Aortic Stenosis and Suitability for Aortic Valve Replacement
Aortic stenosis is the most common form of adult valvular heart disease. In North America, the estimated population incidence of moderate-severe aortic stenosis (valve area <1.5cm2) is 0.4%, rising to 2.8% in subjects aged ≥75 years. In the EuroHeart Survey of 216 elderly patients (≥75 years) who had severe aortic stenosis (≤0.6 cm2/m2) and New York Heart Association functional class III - IV with or without angina, 33% did not undergo intervention. Comorbidity burden, as quantified by the Charlson comorbidity index, was greater in patients who were not accepted for surgery. In a multivariate analysis, increasing age, left ventricular dysfunction and neurological dysfunction predicted the decision not to operate. In North America, the rates of non-intervention in patients with severe aortic valve disease are broadly similar.
For this reason, it is imperative that more therapeutic options become available for elderly patients with symptomatic, severe aortic valve disease.
Percutaneous Aortic Valve Replacement (PAVR)
Alain Cribier and colleagues from Rouen performed the first PAVR in 2002. They used an anterograde trans-septal approach to replace the aortic valve in patients with severe aortic stenosis and cardiac failure. The patient survived for four months then unfortunately died due to complications from peripheral limb ischaemia. Since then, PAVR has progressed with over 200 cases worldwide and is now performed using several different approaches.
Systems Currently used in Humans
The CoreValve ReValving™ system is currently the subject of a multi-national development programme. The first case of CoreValve PAVR was reported by Grube et al., and the authors' have recently performed the first case in North America. Overall, 60 cases have been performed in Europe and Montreal. A case of PAVR simultaneously combined percutaneous coronary intervention has also recently been reported.
The CoreValve Aortic Valve Prosthesis consists of a self-expanding nitinol valve frame designed for the replacement of the native or already in place bioprosthetic aortic heart valve. The CoreValve self-expanding frame is a comprised of three components made from laser-cut nitinol tubing. Each level completes a particular function:
The upper part (aortic level) of the frame increases the prosthesis fixation to the aorta wall and axes the system parallel to the blood flow.
The middle part (commissural level) carries the valve. It is constrained to a given diameter that corresponds to the optimal diameter of the tissue valve. It holds the valve leaflets in their functional position. The convex shape of this level is opposed to the concavity of the coronary sinus to permit normal haemodynamic flow.
The lower conical part (annulus level) expands with high radial force, to anchor the prosthesis firmly to the aortic annulus and prevent migration and paravalvular leaks.
Because of the self-expanding nature of the frame, it can adapt to non-circular local anatomies and does not recoil following expansion.
The valve is made of a standard biological tissue (porcine pericardium). It is fixed to the frame by monofilament sutures. The lower conical part of the frame is covered by pericardium.
The design of this valve utilises the temperature-dependent mechanical properties of nitinol. For mounting, the valve is compressed in chilled saline and retracted into the delivery catheter, and for delivery at the aortic annulus, the sheath is retracted permitting the valve to expand at body temperature. The prosthesis is delivered in a retrograde fashion, usually from the right femoral artery under left femoro-femoral cardiopulmonary bypass (CPB). The multidisciplinary team is made of a cardiac anaesthesiologist, a cardiac surgeon, a cardiologist and nursing staff.
From the 25Fr Generation I system, the CoreValve System has passed through Generation II (21Fr) and presently, the Generation III (18Fr) system is now ready for clinical assessment. The most recent cases have been performed using the TandemHeart® Cardiac Assist device, a left atrial-to- femoral artery bypass system involving a trans-septal approach and a centrifugal blood pump. Use of this system obviates the need for full cardiopulmonary bypass.
In the future, it is anticipated that lower profile devices will permit inclusion of a broader spectrum of high-risk surgical patients for PAVR.
The current Cribier-Edwards Valve (Edwards LifeSciences Inc, Irvine, CA) prosthesis is a trileaflet equine pericardial valve with a tubular stainless steel stent (23mm valve (22Fr sheath = 8mm sheath) and 26mm valve (24Fr sheath = 9mm diameter). A fabric cuff covers the left ventricular part of the prosthesis. The valve is mechanically crimped onto a dedicated valvuloplasty balloon catheter. A deflectable push catheter has also been developed to permit prosthetic valve delivery.
Dr John Webb's group have lead the Cribier- Edwards PAVR programme with approval from the Therapeutic Products Directorate, Department of Health and Welfare, Canada. Valve positioning is performed using fluoroscopy, angiography and transoesophageal echocardiography. At the time of writing, the procedure has been performed on 63 patients under Canadian Special Access Registry for inoperable patients. Initially, peripheral vascular complications were a problem. The first patient sustained iliac artery injury, and the second patient died due to a similar problem. There have been no further deaths with current screening, selection and improvements in equipment and experience. Importantly, Dr Webb's group have changed their approach from anterograde to retrograde PAVR. They have recently reported their experience with the transapical approach. Their balloon-expandable valve may achieve a valve area of 1.7-1.9 cm2.
Emerging Possibilities for PAVR Therapy
Percutaneous heart valve therapy has enormous clinical and commercial potential. Consequently, industry is actively developing novel valves for percutaneous therapy. In the future, valve delivery may be achieved from one of several approaches, such as via the femoral or subclavian arteries, or a left ventricular apical approach. The anterograde PAVR approach with trans-septal access is no longer favoured because of an increased risk of complications. A multi-disciplinary approach will remain essential for optimal patient management. In addition to technical refinements, future advances should include improvements in screening, risk assessment evaluations and post-procedure care. Establishing interventionalist training programmes for percutaneous heart valve therapy will also be crucial to ensure safe and effective dissemination of these therapies. Finally, clinical development of percutaneous heart valve technology should follow the guidelines laid out in the recent position statement guidelines of the Society of Thoracic Surgeons, the American Association for Thoracic Surgery and the Society for Cardiovascular Angiography and Interventions.