Currently, transcatheter aortic valve replacement is considered for high- or extreme-risk patients with severe calcific aortic stenosis; these patients are not considered as appropriate candidates for conventional surgical valve replacements. In general, these procedures have similar benefits as those mentioned above, such as eliminating the need for cardiopulmonary bypass and allowing for shorter patient recovery times. It is conceivable that a transcatheter aortic valve could be delivered by one of four different approaches: (1) transarterially (e.g., via the femoral artery, subclavian artery, or the ascending aorta), (2) transseptally via the right heart, (3) transatrially through the left atrial wall or a port in the atrial appendage, or (4) transapically through the left ventricular wall (Fig. 36.3).

Currently, the transfemoral approach for the delivery of transcatheter aortic valves is the most common access route; this approach is used 69–91 % of the time [9, 10] and is the preferred access route. Primary alternate access routes are transapical (through the apex of the heart in the myocardium), transaortic (through the aorta), and subclavian (subclavian artery) routes. These alternate routes are typically used when the transfemoral approach is deemed not possible due to: (1) the size of the patient’s arteries; (2) the tortuosity of the arteries and/or aorta; and/or (3) the degree of calcification within the aorta, some which could potentially be knocked free by the delivery system. Note that the transfemoral, transaortic, and subclavian routes all require retrograde delivery, while transapical requires an antegrade approach.

In current practice, both the transapical and transaortic approaches require a minimally invasive surgery (see Chap. 35), yet they provide the most direct anatomical approach. The transapical approach must go through the left ventricular wall (myocardium) which can be difficult to close if a large delivery system is used and/or if the myocardial tissue is abnormally frail. The transaortic approach also requires closure of a hole that is placed into the ascending aorta. This can be especially difficult if the aorta is extremely calcified, a condition known as a porcelain aorta . As technology advances, the need for minimally invasive surgery with these approaches may be eliminated. The transseptal approach , which has the advantage of employing transvenous access for the delivery system, has the potential drawback that if the system passing through the interatrial wall is large, the physician may create a septal defect which will require a subsequent repair (see Chap. 37). Additionally, the system must make a turn of approximately 180 degrees in the left ventricle, after passing through the complex structures of the mitral valve, to be positioned in the aortic valve, a maneuver that can be technically challenging. Anatomically, inferior access via a femoral vein is considered advantageous due to the proximity of the inferior vena caval ostium and the fossa ovalis, which is the preferred transseptal puncture location.

It has been reported that rapid ventricular burst pacing can be employed to facilitate transcatheter heart valve implantation. Pacing rates between 150 and 220 bpm with durations of 12 ± 3 s were relatively well tolerated (n = 40) when cautiously used. Rapid pacing was associated with a rapid and effective reduction in systemic blood pressure, pulse pressure, transvascular flow, as well as cardiac and catheter motion [11].

As can be observed in Fig. 36.3, the potential pathways/approaches to place a transcatheter-delivered valve in the aortic position are quite varied and have dramatic differences in the angles and anatomical features that would be required to be navigated within or through, e.g., vessels, chamber walls, valves, or chordae tendineae. Due to this complexity, the flexibility of the delivery system (with the valve loaded inside) will be a major factor to consider when selecting a delivery approach; furthermore, the patient’s cardiac anatomy is a major consideration. This is true for the delivery of the system and also for the positioning of the valve stent into the aortic position, as one must prevent obstruction to the ostia of the coronary arteries.

Currently, there are several transcatheter aortic valves available for clinical use, in clinical trials, or in animal testing. The CoreValve (Medtronic, Inc.) and the SAPIEN XT (Edwards Lifesciences) are FDA approved for high- and extreme-risk patients, and to date, several hundred thousand devices have been implanted in patients worldwide (Fig. 36.4). More specifically, for the transarterial placement of the SAPIEN XT transcatheter heart valve, the surgeon uses the NovaFlex+ delivery system (Edwards Lifesciences), whereas he/she uses the Ascendra+ delivery system (Edwards Lifesciences) for a transapical placement. The CoreValve system is also delivered via a transarterial approach in a specialized delivery catheter. In addition, several other companies that have developed (and continue to develop) competing technologies, including Boston Scientific (Marlborough, MA, USA) and St. Jude Medical (St. Paul, MN, USA) as well as smaller companies such as Direct Flow Medical, Inc. (Santa Rosa, CA, USA) and Heart Leaflet Technologies Inc. (Maple Grove, MN, USA). Nevertheless, nearly all the major players in cardiac valve replacement have a keen interest in these technologies and clinical approaches (Fig. 36.5) [12].

It is important to note that, currently, a percentage of patients that have received transcatheter-delivered aortic valves have elicited conduction abnormalities which may include heart block. Most of the self-expanding or balloon-expanded valve prostheses exert radial forces on the interventricular septal wall and surrounding structures which is required to maintain proper position and to minimize paravalvular leaks. This force, coupled with the close anatomic proximity of the atrioventricular node, the bundle of His, and/or the left bundle branch with the basal annulus of the aortic valve, is the likely explanation for this phenomenon (Fig. 36.6) (see Chap. 13).

Mitral valve dysfunction can be related to several factors including diseased leaflets [13], annular changes [14], abnormal or damaged chordae [15], and ventricular dilatation [16] causing displacement of the papillary muscles. Due to this large variability in disease process, a wide variety of transcatheter devices are being investigated for the mitral valve. These transcatheter devices can be subdivided into five general types: (1) devices for Alfieri-type edge-to-edge repair, (2) indirect annuloplasty devices deployed into the coronary sinus, (3) direct annuloplasty devices placed on or near the mitral annulus, (4) devices for dimensional control of the left ventricle or left atrium, and (5) devices for mitral valve replacement [17, 18].

The edge-to-edge technique involves placing a stitch to join the anterior and posterior leaflets at the location of regurgitation [19–21]. This technique is most commonly used in patients with A2 or P2 prolapse, and the simplicity of the edge-to-edge technique has led to opportunities for percutaneous valve repair [22–24]. More recently, the MitraClip repair system was designed to use transcatheter clips to grasp the ventricular sides of the anterior and posterior mitral leaflets, leaving the clip in place upon deployment (Abbott Vascular, Bloomington, IN, USA). This technology was FDA approved in 2013.

For patients with annular dilatation, many devices are currently being developed to simulate a traditional annuloplasty procedure. These products are classified as either indirect, which typically involves a transvenous coronary sinus approach , or direct, which involves placing the device in direct contact with the mitral annulus. Percutaneous, transvenous mitral annuloplasty is a technology that implants a metal bar with flexible ends and a stiff midsection to reshape the posterior leaflet; it is a reversible procedure (Viacor, Inc., Wilmington, MA, USA). The Monarc system features two self-expanding stents tethered together which reshape the posterior region of the annulus in 2–3 weeks (Edwards Lifesciences). Similar to the Monarc, Carillon XE (Cardiac Dimensions, Inc., Kirkland, WA, USA) utilizes tethered stents in the coronary sinus to reshape the posterior annulus. These technologies rely on the proximity of the coronary sinus to the posterior aspect of the mitral annulus. Additionally, direct mitral annuloplasty is also being investigated; these devices are in direct contact with the mitral annulus either temporarily or permanently. MiCardia Corporation (Irvine, CA, USA) is developing adjustable annuloplasty devices which are currently implanted surgically but can be adjusted on the beating heart. The Mitralign device (Mitralign, Inc., Tewksbury, MA, USA) places implants around the annulus and then cinches them closer together, thereby reducing the orifice area of the valve. Nevertheless, the goal of these devices is to restore valve coaptation and thus eliminate mitral regurgitation.

Indirect approaches to mitral valve repair that influence left ventricular or left atrial dimensions are also under investigation. By changing left ventricular dimensions, products such as Coapsys and iCoapsys (Myocor^® Inc., Maple Grove, MN, USA) are designed to improve mitral valve and left ventricular performance. The PS³ system (Ample Medical, Inc., Foster City, CA, USA) shortens the left atrial dimension between the fossa ovalis and the great cardiac vein and consequently the septal-lateral dimension of the mitral annulus [25].

As described for the aortic valve , it is possible to deploy a transcatheter mitral valve or affect a transcatheter repair using four different approaches: (1) transarterial (e.g., via the femoral artery), (2) transseptal via the right heart, (3) transatrial through the left atrial wall or a port in the atrial appendage, or (4) transapical through the left ventricular wall (Fig. 36.7). The human mitral valve is a very complex, dynamic, and highly variable structure. Therefore, complications in the deployment of a replacement valve or repair device into this position will potentially interact with the (1) valve leaflets themselves, (2) the chordae tendineae, and/or (3) the papillary muscles. As mentioned earlier, the valvular and subvalvular anatomy can be quite variable, with some individuals eliciting bifurcated or trifurcated papillary muscles, distinct chordae tendineae patterns, and numerous scallops of each mitral valve leaflet. Due to the complexity of the mitral valve apparatus and underlying disease processes, it is likely that a combination of the aforementioned devices will be required to provide percutaneous solutions for mitral valve repair.

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