Davidson
M
Ji
,
Baim
D
Si
. Percutaneous Aortic Valve Interventions.
Cohn Lh, ed. Cardiac Surgery in the Adult. New York: McGraw-Hill, 2008:963-971.
Percutaneous Aortic Valve Interventions
Michael J. Davidson/
Donald S. Baim
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INTRODUCTION
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The number of patients with clinically significant aortic stenosis (AS) or regurgitation—2 to 3% of the elderly population with calcific AS, including 1 to 2% with congenital bicuspid aortic valve disease—is far greater than the 50,000 annual surgical aortic valve replacements would suggest. Some patients with aortic valve disease defer surgery in light of mild symptoms, whereas others are deemed too ill to undergo cardiac surgery. The latter currently are treated expectantly or by balloon aortic valvuloplasty (BAV), but this 20-year-old technique offers poor magnitude and durability of the physiologic improvement in aortic valve orifice area.
Recent technological advances, however, now indicate that catheter techniques similar to those used for BAV can be used for percutaneous aortic valve replacement, avoiding open cardiac access or the use of cardiopulmonary bypass. This new era is truly in its infancy, with most relevant devices still in preclinical testing or phase I human clinical evaluation. As the technology matures, though, it is almost certain to alter the treatment landscape for aortic valve intervention significantly. Implementation of this new technology, however, is going to require close ongoing collaboration among cardiac surgeons, cardiac anesthesiologists, and interventional cardiologists.
Given the outstanding current results of open surgical valve replacement, the principal indication for percutaneous aortic valve intervention probably will remain those patients with severe disease who are deemed inoperable because of comorbid conditions. The broader application of these new percutaneous technologies to good surgical candidates will demand prospective, randomized clinical trials with demonstration of similar prolonged safety and efficacy to surgical valve replacement.
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BALLOON AORTIC VALVULOPLASTY
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Following successful percutaneous dilation of pulmonary and mitral valves, the first adult BAV was performed by Alain Cribier in 1985.1 Early experience in the mid-1980s showed that BAV was safe, but it also showed that the technique provided a far smaller increment in aortic valve area (AVA) (i.e., from 0.6 to 0.9 cm2) than provided by surgical valve replacement.2,3 The basic technique has not changed significantly since this time, but the procedure has been aided by advances in guidewire and balloon design, as well as newer imaging modalities such as transesophageal and intracardiac echocardiography. The classic retrograde approach has been used most commonly but may present difficulty with crossing a severely stenotic valve or complications caused at the arterial entry site by insertion of large-caliber devices. The antegrade approach is a more recent alternative, in which left atrial access is obtained via the femoral vein using standard transseptal puncture, following which a balloon flotation catheter and guidewire are advanced through the mitral valve, to the left ventricular apex, and then through the aortic valve.
Once a 0.035 to 0.038 inch guidewire has been positioned appropriately using either approach, an 18 to 23 mm valvuloplasty balloon then is advanced over the wire and across the stenotic valve, and it is inflated with dilute contrast material (Fig. 40-1). Rapid ventricular pacing (i.e., 180 to 200 beats per minute) can be initiated to transiently lower cardiac output and allow balloon inflation without the risk of balloon migration. The mechanism of action initially was assumed to be reopening of fused valve commissures, but pathologic investigation reveals little commissural fusion, with the dominant mechanism being fracture of calcified nodules along the leaflets with elastic expansion of the aorta.4 While nodule fracture allows initially greater leaflet mobility, the effect is modest and relatively short lived as the aorta recoils and the nodule fractures go on to heal.
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Figure 40-1 Percutaneous balloon aortic valvuloplasty using an antegrade transseptal approach. Native aortic valve calcium is visible at level of balloon "waist."
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Early enthusiasm for the technique stemmed from its relative ease compared with surgical valve replacement, and some proponents hoped that BAV might supplant many elective valve replacements. On average, however, AVA increased from 0.6 to 0.9 cm2 and frequently reverted to severe stenosis over a matter of months. The overall 1-year survival was 65%, and the 1-year survival free of death, aortic valve replacement (AVR), or repeat BAV was 40%.5 In addition, given the selected patient population, hospital mortality may be as high as 14%, with one-third having periprocedural complications,3 including vascular access-site problems, arrhythmia, heart block, and stroke. At the present time, then, BAV is indicated only in select patients with severe AS and no surgical options. If stenosis recurs, repeat BAV can be performed.6 BAV also may be used as a bridge to AVR in hemodynamically unstable patients or in those requiring urgent noncardiac surgery.7
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VALVED STENTS
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The impetus for the development of percutaneous AVRs thus lies in the need for an intervention that is more durable than BAV and that can be used in patients who are too high risk for surgical valve replacement. As in the percutaneous treatment of coronary artery disease (CAD), where simple balloon dilation was replaced by stent implantation to resist vessel recoil, the basic concept of percutaneous valve replacement hinges on the use of an outer stentlike structure to resist the tendency of the aortic annulus and diseased native leaflets to recoil following BAV. In addition, that stentlike structure is used to support three internal leaflets that together constitute a functioning valvular prosthesis. The first known embodiment of a stent-mounted valve was the
Anderson valve, as described in 19927 (Fig. 40-2). While not initially optimized for orthotopic catheter-based valve replacement, the concept of a tissue valve mounted within a balloon-expandable stent formed the basis of subsequent percutaneous aortic valve intervention. Refinements in this concept ultimately led to the first human percutaneous aortic valve replacement, performed by Cribier in France in April 2002.8
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Figure 40-2 The Anderson valve consists of a trileaflet porcine valve mounted in a steel stent structure. The stent could be crimped onto a delivery balloon for percutaneous delivery. Initial use was in the heterotopic (i.e., Huffnagel) position.
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Two devices are currently in active clinical use for percutaneous AVR: the balloon-expandable Cribier-Edwards valve and the self-expanding Corevalve prosthesis. A number of other second-generation devices are in various stages of development. All, however, face a common set of technical challenges.
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PROCEDURAL AND DEVICE DESIGN CHALLENGES
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The first procedural challenge concerns vascular access. The valved stents are, by necessity, significantly larger than most existing percutaneous cardiac catheters and devices. The first-generation delivery catheters are on the order of 22F to 24F, requiring direct femoral or iliac arterial access via surgical exposure if a retrograde approach is used. While an antegrade transseptal approach may enable percutaneous femoral venous access with some, the large sheath size still may predispose to vascular injury. While second-generation devices have reduced the size of the delivery catheter below 20F (by using the lower-profile self-expanding platform and by modifying the valve leaflets), issues with aortic, iliac, and femoral anatomy still may limit the retrograde introduction of even these smaller devices. Other options include retrograde implantation via axillary artery cutdown or the descending thoracic aorta or antegrade introduction via minimally invasive or thoracoscopic left ventricular transapical puncture.10 Overcoming vascular access issues is likely to be a central concern as device development continues.
A second design challenge for these devices is the potential for interference with other cardiac structures. The combination of fluoroscopy and transesophageal echocardiography offers limited control of the delivery catheter and makes precise three-dimensional (3-D) positioning difficult. Only the position of the valve prosthesis above the native aortic annulus can be adjusted, with no control of the orientation of the prosthetic leaflets relative to the native commissures. Unlike the pulmonary valve, a prosthesis within the aortic annulus has the potential to impede coronary flow, impinge on anterior mitral leaflet mobility, and apply pressure against the atrioventricular conduction system in the upper ventricular septum.11,12 In current designs, avoiding interference with the coronary ostia relies on the calcified native aortic leaflets serving as "stand-off " from the coronary ostia; i.e., they preserve the space between the stent wall and the sinuses of Valsalva into which blood can flow from the ascending aorta to the coronary ostia. Rare cases have been described, however, where a large calcified nodule can block a coronary ostium or embolize calcium fragments into the coronary artery. Some stent architectures may extend to the tubular ascending aorta above the sinuses and potentially interfere with future catheter access to the coronary arteries and thus future catheter-based coronary artery interventions. Placement of the valve prosthesis too low in the left ventricular outflow tract (LVOT) can impede mitral leaflet mobility or cause heart block by impinging on the conduction system. Moreover, the first-generation devices are single-shot implants, with no possibility for recapture and repositioning, if needed. Second-generation designs, however, emphasize the ability to retrieve and reposition the device as needed during deployment. In addition, newer imaging modalities, including real-time computed tomography (CT) and magnetic resonance imaging (MRI) and 3-D echocardiography may aid in more accurate device placement.
The third challenge facing these devices is that of secure seating within the aortic annulus. First, the valve must be able to deploy accurately in the nonarrested heart. Initial approaches to this have included rapid ventricular pacing (at rates of up to 220 beats per minute) and the use of peripheral cardiopulmonary bypass to temporarily suspend left ventricular ejection. Each of these has potential pitfalls, and some second-generation devices thus are designed to allow stable seating of the valve prosthesis without interrupting cardiac output. Once the valve is placed, it must maintain a stable position within the annulus without risk of embolization. At this point, the valve is not retrievable, necessitating placement in a nonanatomic position (i.e., descending aorta) as a salvage maneuver. Moreover, the irregular surfaces of native calcified aortic leaflets may provide an advantage in serving as a "spacer" from the coronary ostia, but they also predispose to
perivalvular leak (Fig. 40-3). In this sense, percutaneous AVR differs from open surgical AVR, where complete decalcification is performed prior to valve insertion to ensure proper valve seating. Options to improve sealing include the use of larger-diameter devices (i.e., 26 mm rather than 23 mm) to overstretch the aortic root into a rounder configuration or use of static or dynamic seals on the outside of the support stent to block potential paths for perivalvular leakage.
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Figure 40-3 Postmortem specimen following placement of Cribier-Edwards valve. This patient suffered from severe perivalvular leak and resulting aortic regurgitation. The valve is positioned appropriately, but there is space present between the stent and the native valve leaflets.
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Finally, the durability of these devices must be investigated thoroughly prior to their widespread use. While the initial applications have been in nonsurgical candidates with limited life expectancy, these valves may find use in a broader population. The biomaterials used for leaflet construction often have limited prior clinical application; these materials include equine and porcine pericardium, bovine jugular vein, and tissue-engineered leaflets. Furthermore, the long-term effects of bioprosthetic leaflets striking a rigid metal frame have not been established. Experience with aortic root replacement suggests that accelerated leaflet degradation may occur when valve cusps repeatedly strike a foreign surface.13 In addition, if indications for these new devices move beyond nonsurgical candidates, the inevitable ingrowth of stent into aortic wall may preclude future simple surgical valve replacement and instead will necessitate full root replacement.
The initial clinical successes with these devices, however limited, have demonstrated that percutaneous AVR is possible and thereby have spawned an enormous professional and commercial impetus to overcome the challenges just described. The second-generation devices now in preclinical testing have begun to address many of the problems faced by early designs, and the pace of innovation in this arena is accelerating.
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BALLOON-EXPANDABLE VALVES
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The largest single-center experience to date has been reported from France by Alain Cribier using what is now termed the Cribier-Edwards 23 mm balloon-expandable aortic valve prosthesis9,14,15 (Edwards Lifesciences, Irvine, CA) (Fig. 40-4). The valve consists of three pericardial leaflets (originally equine, now bovine) sewn inside a stentlike stainless-steel structure. The valve is stored in the open position to avoid damage to the leaflets and must be hand-crimped onto a deployment balloon catheter just prior to insertion. The initial approach in the most recent series from France almost exclusively has been antegrade. A transseptal puncture is performed, and a stiff guidewire is passed via the left ventricle and aortic valve into the descending aorta, where it is externalized through a femoral arterial sheath using a snare. After initial antegrade BAV over this wire, the Cribier-Edwards valve is crimped onto a Numed delivery balloon and advanced antegrade until it lies at the level of the native aortic valve calcifications. During rapid ventricular pacing, the balloon is inflated and deflated rapidly, leaving the prosthesis deployed at the level of the aortic annulus (once the valve has been deployed, it may not be retrieved or repositioned). Injection of contrast material into the aortic root confirms valve competence (Fig. 40-5).
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Figure 40-4 The Cribier-Edwards valve consists of three pericardial leaflets sewn to a stainless-steel stent. The valve is stored in the open position to avoid damage to the leaflets (left panel) and must be hand-crimped to the delivery balloon (right panel) immediately prior to implantation.
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Figure 40-5 Fluoroscopic appearance of Cribier-Edwards valve placement. Stent is expanded by balloon at the level of the native aortic valve using calcification as a guide (left panel). Rapid ventricular pacing at 220 beats per minute transiently inhibits cardiac output to allow accurate valve placement. Aortic root injection after successful placement of the valve (right panel). Note nonobstructed flow to the coronary arteries and the presence of aortic insufficiency, suggesting perivalvular leak.
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Both the pilot phase of the clinical trial (termed I-REVIVE) and the subsequent trial (termed RECAST) have enrolled patients deemed not to be surgical candidates, having been turned down for valve replacement by two independent surgeons. Accordingly, this group of 40 patients had multiple comorbidities and had an average Euroscore of 13 and Parsonnet score of 47. Technical success was reported in 17 of the initial 20 patients (85%), with failures including two procedural deaths.16 The mean gradient fell from 43 to 8.5 mm Hg, with AVA rising from 0.56 to 1.69 cm2. This large effective orifice area is achieved by associated expansion of the aortic annulus and the absence of any struts or sewing ring so that it approaches the orifice of the best stentless surgical bioprosthetic valves. While no patients have experienced prosthetic valve dysfunction per se up to 26 months, moderate to severe perivalvular leak has been documented in one-half the patients.16
The North American experience with this approach, however, has been mixed. A small U.S. pilot study used an antegrade approach and the 23 mm Cribier-Edwards valve (analogous to that used in I-REVIVE and RECAST) but encountered a number of complications due to both the antegrade access and the limited valve size. Two cases of pericardial tamponade and two cases of injury to the anterior mitral leaflet caused periprocedural mortality, whereas several patients experienced significant perivalvular leak and valve migration. In contrast, a series of more than 20 insertions performed by Dr. John Webb in Vancouver, British Columbia, used the retrograde approach with a new, lower-profile deflectable delivery catheter that allows translation around the aortic arch and assists in crossing the stenotic valve. In addition, the Vancouver trial has used a larger 26 mm valve, which has reduced the number of perivalvular leaks. Complications have included two valve embolizations (treated by completing implantation in the descending aorta) and one case of fatal obstruction of the left main coronary artery owing to adverse positioning of a large calcified nodule during device expansion.17 The U.S. pilot trial has resumed (December 2005) using a similar approach. The total experience with the Cribier-Edwards bioprosthesis—a first-generation device—demonstrates that percutaneous transcatheter AVR is possible but illustrates the significant technical challenges described earlier.
Additional balloon-expandable valved stents have been developed but have not yet entered clinical use.18 Bonhoeffer and colleagues—the first to use valved stents successfully in humans—have modified the bovine jugular vein valve used in the pulmonary position for aortic use (Fig. 40-7C) and are in the stage of preclinical testing.11 Other balloonexpandable valve stents likely will enter preclinical testing, but design emphasis has shifted toward self-expanding stent design because of the potentially lower device profile.
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Figure 40-7 Percutaneous aortic valve replacement devices in preclinical development. (A) The Sadra self-expanding Lotus valve uses a nitinol frame that shortens as it is deployed, generating radial force for anchoring. (B) The AorTx valve uses a solid frame structure rather than traditional stent architecture to avoid damage to biologic leaflets. (C) The Bonhoeffer valve uses both self-expanding nitinol and balloon-expandable platinum-iridium components to achieve anchoring in the annulus. (D) The eNitinol thin membrane PercValve is a completely mechanical valve using nitinol leaflets that could become covered with native endothelium.
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SELF-EXPANDING VALVES
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A number of other devices have used a self-expanding frame rather than the balloon-expandable platform.19–25 Of these, the Corevalve revalving system has undergone the largest amount of clinical application. This system uses a nitinol cage housing a valve constructed of porcine pericardial leaflets (Fig. 40-6). The delivery sheath is 21F and has been used in a retrograde fashion from surgical exposure of either the iliac artery, femoral artery, or axillary artery. The patients enrolled in this trial, like those receiving the Cribier-Edwards valve, have been deemed nonsurgical candidates, with an average Euroscore-predicted mortality of 22%. The technique uses peripheral cardiopulmonary bypass in lieu of rapid ventricular pacing for stable device deployment. To date, at least 20 patients have undergone attempted Corevalve placement by Dr. Eberhard Grube. Of the initial 21 patients, 17 had acute procedural success, and 10 of these were discharged from the hospital without major adverse cardiovascular events. There were seven periprocedural deaths (33%) in the early patients but a 90% procedural success rate and 10% mortality in the most recent 10 patients.25
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Figure 40-6 The Corevalve system consists of pericardial leaflets attached to a self-expanding nitinol frame. In the deployed state. The flared distal end assists in anchoring in the ascending aorta. The stent covers the coronary ostia, but cell size is designed to allow later coronary catheterization.
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A number of other devices using the self-expanding frame concept are in various stages of development and preclinical testing but have yet to be implanted in humans (Fig 40-7). These valve designs attempt to overcome several of the limitations faced by earlier generation valves, including embolization, perivalvular leak, inaccurate positioning, and unproven durability. The Sadra Lotus valve (Fig. 40-7A) uses a unique nitinol wire frame whose intrinsic self-expansion can be enhanced by active shortening of the axial length to generate considerable radial force to aid in seating. This can be reversed to allow the valve to be retrieved and repositioned during deployment, and the device includes an outer dynamic sealing system to reduce perivalvular leak. The AorTx design (Fig. 40-7B) suspends pericardial leaflets in a convex triangular nitinol frame that unrolls into a cylindrical shape to anchor in the annulus. This design is also repositionable and retrievable and may confer a leaflet durability advantage because the valve cusps do not come in contact with the frame during valve opening. Another design also achieves this goal but avoids the use of a metal frame altogether; the Direct Flow Medical device uses an inflatable cylindrical support that anchors within the native aortic valve. Additional self-expanding valves intended for aortic use have been developed for a direct left ventricular transapical approach via left thoracotomy or thoracoscopy.10 While not currently adapted for catheter-based use, it is likely that these and other balloon-expandable valved stents will be modified for a percutaneous platform. Finally, while the vast majority of percutaneous AVR concepts have used biologic tissue for valve leaflets, some early preclinical attempts have been made to implant a mechanical prosthesis percutaneously.26
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OTHER CONCEPTS
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More radical designs include one by Palmaz and Bailey that uses nanoprocessing to create a valve consisting solely (including the leaflets) of nitinol (Fig. 40-7D). This might allow for rapid ingrowth of native endothelium and has the advantage of deliverability through a 10F sheath. A final percutaneous aortic valve intervention is that conceived at Corazon, which uses a chemical demineralization of the leaflets rather than valve replacement. This concept is to bathe the aortic valve in hydrochloric acid at a pH of 1 for 30 minutes to allow leaflet decalcification. This has been tested in humans during concomitant cardiac surgery, and a percutaneous embodiment was in development before initial capital was exhausted.
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TRANSAPICAL APPROACH
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The transapical approach offers to overcome vascular access issues as well as the challenges with crossing a stenotic valve and accurate positioning. A small left anterior thoracotomy is performed, and a sheath is placed in the left ventricular apex with a purse-string suture. A guidewire is passed through the native valve under fluoroscopic guidance, and the remainder of the procedure is performed in a manner similar to the transfemoral approach. This approach was validated initially in an animal model10,27 and has been used clinically in at least 50 patients using the Cribier-Edwards valve.28 While necessarily more invasive than a transfemoral approach, the shorter catheter length and antegrade approach may afford more stable control of the device for deployment.
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CONCLUSION
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Although definitive catheter-based AVR is clearly in its infancy (with a total of fewer than 200 patients treated worldwide over the past 3 years), it is clear that a number of percutaneous approaches can allow this procedure. The field is likely to develop rapidly over the next several years, with refinement of the early approaches and emergence of still newer technologies. Guided by clinical results, these devices ultimately may expand beyond the current target of patients with no surgical option to treat high-risk patients, patients with stenosis of a prior aortic valve bioprosthesis, or even as an alternative to surgery. Indeed, pilot studies of valved stent deployment inside previously placed bioprosthetic valves have been promising. Procedures likely will be performed by only a subgroup of "structural" cardiologists and surgeons who possess the training and skills to understand valve pathology more completely, to work transseptally, and to integrate fluoroscopic with 3-D transesophageal and intracardiac echocardiographic images. The remaining engineering obstacles do not appear to be insurmountable but clearly will require the close collaboration of interventional cardiologists with cardiac surgeons (who best understand the valve pathology and repair methods) and engineering teams who must turn these concepts into clinically usable devices.29
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References
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