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Ann Thorac Surg 2004;78:2199-2206
© 2004 The Society of Thoracic Surgeons

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Review

Percutaneous Valve Replacement: Current State and Future Prospects

^a Department of Cardiovascular Surgery, University of Kiel, Kiel, Germany
^b Department of Internal Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland
^c Department of Pediatric Cardiology, Great Ormond Street Hospital, University of London, London, Great Britain

^* Address reprint requests to Dr Lutter, Department of Cardiovascular Surgery, Christian-Albrechts-University Kiel, School of Medicine, Arnold-Heller-Strasse 7, Kiel, D-24105 Kiel, Germany (E-mail: lutter{at}kielheart.uni-kiel.de).

	Abstract

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

 
Percutaneous valve implantation is the development of a foldable heart valve that can be mounted on an expandable stent delivered percutaneously through standard catheter-based techniques and implanted within a diseased valve annulus. In cases with severe aortic stenosis, the diseased valve has to be pre-dilated. To perform a true replacement the diseased valve has to be ablated and removed. In this article, we review the development of percutaneous valve replacement technology and discuss future prospects in this field.

	Introduction

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

 
The field of interventional cardiology is one of the most rapidly evolving and vibrant areas in today's medicine. Although surgical treatment of diseased heart valves has proven to be the ultimate curative approach for these patients, recent reports indicate that endovascular procedures may provide an alternative to open heart operations [1–4].

Percutaneous valve implantation is the development of a foldable heart valve that can be mounted on an expandable stent [1–4], delivered percutaneously through standard catheter-based techniques [5, 6] and implanted within a diseased valve annulus [3, 4]. In cases with severe aortic stenosis the diseased valve has to be pre-dilated [3], although to perform a true replacement, the diseased valve has to be ablated and removed [4]. It is imperative that such an implant has a fixed and stable intraluminary position, that it provides adequate hemodynamics [4], and in the case of an aortic valve, that it does not compromise coronary flow [1, 2, 4]. This novel concept has evolved by extensive trial and error and has been tested in several animal models with success [1, 2, 4–6]. Furthermore, recent reports of percutaneous implantation of pulmonary valves in pediatric patients [7, 8], as well as aortic valves in adults [3], indicate that percutaneous valve implantation may become an effective and versatile procedure, which could benefit a large patient population [1, 4, 9, 10]. In this article, we review the development of percutaneous valve replacement technology and discuss future prospects in this field.

In this review, books, journal articles, reviews, and meeting abstracts reporting on experimental and clinical work were analyzed and are to some extent included. The literature acquisition was performed in the 1966 through May 2004 database of Medline and the "all years" database from the Web of Science. The following keywords were used in the searches (both in American and Oxford English): percutaneous valve replacement, transluminal valve replacement, transcatheter valve replacement, percutaneous valve implantation, percutaneous valve insertion, transluminal valve implantation, transcatheter heart valve implantation, percutaneous balloon valvuloplasty, transluminal balloon valvuloplasty, percutaneous balloon dilatation, valved stent, expandable stent-valve, percutaneous heart valve, endovascular stent graft, prosthetic heart valve, prosthesis, heart valve disease, valves, stents, prosthesis, filters, catheterization, catheter-based procedures, catheter-mediated treatment, catheter-based intervention, interventional procedures, percutaneous heart valve excision, and percutaneous heart valve ablation.

	Evolution of Treatment Options for Valvular Heart Disease

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

 
The evolution of percutaneous aortic valve implantation parallels that of its surgical development. In 1951, Hufnagel and colleagues [11] surgically implanted a prosthetic ball valve device in the proximal descending aorta of patients with aortic insufficiency and prevented regurgitation into the left ventricle. In 1992, 40 years later, similar attempts were made by two separate groups to implant prosthetic heart valves transluminally in closed chest animals. Since then several groups have made significant contributions to this development (Table 1). In 1992, Andersen and coworkers [1] demonstrated that artificial aortic valves could be implanted in closed-chest pigs by means of a transluminal catheter technique for the first time. The expandable aortic valve was mounted on a balloon catheter and was inserted retrogradely into the descending, ascending, or the aortic root in anesthetized pigs. Although implantation in the ascending aorta revealed adequate hemodynamics across the valve, subcoronary implantation resulted in restriction of the coronary blood flow. That same year, Pavcnik and colleagues [5] showed that a percutaneous transcatheter placement of an artificial caged-ball valve in dogs is feasible. However, a major difficulty associated with this design was the secured positioning of the cage and the ball. In all tested animals, the assembly failed to remain stable in a dynamic environment, leading to the escape of the ball through the top of the cage.

Despite efforts to minimize the invasiveness of operations, a large number of risks associated with such surgical procedures still exist. An approach that would offer a similar or superior outcome to the conventional surgical technique only involves minimal intervention without general anesthesia and cosmetic scarring, and lower overall cost has been the basis for the emergence of percutaneous valve replacement.

	Percutaneous Valve Replacement: Novel Ideas Turned Into Reality

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

 
Until 1982 the traditional method of treatment for congenital pulmonary stenosis was surgical valvotomy, when Kan and colleagues [17] first reported successful application of percutaneous balloon valvuloplasty. The results of the procedure have been so successful that in recent years it has largely replaced surgical valvotomy except in patients with dysplastic valves [17, 18]. Percutaneous valvuloplasty has now become the routine and effective procedure of choice for congenital stenotic valves [19, 20], which has been widely performed in the pediatric population with aortic or pulmonary valve stenosis [9, 21]. Percutaneous transseptal mitral commissurotomy has also supplanted surgical commissurotomy for most patients with rheumatic mitral stenosis and has become an excellent therapy option for many who previously had none [22].

Considering the limitations associated with an interventional approach to valvular heart disease in adults, valvuloplasty remains a viable alternative to surgery only in a subgroup of patients [22]. Those with noncalcified mitral stenosis benefit from temporary symptomatic relief, whereas in others it is only accepted as a palliative procedure or as a bridge to valve surgery in critically ill patients with advanced aortic stenosis [3, 23].

Furthermore, recent percutaneous approaches to mitral valve repair include placement of a mitral annular constraint device in the coronary sinus to reduce mitral regurgitation in an experimental model of dilated cardiomyopathy [18, 24]. In addition, St. Goar and coworkers [25] are successfully working on percutaneous mitral valve intervention, consisting of an expandable capture device for grasping and positioning the mitral leaflets, and a fixation instrument that clips both leaflets together (percutaneous edge-to-edge operation).

In the late 90s, one of us began a series of animal studies in pursuit of successful percutaneous valve replacement [2]. The system was comprised of glutaraldehyde-treated, bovine, venous jugular valves that were sewn into a balloon expandable stent that was delivered percutaneously to the pulmonary artery of lambs (Fig 1). The valved stent was deployed in the native pulmonary valve to affect functioning of the native valve and fix the device to the pulmonary wall. Transcatheter implantation of the valved stent was successful, and the long-term study in lambs revealed stable positioning of the stent with competent valves at the end of 2 months [2]. The same stent design was later successfully implanted in the descending aorta in a group of lambs with traumatically-created massive aortic insufficiency [6]. This valve was able to normalize pressures in the distal descending aorta and remained competent. However, all experiments failed when the valved stent was positioned into the native annulus as its walls either obstructed the coronaries or interfered with the function of the mitral valve.

View larger version (164K): [in this window] [in a new window]   Fig 1. One of Bonhoeffers' first trileaflet bovine jugular venous valved stents which was used for percutaneous pulmonary implantation.

In 2000, the same group reported two studies describing the first successful percutaneous pulmonary valve replacements in man [7, 8] in which 8 patients with a failing artificial pulmonary outflow tract received bovine jugular-vein valve (Venpro Corp, Irvine, CA) fitted in a platinum stent (Numed Inc, Hopkinton, NY). The current clinical experience of 56 patients (median age, 16 years, range, 9 to 41), including this early series, confirms a 98% freedom from pulmonary regurgitation at median follow-up of 4 months (range, 2 weeks to 36 months) with zero mortality (Fig 2). Procedural complications requiring emergency surgery have occurred in 3 patients, 2 due to embolization of the valved stent and 1 due to homograft rupture (personal communication). The predominant indication remains homograft–conduit stenosis with or without valvular regurgitation due to limitation of device size. The recent development of an infundibular reducing device in animals [26] is a promising advance for those patients with dilated right ventricular outflow tracts who are currently unsuitable for this technology.

View larger version (178K): [in this window] [in a new window]   Fig 2. Pulmonary angiogram of valvular competence of a newly implanted valve after percutaneous delivery in the last series of Bonhoeffer and colleagues.

View larger version (59K): [in this window] [in a new window]   Fig 3. (A, B) View of the homologous valved segment in a vascular memory stent. Note that the anchoring barbs at the outside of the valved stent ensure the desired position after embedding. (Lutter and colleagues.)

View larger version (83K): [in this window] [in a new window]   Fig 4. First human implantation of a valved stent within the native calcific valve. (Left) Maximal balloon inflation for valve delivery (Reprinted from Cribier A, et al. Circulation 2002;106:3006-8 [3], with permission.) (Middle) The percutaneous heart valve (PHV) in position at mid part of the native aortic valve. (Right) Angiogram showing no aortic regurgitation across the PHV and a mild paravalvular regurgitation (arrow). (LCA = left coronary artery; RCA = right coronary artery.)

An additional 11 patients with end-stage aortic stenosis received percutaneous aortic pre-dilation and aortic implantation by the same group (Table 2). The valved stents were introduced percutaneously into the right femoral vein (or more recently inside the femoral artery) through a 24-French introducer under local anesthesia and mild sedation. Cribier and coworkers (personal communication) have reported nine successful cases, and two technical failures. In one case, an attempt to implant a valve in a patient with aortic stenosis associated with massive aortic regurgitation resulted in migration of the valve assembly during the delivery, due to the ruptured native valve postballoon valvuloplasty. In another case, the group was unable to cross the aortic valve while attempting for the first time a valve implantation through the retrograde arterial approach; the delivery catheter was too short at a length of 100 cm.

These preliminary studies suggest that implantation of the percutaneous valved stent in the aortic position can be achieved in patients with end-stage calcific aortic stenosis and may become an important therapeutic option for patients not suitable for surgical valve replacement [28].

Therefore the field of catheter-based techniques for valve replacement has seen much improvement in the last decade. Initially considered only a novel idea, it has been critically investigated by a number of groups, and we have recently witnessed its application in human subjects [3, 7, 8, 28]. This approach may open a new chapter in the treatment of valvular heart disease. As the population ages and valvular heart disease becomes more common, a less invasive treatment option in such a critically-ill patient population may reduce morbidity and improve overall results. However the current model is far from being ideal and there are a number of problems to address, especially in the human aortic valve implantation series.

	Limitations and Technical Difficulties

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

 
A number of technical difficulties were encountered in the early phases of percutaneous valve replacement [1], which to varying degrees still exists. These difficulties include optimal attachment of the valve into the stent, preservation of the function of the valved stent after compression and re-expansion, a suitable visualization method, functional anchoring mechanism, and avoidance of paravalvular regurgitation and obstruction of coronary orifices in aortic implantation [1, 2, 4, 6].

From the different valve designs used experimentally and clinically (see Table 1), a biological valve sutured to an expandable stent (self-expandable, see Fig 3, or balloon expandable, see Figs 1, 2, and 4) has been used frequently in the reported studies. Other types of valves, including synthetic polymer-based material, may also provide alternatives, but they carry the risk of hemo-incompatibility and bio-incompatibility, as well as uncertainty about their durability [29]. The biological valves presently used have a collapsible valved-stent that can be crimped onto a carrier catheter and inserted through the vascular access. However the current designs require large vascular access and delivery systems for deployment [28, 30]. This could potentially limit the application of this system especially in the pediatric population in which a large vascular access is difficult to obtain [8, 9].

Furthermore it is important to determine the most appropriate site for vascular access. Several previously published studies utilized the carotid artery as the entrance site for the guidewire and the catheter [5, 6]. In a clinical setting of a patient with aortic stenosis who is at high risk for carotid plaque rupture and further embolization upon intraluminal manipulation, efforts must be made to use the femoral vein [2, 28], artery [1, 4], and jugular vein as the standard vascular access points.

The stable positioning of the stent onto the catheter is not only important for safe and accurate deployment, but also to avoid movement during catheter advancement [4]. Once the stent is deployed into the desired position, it is essential that it be anchored in position to avoid migration or dislodgement [4, 6, 28]. Stents with a diameter larger than that of the vessel wall tend to anchor to the wall tightly, but in an area of high flow and shear stress such as the aortic root, a more secure fixation to the surrounding friable tissue in patients with aortic insufficiency is desired. Whereas the anchoring barbs attached to the outside of the stent may attach to the tissue [4], a more ideal approach would include a sutureless technique for securing the valved-stent into the desired position. Self-closing clips and sutureless techniques are gaining popularity in some surgical procedures [15, 16], and a similar approach for fixing a valved-stent in aortic anulus, for instance, can also be applied in percutaneous procedures.

Another major obstacle faced by many investigators has been the unfeasibility of the aortic valved stent implantation into the aortic anulus [1, 4, 6, 27, 31]. The close proximity of the coronary ostia to the aortic valve compromises coronary perfusion when the valved stent is implanted in the orthotopic position. Coronary flow restriction can occur either by direct blocking from the implanted stent, or from the native leaflets immobilized against the coronary ostia [4, 6, 30, 31]. Whereas the majority of previous experiments included valve implantation in the heterotopic position (ascending or descending aorta, see Table 1), Bonhoeffer and colleagues [6] reported a novel technique in sheep that allows precise transannular placement of a two-layer stent that would capture and shelter the native aortic leaflet at the time of implantation. However, in a more realistic model of a stenotic and calcific aortic valve, placement of a valved stent in the native position is technically very difficult.

Even more demanding is positioning a valved stent in a noncalcified annulus surrounded by friable tissue without obstructing the coronary ostia or causing significant paravalvular regurgitation. The stable positioning and anchoring within the annulus without generation of calcific lesions that could cause distal embolization is no longer a challenge; Cribier and coworkers [28] presented acute and early follow-up results of the only series of human implantations in patients with calcific aortic stenosis in 2004. They successfully showed that a bioprosthetic valve can be implanted percutaneously within a diseased stenotic aortic valve without obstructing the coronary arteries. At early follow up, the function of the implanted valves remained normal and the mild or severe paravalvular aortic regurgitation was stable.

A novel and challenging approach to overcome the previously described problems is in situ removal and ablation of the native aortic valve by transluminal techniques (Fig 5). This would include ablating the diseased valve in a nonbeating or beating modus and replacing it with the valved stent. Such a catheter-based system would provide a space between the mitral valve and the ascending aorta (aortic valve ablation chamber), in which the native valve can be removed while preventing embolization of remnant particles by a filtering mechanism. Preliminary in vitro studies demonstrated the possibility of ablating human calcified aortic valves with different types of lasers [4].

View larger version (94K): [in this window] [in a new window]   Fig 5. An in vitro model of a scaffold (light blue) in the ascending aorta (acrylic tube) in which different ablation tools may be positioned and fixed for the ablation of a calcified aortic valve (yellow).

	Future Direction and Potential Clinical Applications

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

The current methods for substituting heart valves are burdened with considerable problems. These include the need for anticoagulation, susceptibility to infection, inability to grow, and auto repair [3, 8]. A number of strategies to design an ideal tissue-engineered valve are being pursued. One interesting approach is the use of biodegradable scaffolds configured to the shape of the valve, seeded with cells potentially derived from stem cells. The seeded cells proliferate, organize, and produce cellular and extracellular matrix, whereas the starter scaffold is resorbed [33, 34]. Others have designed decellularized allograft and xenograft valves that are either pre-seeded with autogenous cells, or are repopulated by adaptive remodeling in vivo [35]. The clinical evaluation of tissue engineered valves is currently in progress and the outcome holds promise for percutaneous intervention.

As we march toward the regular use of percutaneous valve replacement, the question remains as to who would be the first to benefit from such technology? Successful human studies have indicated that pediatric patients with pulmonary regurgitation or right ventricular outflow tract stenosis would greatly benefit from such a minimally invasive procedure [7, 8]. This technique could potentially substitute or reduce the number of surgical interventions normally performed to replace the malfunctioning conduits that provide continuity between the right ventricle and pulmonary artery. In addition to ameliorating aortic insufficiency, a valved stent could act as an aggressive treatment for aortic stenosis without the risk of incurring harmful regurgitation. Besides children, an ever growing number of older patients with calcific aortic stenosis could be appropriate candidates for this technique, once the technical difficulties have been addressed [3, 4]. At times due to comorbid risks, this very small group of patients is often not considered candidates for surgical intervention. Currently, balloon dilation of the aortic valve serves as a palliative measure for these patients [23, 32]. A nonsurgical technique to replace the stenotic aortic valve may prove to be ideal and a valuable source of therapeutic intervention for such candidates.

Percutaneous valve implantation, once considered only a novel idea, has become a reality. This nonsurgical approach has been proven to be feasible and hold promise, although many obstacles still exist [3, 4, 6, 8, 28]. The extent to which these techniques will become part of conventional therapies greatly depends on their ability to deliver improved clinical endpoints in human studies compared with the gold standard of surgical valve replacement. To date, we are encouraged by our preliminary results achieved in animal studies, and we are striving to validate these observations in both the pediatric and adult populations.

	Acknowledgments

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

 
Dr Lutter's project of percutaneous valve replacement is supported by the German Research Foundation (Grant LU 663/4-1, LU 663/4-2). Dr Bonhoeffer's percutaneous valve program is supported by the British Heart Foundation. The authors thank Wolfgang Menz, Institute for Microsystem Technology, Freiburg, Germany (Fig 5), and Dr Louise Coats, British Heart Foundation Fellow, Institute of Child Health, London, for their contributions to this article.

	References

Top Abstract Introduction Evolution of Treatment Options... Percutaneous Valve Replacement:... Limitations and Technical... Future Direction and Potential... Acknowledgments References

 

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