Summary
It is two decades since the first report of transcatheter implantation of a stented aortic valve in an animal. The first implantation of a transcatheter aortic valve in a human was accomplished just one decade ago dramatically demonstrating the promise and feasibility of this new therapy. Over the past 10 years, there have been rapid developments in valves, delivery systems and technical approaches. Today, transcatheter valve implantation is a technical possibility for the great majority of patients with aortic stenosis. The next 10 years may well see this become the dominant therapy for aortic stenosis.
Résumé
Deux décennies se sont écoulées depuis la première implantation d’une valve stentée chez l’animal. La première implantation d’une valve aortique percutanée chez l’homme a été réalisée, il y a juste dix ans, avec une faisabilité démontrée et des perspectives prometteuses. Ces dix dernières années ont connu un développement technologique rapide que ce soit sur les valves elles-mêmes, les dispositifs de délivrance ou les voies d’abord. Aujourd’hui, l’implantation de valves aortiques percutanées est techniquement accessible à une majorité de patients porteurs d’un rétrécissement aortique. Il est possible que dans les dix prochaines années, cette technique devienne le traitement prédominant du rétrécissement aortique.
Background
Almost 50 years ago, in 1965, Hywel Davies placed a catheter-mounted valve via the femoral artery into the descending aorta of a dog . The parachute-like valve was oriented so as to passively open in diastole and obstruct retrograde flow in the patients with aortic regurgitation. Over the subsequent few decades, a number of groups experimented with various catheter-mounted passive and active temporary valves ( Fig. 1 ). These valve designs were intended as temporary palliation for aortic regurgitation and were never utilized in humans .
The first catheter intervention specifically targeted at the stenotic aortic valve was aortic balloon valvuloplasty; developed initially as a therapy for congenital aortic stenosis, and described for degenerative aortic stenosis in adults in 1986 by Alain Cribier . The feasibility of dilation of the stenotic calcific valve provided the seed for future developments. In 1992, Henning Andersen described the first transcatheter aortic stent valve . This was constructed of a handmade wire frame within which was sewn a porcine aortic valve ( Fig. 2 ). The assembly was crimped onto a balloon catheter and implanted transarterially into a pig. Subsequently, a number of groups, including our own, pursued the development of a practical and reliable transcatheter stent-mounted valve suitable for implantation in humans . In 2000, Philippe Bonhoeffer described a bovine jugular venous valve sewn within a large-diameter stent, which was implanted percutaneously in children with pulmonary conduits ; proving the feasibility of transcatheter valve implantation in humans.
However, it was in 2002, 10 years ago, that Alain Cribier accomplished the first transcatheter aortic valve implantation (TAVI) for aortic stenosis ; opening the door to a new age in the management of this relatively common disease.
Evolution of transcatheter valves
Balloon-expandable valves
The original Cribier-Edwards valve (Edwards Lifesciences Inc., CA, USA) was constructed from a laser-cut stainless steel tubular frame within which was sewn valve leaflets constructed from equine pericardium, with the inflow covered with a fabric sealing cuff ( Fig. 3A ). This valve was subsequently modified as the Edwards SAPIEN transcatheter heart valve (THV) ( Fig. 3B ), incorporating more durable bovine pericardium, and a higher sealing cuff to reduce paravalvular leaks .
The SAPIEN valve was subsequently superseded by the low-profile Edwards SAPIEN-XT valve ( Fig. 3C ); in which the stainless steel was replaced with a cobalt chromium alloy frame . This alloy allows for a thinner, stronger, more open and compressible frame, while the reengineered leaflets ensure valve closure even when closing pressures are low, along with increased durability. Importantly, this valve was designed to allow the use of smaller diameter 18 French delivery systems. The SAPIEN-XT valve is currently manufactured with diameters of 20, 23, 26 and 29 mm. Next-generation SAPIEN valve systems are currently undergoing evaluation, and will incorporate additional leaflet, frame and sealing enhancements to further reduce delivery profile and paravalvular leaks.
CoreValve
In 2005, following initial implants in India, early German experience with the CoreValve ( Fig. 4 ) was reported by Grube et al. . The valve frame was constructed of nitinol; a nickel-titanium alloy that can be manufactured so as to be malleable when cool, but to become relatively rigid at body temperature. The valve leaflets and annular seal were constructed of porcine pericardium. The valve is cooled and compressed within a delivery catheter, which is then placed within the diseased valve. As a covering sheath is withdrawn, the valve is released and expands, returning to its preset shape. A long multi-staged frame is anchored within the aortic annulus as with the SAPIEN valves, but also extends superiorly to anchor in the supracoronary aorta. The supracoronary portion serves to self-centre the valve and provides an additional point of fixation. The CoreValve (Medtronic Inc., MN, USA) is currently available with diameters of 26, 29 and 31 mm.
Only limited non-randomized comparisons are available . Deployment of the CoreValve device may be more intuitive, and does not require rapid pacing, while deployment of the SAPIEN device may be more targeted and clinical benefit is better documented. The CoreValve device can, up to a point, be repositioned or retrieved, although this process is not without complications . Coronary obstruction is rare, but more of a concern with the SAPIEN-type valves. Atrioventricular block requiring pacemaker implantation is much more common with CoreValve. Both valves can offer dramatic clinical benefit and both systems continue to improve.
Newer valves
Several newer transcatheter valves ( Fig. 5 ) are in various stages of evaluation. Some are in clinical use, although experience remains very limited . Generally, these newer valves attempt to improve on the widely available SAPIEN-XT and CoreValve devices by enhancing deliverability, positioning, sealing, or facilitating repositioning or removal.
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