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Ann Thorac Surg 2006;82:110-116
© 2006 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Transapical Aortic Valve Implantation: An Animal Feasibility Study

^a Cardiopulmonary Research Science and Technology Institute, Dallas, Texas
^b Heart Center Leipzig, Leipzig, Germany
^c Department for Thoracic and Cardiovascular Surgery, JW-Goethe University, Frankfurt, Germany
^d Cleveland Clinic Foundation, Cleveland, Ohio

Accepted for publication February 13, 2006.

^_* Address correspondence to Dr Dewey, 7777 Forest Lane, Suite A323, Dallas, TX 75230 (Email: tdewey{at}csant.com).

Presented at the Basic Science Forum of the Fifty-second Annual Meeting of the Southern Thoracic Surgical Association, Orlando, FL, Nov 10–12, 2005.

Drs Dewey, Doss, and Wimmer-Greinecker disclose that they have a financial relationship with Edwards Lifesciences.

 

	Abstract

Top Abstract Introduction Material and Methods Results Comment The Society of Thoracic... Acknowledgments References

 
BACKGROUND: Percutaneous aortic valve implantation has recently been performed in nonsurgical patients with severe aortic stenosis. Retrograde valve delivery has been problematic because of the size of the delivery system and concomitant peripheral vascular disease. We investigated a minimally invasive approach through the left ventricular apex for antegrade placement of a device-deliverable valve.

METHODS: Transapical aortic valve implantation was performed using a 23-mm equine valve mounted on a stainless steel stent in 24 swine (weight range, 35 to 45 kg). A limited or full sternotomy approach was used to access the apex of the heart. The crimped valve was introduced through a sheath in the left ventricular apex. Fluoroscopy and echocardiography were used for guidance. Deployments were performed on the beating heart either with ventricular unloading using femoral extracorporeal circulation or rapid ventricular pacing.

RESULTS: All valves were successfully delivered at the selected target site with acceptable visualization of the noncalcified aortic annulus. Valve migration occurred during eight deployments (two distal and six retrograde) secondary to persistent cardiac output, unfavorable annular anatomy, and dislodgement by the delivery catheter. Exact positioning of the nonmigrated valves at the aortic annulus was examined by necropsy of all animals at the end of the procedures. Paravalvular leak was noted in 14 of 18 (77.8%) valves remaining in situ.

CONCLUSIONS: The transapical approach was used for the successful antegrade placement of a stented valve, obviating the technical problems associated with a large delivery system transiting the peripheral vascular system. Stent design contributing to paravalvular leak remains problematic.

Aortic valve replacement using cardiopulmonary bypass and cardioplegic arrest has been the standard approach for the treatment of severe aortic stenosis for decades. Increases in overall life expectancy combined with a rapidly growing elderly population have increased the numbers of patients presenting with calcific degenerative aortic stenosis. Patients now routinely present with comorbid illnesses that increase their operative risk with standard valve surgery. Although most surgeons believe that the preponderance of patients with critical aortic stenosis are referred for surgery, there likely exists a sizable subset of patients with critical aortic stenosis that are never referred for surgical evaluation. Reasons identified for nonreferral include an apparent lack of patient symptoms, a wish to avoid major surgery on the part of the patient, or a perceived prohibitive operative risk by the referring cardiologist. Balloon valvuloplasty has been used selectively in some patients considered nonoperative candidates. Valvuloplasty achieves an increase in aortic valve area by the fracturing of calcific deposits, separation of commissural fusion, and stretching of the aortic root. Unfortunately, widespread adoption of this technique remains low because of a high return of symptoms and restenosis within months of the procedure [1–3].

Recent advances in the field of aortic valve replacement have focused on avoiding a sternotomy and minimizing the incision size required to reach the valve. Comparative reports have demonstrated equivalent perioperative outcomes with corresponding reduced length of hospital stay using these minimally invasive techniques. Unfortunately, the greatest source of potential complications in a high-risk population, ie, the use of extracorporeal circulation with cardioplegic arrest, remains unchanged.

Advances in the integration of bioprosthetic valve technology and balloon-expandable stainless steel stents have made intervention of the nonoperative patient with severe aortic stenosis feasible [4]. Once successfully delivered, the stent valves have demonstrated a significant reduction in transvalvular gradients [5]. Additionally, valve fixation in the annulus has been stable as evidenced by no reports of migration or embolization of the devices. Unfortunately, actual delivery of the device to the aortic annulus has been extremely problematic. Retrograde delivery of this prosthesis has been difficult because of the obligatory size of the delivery system and anatomic factors such as the diameter of the patients' peripheral arterial tree and concomitant occlusive disease. Thus, the majority of the procedures to date have been performed antegrade using a challenging transseptal approach to the aortic valve. Although possible, this approach places a premium on the individual practitioners' experience and skill level with transseptal puncture and may not be widely applicable to the average operator.

Previous reports have described the use of the left ventricular apex as a reproducible route for minimally invasively accessing the aortic annulus [6]. We performed a series of animal studies validating this approach as a valuable technique for facilitating the placement of a catheter-deliverable aortic stent valve in the aortic annulus.

	Material and Methods

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Experimental Group and Protocol
Transapical aortic valve implantation of twenty-six 23-mm Cribier-Edwards Aortic Bioprosthesis (Edwards Lifesciences, Irvine, CA; Fig 1) equine valves mounted on a stainless steel stent was performed in twenty-four 35- to 46-kg juvenile swine. Ten procedures were performed at the Edwards Lifesciences Biological Resource Center, (Irvine, CA), and 14 animals were operated on in the experimental animal laboratory at the Heart Center, Leipzig, Germany. All animals received humane care in compliance with the NIH "Guide for the Care and Use of Laboratory Animals" (revised 1996). All components of the transapical animal work that were performed at the Edwards Lifesciences facility in Irvine were conducted in an American Association for Accreditation of Laboratory Animal Care–accredited facility and reviewed by the Edwards Institutional Animal Care and Use Committee. Experiments in Leipzig had been approved by the local government offices.

View larger version (141K): [in this window] [in a new window]   Fig 1. Cribier-Edwards Aortic Bioprosthesis Model 9000.

View larger version (107K): [in this window] [in a new window]   Fig 2. Fully crimped valve on delivery catheter.

The apex of the heart was exposed either through a ministernotomy that extended from the subxiphoid notch cranially for two rib spaces or a full sternotomy to facilitate epiaortic ultrasound. The pericardium was incised and tacked to the chest wall. A double pursestring suture of 3-0 polypropylene was placed in the apex of the heart to provide hemostasis. The animals were then anticoagulated with 300 IU/kg of heparin. Activated clotting times were measured every 15 to 20 minutes to maintain adequate anticoagulation. Cineangiography of the aortic root with a calibrated catheter was then performed to size the aortic annulus. Confirmation of the measured annular size was achieved by echocardiography. The animals were selected by weight to provide an aortic annulus of between 15 and 21 mm in size. Landmarks from the cineangiogram were used to identify the noncalcified porcine annulus. Additionally, small puffs of contrast were given during implantation to locate the annulus for deployment.

The left ventricle was accessed with an 18-gauge needle through the pursestring sutures. A 5F introducer sheath was inserted over a small guide wire into the left ventricle and secured with snares. This introducer was then used to place a 0.035-inch superstiff guidewire across the aortic valve and down the descending thoracic aorta (Fig 3). Once the stiff wire was positioned, the small introducer sheath was then removed. A cruciate incision was then made in the apex, and the 24F introducer sheath was placed into the left ventricle below the aortic valve under fluoroscopic guidance (Fig 4). The crimped valve on the delivery catheter was then introduced through the delivery sheath and into the left ventricle. The catheter has two radiopaque markers to identify the margins of the deployment balloon. Cardiac output was then reduced, either by initiating cardiopulmonary bypass or by instituting rapid ventricular pacing. Once transaortic valve flow had been reduced, as confirmed by loss of systolic pressure spike on arterial monitoring, the valve was positioned so that the annulus bisected the stent. Small boluses of contrast were given to aid in identifying the noncalcified porcine annulus and coronary ostia. Once positioned, the valve was deployed by inflating the delivery balloon with saline mixed with contrast to achieve full expansion of the stent (Fig 5). Once deployed, the balloon was deflated and rapid ventricular pacing discontinued. The stent-valve sits within the confines of the native porcine annulus and leaflets. The delivery catheter, sheath, and guidewire are then completely removed. Pigs on cardiopulmonary bypass are then weaned from extracorporeal support. A completion cineangiogram was then performed to assess for paravalvular leak (Fig 6) and positioning. Additionally, echocardiography was used to evaluate valve function, leaflet motion, and regurgitation. The animals were then sacrificed, and an examination of the heart was performed to evaluate the positioning of the valve and to identify any damage to the annulus or adjacent structures.

View larger version (55K): [in this window] [in a new window]   Fig 3. (A) Illustration of guidewire crossing the left ventricular cavity and into the descending thoracic aorta. (B) Fluoroscopic image of guidewire crossing the left ventricular cavity and into the descending thoracic aorta.

View larger version (59K): [in this window] [in a new window]   Fig 4. (A) Illustration of valve on the delivery catheter crossing the aortic annulus. (B) Fluoroscopic image of valve on the delivery catheter crossing the aortic annulus.

View larger version (115K): [in this window] [in a new window]   Fig 5. Fluoroscopic image of valve during deployment.

View larger version (126K): [in this window] [in a new window]   Fig 6. Completion angiogram after valve deployment.

	Results

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Paravalvular leak or aortic regurgitation was noted in 14 of the 18 (77.8%) valves that remained intraannular and did not migrate. The degree of regurgitation was visually estimated from the completion aortogram performed after implantation and retention of the valve in the annulus. A scale was derived in which mild regurgitation was denoted as 1+, moderate 2+, moderate to severe 3+, and wide open reflux of contrast into the ventricle as 4+. The mean degree of regurgitation was 1.8 ± 1.4. Only two valves showed severe (4+) regurgitation secondary to nonfunction of one of the leaflets that became trapped by a fold of aorta at the sinotubular junction. This occurred in 2 animals with smaller than average ascending aortas. The remaining animals exhibited lesser degrees of paravalvular leak across the annulus into the left ventricle on completion angiogram. The leak primarily appeared to be caused by regurgitation of blood back through the stent interstices, thereby going around the leaflets in an area not covered by cloth. Central leak through the valve was not noted in the majority of animals.

	Comment

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Percutaneous implantation of a stent type aortic valve became a clinical reality with the first reported successful human case by Cribier and colleagues in 2002 [4]. Subsequently, intense interest has formed toward the development of a catheter-delivered valve for use in patients with critical aortic stenosis declined for surgery. These first-generation devices have now been implanted in selected patients worldwide. As with any new device, significant questions regarding patient selection, implantation technique, potential for valve migration, long-term affect of paravalvular regurgitation, valve durability, and what constitutes an acceptable result remain to be answered.

This study was initiated to validate the transapical technique as a viable alternative to either an antegrade transseptal or a retrograde approach to the aortic valve. The majority of cases documented in the literature have been performed antegrade, in which the valve travels over a superstiff guidewire across the atrial septum, through the mitral valve, and ultimately across the aortic valve [7]. The wire by necessity forms a loop in the left ventricle so that the guidewire becomes coaxial with the ventricular outflow tract and the valve is not angulated across the annulus. Dramatic hemodynamic deterioration has been documented when the loop of wire in the ventricle becomes too large and places traction on the anterior leaflet of the mitral valve, causing severe mitral regurgitation [7]. Another limitation of the transseptal technique is that the operator has reduced fine motor control over the valve as the intervening loop provides significant "play" in the system. Most of the described morbidity and mortality of the procedure can be traced to the technical difficulty of this challenging approach. Likewise, the retrograde approach can be problematic because of the obligatory size of the 24F delivery sheath. Many of these elderly patients have coexisting peripheral vascular disease that precludes passing large size sheaths or catheters from the groin, around the arch of the thoracic aorta, and across the aortic valve annulus. Moreover, this technique also runs the risk of embolizing atherosclerotic material from the aorta into the distal circulation. However, in patients with adequate vessel size and without peripheral vascular disease, a retrograde approach would be the easiest and most direct of the percutaneous routes. We believe that the transapical approach provides a reliable alternative to either of these techniques. The distance of the aortic valve from the left ventricular apex is straight and relatively short, which provides good control over the delivery catheter. Additionally, in this animal series, control over the placement of the valve was believed to be optimal, and all valves were deployed at the intended target site.

Valve migration after deployment was seen in eight valves for clearly identifiable reasons. The two distal embolizations were in animals in which no attempt was made to decrease cardiac output during deployment. It became quickly obvious that with maintained blood flow across the aortic valve, the deployment balloon acts like a sail and carries the valve distally into the ascending aorta. Once measures such as cardiopulmonary bypass or rapid ventricular pacing were instituted to decrease cardiac output during deployment, no further distal embolizations were noted. Likewise, the six episodes in which the valve migrated into the ventricle could primarily be attributed to the animal model. On one occasion, early in the experience, a valve was crimped too tightly to the delivery catheter, so that on deployment the valve stuck to the balloon and was pulled back into the ventricle on removal of the catheter. The rest of the valve migrations were thought to be caused by the fact that the annular model was anatomically normal and not calcified. The primary fixation of the valve within the annulus is predicated on the resistance of a calcified annulus opposing the radial expansion forces of the valve. Additionally, friction between the stent interstices and the irregular surface of a calcified aortic annulus helps to anchor the valve. The juvenile swine annulus is highly elastic in nature and has a tendency to "watermelon seed" the valve back into the ventricular cavity when afterload is applied to the valve. Several times during these experiments, well-placed valves dislodged back into the ventricular cavity once cardiopulmonary bypass or rapid ventricular pacing was discontinued and afterload increased. Currently, there are no reports in the literature of valves embolizing either distally into the ascending aorta or proximately back into the ventricle once accurately placed in the annulus.

In the limited number of patients reported, particulate embolization leading to stroke has also not been noted to be a significant risk with this procedure; it does, however, remain a theoretical possibility. Assuming a stroke rate of 1% to 4% as reported in the balloon valvuloplasty literature [2, 8], the use of filters or other embolic protection devices may be a useful adjunct to the procedure.

In most reported series, paravalvular regurgitation is noted in nearly all patients [9, 10]. The stainless steel skeleton of this stent expands radially to become a perfect sphere with little deformability. The radial strength is an asset to the device in that it provides tremendous strength to force and hold open tightly stenotic valves. Unfortunately, it also contributes to paravalvular leak in that it cannot conform to the irregular annulus formed by calcific aortic stenosis. Areas of poor coaptation between the stent and calcium nodules within the annulus provide points for blood to leak back across the annulus and into the ventricle. The long-term significance of such leaks, provided they are not severe and do not lead to hemolysis, remains to be examined. Theoretically, exchanging critical aortic stenosis and obstructive physiology for mild to moderate regurgitation with minimal transvalvular gradient could be well tolerated for many years, and may never be an issue as current percutaneous candidates have such severe comorbidities that overall life expectancy is severely reduced.

Paravalvular leak was seen in 14 of the 18 valves that remained in situ after deployment in the aortic annulus. Again, the animal model was identified as the primary cause of the observed regurgitation. The Cribier-Edwards Aortic Bioprosthesis, as designed for the human aortic annulus, is 14 mm in height, and has a cloth covering the proximal 6 mm of the valve. The cloth-covered portion of the valve must sit within the aortic annulus to prevent blood from flowing back into the left ventricle by way of the stent interstices in an uncovered area. Swine have a short annulocoronary distance of approximately 5 to 7 mm, which obligates that the stents be placed relatively low to avoid coronary obstruction. Three episodes of coronary obstruction directly contributed to the demise of the animals from myocardial ischemia shortly after valve deployment. Low placement resulted in the covered portion of the stent being below the annulus in the majority of animals, thereby resulting in frequent trans-stent regurgitation. Additionally, the sinotubular junction diameter in several animals was smaller than the aortic annulus. On two occasions in animals with narrow sinotubular junctions, the aortic wall invaginated over the edge of the stent and trapped one of the leaflets, resulting in severe central insufficiency. In the animals in which the cloth-covered portion was within the annulus, no paravalvular or central leak was identified. Finally, coronary obstruction from the valve has not been shown to be a problem in the clinical cases reported in the literature.

In vivo valve durability remains unknown. Accelerated wear testing in bench-top pulse duplicators demonstrates acceptable durability up to 200 million cycles (5 years) [9]. What effect crimping the valve onto the delivery catheter has on the leaflet tissues and the longevity of the valve remains to be seen. However, the fact that these first-generation devices are unlikely to have the same durability as current surgically implanted valves should be used in selecting appropriate candidates for the procedure.

That being said, identifying appropriate patients for percutaneous valve implantation remains difficult. To date, patients having undergone percutaneous valve implantation have all been refused conventional aortic valve replacement because of either the presence of severe comorbidities or hemodynamic instability. Until this technology is scientifically validated by prospective trials, good-risk patients, as determined by surgeons, should undergo conventional valve replacement and not be offered percutaneous therapy. Patients turned down for standard valve replacement for contraindications to the procedure (eg, porcelain ascending aorta) or those believed to have an excessive operative risk, using an accepted risk-scoring algorithm, could be considered candidates for inclusion into early feasibility trials. The construction of larger pivotal studies remains challenging in the sense of what current therapy do you compare this technology with for safety and efficacy. Medical therapy and balloon valvuloplasty have notoriously poor long-term outcomes in patients with critical aortic stenosis. Furthermore, is it ethical to randomize patients who might be considered candidates for conventional surgery to a device with an unproven track record of durability and high rates of postimplant paravalvular regurgitation? These and other questions remain to be answered as this technology nears the point of large multi-institutional studies.

In summary, despite the limitations of the animal model, we believe that the transapical approach for implantation of a stented aortic valve provides a reliable method for prosthesis delivery. Furthermore, this approach obviates the risk and complications associated with the extremely challenging transseptal technique, and provides an alternative to a retrograde delivery should patients have severe coexisting peripheral vascular disease. Other approaches to the aortic annulus such as retrograde from the left subclavian artery may also offer advantages over peripheral techniques, and have been successfully reported in small animal series [11]. Valve performance, in regards to proclivity for migration and propensity for paravalvular leak, is more reliably gauged by the reported clinical experience than in the noncalcified animal annulus.

	The Society of Thoracic Surgeons Policy Action Center

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The Society of Thoracic Surgeons (STS) is pleased to announce a new member benefit—the STS Policy Action Center, a website that allows STS members to participate in change in Washington, DC. This easy, interactive, hassle-free site allows members to:

• Personally contact legislators with one's input on key issues relevant to cardiothoracic surgery

• Write and send an editorial opinion to one's local media

• E-mail senators and representatives about upcoming medical liability reform legislation

• Track congressional campaigns in one's district—and become involved

• Research the proposed policies that help—or hurt— one's practice

• Take action on behalf of cardiothoracic surgery

This website is now available at www.sts.org/takeaction.

	Acknowledgments

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The authors wish to acknowledge the research support of Edwards Lifesciences, including providing research funding and supplying the tested valves. Additionally, Jane Olin, DVM, Petra Böske, DVM, Cris Ullmann, PhD, and Fabian Emrich also provided excellent technical support during the procedures.

	References

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1. Block PC, Palacios IF. Clinical and hemodynamic follow-up after percutaneous aortic valvuloplasty in the elderly Am J Cardiol 1988;62:760-763.[Medline]
2. Serruys PW, Luijten HE, Beatt KJ, et al. Percutaneous balloon valvuloplasty for calcific aortic stenosisA treatment ‘sine cure'?. Eur Heart J 1988;9:782-794.[Abstract/Free Full Text]
3. Lieberman EB, Bashore TM, Hermiller JB, et al. Balloon aortic valvuloplasty in adultsfailure of procedure to improve long-term survival. J Am Coll Cardiol 1995;26:1522-1528.[Abstract]
4. Cribier A, Eltchaninoff H, Bash A, et al. Percutaneous transcatheter implantation of an aortic valve prosthesis for calcific aortic stenosis. First human case description Circulation 2002;106:3006-3008.[Abstract/Free Full Text]
5. Bauer F, Eltchaninoff H, Tron C, et al. Acute improvement in global and regional left ventricular systolic function after percutaneous heart valve implantation in patients with symptomatic aortic stenosis Circulation 2004;110:1473-1476.[Abstract/Free Full Text]
6. Huber CH, Cohn LH, Von Segesser LK. Direct access valve replacement. A novel approach for off-pump valve implantation using valved stents J Am Coll Cardiol 2005;46:366-370.[Abstract/Free Full Text]
7. Eltchaninoff H, Tron C, Cribier A. Percutaneous implantation of aortic valve prosthesis in patients with calcific aortic stenosistechnical aspects. J Interv Cardiol 2003;16:515-521.[Medline]
8. Dorros G, Lewin RF, Stertzer SH, et al. Percutaneous transluminal aortic valvuloplastythe acute outcome and follow-up of 149 patients who underwent the double balloon technique. Eur Heart J 1990;11:429-440.[Abstract/Free Full Text]
9. Cribier A, Eltchaninoff H, Tron C, et al. Early experience with percutaneous transcatheter implantation of heart valve prosthesis for the treatment of end-stage inoperable patients with calcific aortic stenosis J Am Coll Cardiol 2004;43:698-703.[Abstract/Free Full Text]
10. Hanzel GS, Harrity PJ, Schreiber TL, O'Neill WW. Retrograde percutaneous aortic valve implantation for critical aortic stenosis Catheter Cardiovasc Interv 2005;64:322-326.[Medline]
11. Ferrari M, Figulla HR, Schlosser M, et al. Transarterial aortic valve replacement with a self expanding stent in pigs Heart 2004;90:1326-1331.[Abstract/Free Full Text]

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Todd M. Dewey Thomas Walther Mirko Doss David Brown William H. Ryan Lars Svensson Tomislav Mihaljevic Gerhard Schuler Gerhard Wimmer-Greinecker Friedrich W. Mohr Michael J. Mack Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Dewey, T. M. Articles by Mack, M. J. Search for Related Content PubMed PubMed Citation Articles by Dewey, T. M. Articles by Mack, M. J. Related Collections Valve disease