure, rhythm disturbances occurred in all animals and the mean arterial blood pressure dropped from 83.9 ± 26.0 mm Hg to 68.3 ± 22.3 mm Hg (p = 0.006). Five minutes after deployment, the mean arterial blood pressure recovered to 83.5 ± 23.7 mm Hg (p = 0.002) (n = 5). There were no significant changes in the contractility or relaxation behavior of the right and left ventricle.

The peak-to-peak gradient across the valved stents was 5.1 ± 4.0 mm Hg at 5 minutes after the implantation and 3.6 ± 1.6 mm Hg at the 3-month follow-up (n = 6) (Table 1).


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  		Table 1. Hemodynamic Data
 
Macroscopic Examination
Postmortem examination after 3 months confirmed the correct position of the valved stents in 5 sheep. In 1 animal the valved stent was found in the pulmonary artery at approximately 1 cm distal to the pulmonary valve.

No paravalvular defects were visible. The valved stents were pliable and the leaflets were thin without indurations. In one valve there was an evident lack of coaptation with one retracted leaflet. Slight fibrous overgrowth was seen at the inflow portions of two valved stents (Fig 4). Cardiac structures were unscathed. No macroscopic damage of the pulmonary artery was noted. In particular there was no penetration of stent struts.


Figure 4
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  		Fig 4. Representative macroscopic image of an ovine heart at postmortem. Right ventricle and its outflow tract are opened. Valved stent in pulmonary position with thin and transparent leaflets. The visible central defect of the valve disappeared during water rinse testing.

 
Roentgenogram Assessment
The wall of explanted stent valves appeared mildly calcified at roentgenogram examination. No calcification of valve leaflets could be detected (Fig 5).


Figure 5
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  		Fig 5. Representative roentgenogram images of a valved stent after 3 months of pulmonary implantation: (a) side view and (b) view from above. Note only minimal calcification of the wall of the bovine jugular vein and absent calcification of the leaflets.

 

   Comment
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The surgical reconstruction of the RVOT in children with congenital heart defects including pulmonary valve replacement has a low morbidity and mortality [7]. The implantation of a pulmonary homograft is considered to be the gold standard. However graft deterioration is very common and therefore reoperations are often unavoidable in adolescents [8]. Percutaneous pulmonary valve implantation is emerging as an alternative or additional option for a successful surgical scheme, recently even being introduced into clinical practice [1, 9]. Bonhoeffer and colleagues' [1] group successfully performed 35 procedures with no early deaths and only three procedural complications. Early and late outcome was comparable with that of 94 surgically treated patients in the control group [3]. The advantages of minimally invasive procedures are obvious (i.e., smaller incisions, less postoperative pain, shorter hospital stay, and lower costs).

This is one of the first reports of evaluation of percutaneously implanted self-expanding valved nitinol stents in the pulmonary position. So far, balloon-expandable valved stents are used in clinical practice [3]. The potential advantages of nitinol stents concerning the preservation of the valve and the deployment maneuver were previously discussed [6]. Interestingly in the present study, the echocardiographic examinations nicely demonstrated adaptation in the diameter of the valved stent to the radial motion of the RVOT during the cardiac cycle. The soft nitinol stent's ability to adapt to the surrounding structures could be beneficial for a physiologic blood flow in the proximal great arteries. However the radial force of this stent will not be high enough to overcome calcified valves. To reach a sufficient valve area, a preceding balloon dilation or resection procedure would be required [6, 9].

For the first time meticulous hemodynamic monitoring is reported for percutaneous heart valve replacement. The contractility of both right and left ventricle and their relaxation properties were unchanged at the end of the study. Thus the neovalves had no negative effect on the cardiac function. During the implantation, rhythm disturbances and a significant decrease of mean arterial pressure was observed without affecting the contractility as indicated by the left ventricular dP/dtmax.

The Millar catheter measurements revealed a low transvalvular pressure gradient shortly after the implantation as well as after 3 months. Macroscopy and roentgenogram assessment revealed no calcification of the leaflets, which would affect the valves function.

Limitations
The authors wish to address several limitations of this study. First, the large diameter of the application device has to be mentioned. Two animals died shortly after the percutaneous procedure due to hemorrhage from venous structures after injury by the large introducer sheath. This problem was not observed in our acute experiments, possibly because of the short follow-up period [6]. Only a further reduction of our catheter system's size will allow the clinical application in children without an inguinal cut down. Recently, Ruiz and coworkers [10] published a remarkable long-term study in pigs of catheter-placed low-profile biodegradable pulmonary valves made of small intestinal submucosa [10]. Their square stent-based valve required only an 8-French delivery system. Such a low-profile valve combined with the small flexible delivery system is ideal for use in pediatric cardiology.

Second, tissue deterioration and ensuing loss of function is a major problem with biological heart valve prostheses. In the present study, one of six valves was slightly affected. The 3-month observation period may be too short to provide valid information on the durability of the biological valve. For that reason the development of catheter-based techniques for the implantation of mechanical heart valves that are not affected by this problem should not be overlooked. An interesting feasibility study of percutaneous aortic disc valve prosthesis was reported by Sochman and coworkers [11]. Further studies in this field should be undertaken.

Third, bovine jugular veins have a limited maximal diameter of approximately 24 mm. Therefore they are not suitable for an enlarged RVOT or pulmonary trunk. Boudjemline and colleagues [5] are working intensively on this problem. In an ovine study they successfully tested a two-stage surgical and percutaneous procedure to overcome the previously mentioned problem [5]. Others chose a transthoracic beating heart approach to introduce a self-expanding valved stent of large size in the pulmonary position of pigs [4]. In contrast, our transfemorally inserted cylindrical valved stent can not provide a solution on this matter.

Nonetheless, percutaneously implanted memory nitinol valved stents are evaluated during a 3-month period in an ovine model in the present study showing a good structural and functional outcome.


   Disclosures and Freedom of Investigation
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Dr Lutter's project of percutaneous valve replacement is supported by the German Research Foundation, Bonn, Germany (Grants LU 663/4-1, LU 663/4-2). The authors of this article had full control of the design of the study, methods used, outcome measurements, analysis of data, and production of the written report. The authors had no financial relationship with any companies.


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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.


   Acknowledgments
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The authors are indebted to Beata Hoffmann, Marion Frahm, Christian König, Florian Alten, Andrea Freistedt, Kristin Rumberg, and Andreas Bohlen for their technical and operative assistance. Fritz Schaefer, MD, contributed to the roentgenogram assessment.


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  1. Bonhoeffer P, Boudjemline Y, Saliba Z, et al. Percutaneous replacement of pulmonary valve in a right-ventricle to pulmonary-artery prosthetic conduit with valve dysfunction Lancet 2000;356:1403-1405.[Medline]
  2. Cribier A, Eltchaninoff H, Bash A, et al. Percutaneous transcatheter implantation of an aortic valve prosthesis for calcific aortic stenosisfirst human case description. Circulation 2002;106:3006-3008.[Abstract/Free Full Text]
  3. 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]
  4. Zhou JQ, Corno AF, Huber CH, Tozzi P, von Segesser LK. Self-expandable valved stent of large sizeoff-bypass implantation in pulmonary position. Eur J Cardiothorac Surg 2003;24:212-216.[Abstract/Free Full Text]
  5. Boudjemline Y, Schievano S, Bonnet C, et al. Off-pump replacement of the pulmonary valve in large right ventricular outflow tractsa hybrid approach. J Thorac Cardiovasc Surg 2005;129:831-837.[Abstract/Free Full Text]
  6. 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]
  7. Kanter KR, Budde JM, Parks WJ, et al. One hundred pulmonary valve replacements in children after relief of right ventricular outflow tract obstruction Ann Thorac Surg 2002;73:1801-1806.[Abstract/Free Full Text]
  8. Wells WJ, Arroyo Jr H, Bremner RM, Wood J, Starnes VA. Homograft conduit failure in infants is not due to somatic outgrowth J Thorac Cardiovasc Surg 2002;124;:88-96.[Abstract/Free Full Text]
  9. Lutter G, Ardehali R, Cremer J, Bonhoeffer P. Percutaneous valve replacement current state and future prospects Ann Thorac Surg 2004;78:2199-2206.[Abstract/Free Full Text]
  10. Ruiz CE, Iemura M, Medie S, et al. Transcatheter placement of a low-profile biodegradable pulmonary valve made of small intestinal submucosaa long-term study in a swine model. J Thorac Cardiovasc Surg 2005;130:477-484.[Abstract/Free Full Text]
  11. Sochman J, Peregrin JH, Pavcnik D, et al. Percutaneous transcatheter aortic disc valve prosthesis implantationa feasibility study. Cardiovasc Intervent Radiol 2000;23:384-388.[Medline]



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