Review articleTranscatheter aortic valve replacement: History and current status
Introduction
Calcific aortic stenosis is the most common acquired valvular disorder in developed countries, with a prevalence between 2 and 4% in adults over the age of 65 (Freeman and Otto, 2005). The pathophysiology of calcific aortic stenosis is believed to be related to lipid accumulation, inflammatory changes and infiltration of macrophages and T lymphocytes, which ultimately leads to bone formation by osteoblasts and immobilization of the valve leaflets (Rajamannan, 2009). Leaflet immobility causes increased afterload, left ventricular hypertrophy, diastolic dysfunction and may even progress to left ventricular dilation and systolic dysfunction with decreased cardiac output. While there is a relatively long period of asymptomatic, disease latency, the mortality rate rises rapidly to around 50% over 2 years after symptom onset (Kelly et al., 1988, Otto et al., 1989, Ross and Braunwald, 1968, Turina et al., 1987).
Surgical aortic valve replacement (AVR) has been shown to improve survival in patients without coexisting conditions (Lund, 1990). Given the high prevalence of the disease, low perioperative mortality of isolated AVR (less than 5%), and improved outcomes, AVR has become the standard of care for symptomatic patients with severe AS. It is now the most common valvular surgery, with an estimated 50,000 surgeries performed annually in the United States (Astor et al., 2000). The risk of AVR is increased by a number of factors, such as elderly age, prior cardiac surgery and several comorbidities, including chronic lung disease, peripheral vascular disease, prior stroke, renal failure, heart failure (NYHA class III or IV), systolic dysfunction, frailty, and concomitant coronary disease (Nowicki et al., 2004, Sharony et al., 2003, Thourani et al., 2011). These risk factors, as well as absolute contraindications such as porcelain aorta and cirrhosis, have led to as many as one-third of patients being denied aortic valve replacement surgery (Iung et al., 2005). This large population of inoperable and high-risk patients prompted investigation into alternate therapies, including percutaneous balloon aortic valvuloplasty (BAV). Unfortunately, the results for BAV have been disappointing, with midterm improvements in quality of life, but recurrence rates of 80% at 1 year and a lack of survival benefit (1, Letac et al., 1988, O’Neill, 1991). Accordingly, it has been used primarily as a bridge to surgery in critically ill patients or for palliation. Exciting progress has been made in the last 10 years on a transformative technology for severe aortic stenosis: transcatheter aortic valve replacement (TAVR). In this review, we briefly describe the development, key clinical trials, current outcomes and future directions of TAVR. A detailed description of the technique and current indications is available elsewhere (Holmes et al., 2012).
Section snippets
Early history of transcatheter valve replacement
The large, unmet need for treatment options in inoperable and high-risk patients set the stage for the development of transcatheter approaches to aortic valve replacement. The first percutaneously implanted cardiac valve was described by Davies et al. in 1965 to treat experimentally induced aortic insufficiency in dogs (Fig. 1A) (Davies et al., 1965). Andersen subsequently reported successful delivery of a catheter-mounted, balloon-expandable stent valve in the aortic position in pigs (Andersen
The next generation of transcatheter valves
These exciting data spawned the next generation of transcatheter aortic valves. The PVT valve was modified to create the Edwards SAPIEN valve (initially Cribier-Edwards), which consists of a tri-leaflet bovine pericardial valve in a stainless steel balloon-expandable stent (Fig. 1C). The valve is currently available in 23- and 26-mm annular diameters, with a 29-mm size in development. The device can be delivered via a transfemoral arterial approach using a 22- or 24-French sheath for the 23-
Early observational studies of TAVR
Several observational trials and registries have shown reasonable procedural success with acceptable morbidity and mortality. Webb et al. published mid- and long-term follow-up of 168 patients with severe aortic stenosis who underwent placement of an Edwards SAPIEN or SAPIEN XT aortic valve after being deemed inoperable, secondary to a high-risk Society of Thoracic Surgeons (STS) score (median of 9.1%), comorbid conditions or absolute contraindications (e.g. 21% of patients declined for
Randomized control trials of TAVR vs. standard therapy and AVR
The Placement of Aortic Transcatheter Valves (PARTNER) trials were two parallel, multicenter, randomized clinical trials comparing TAVR with standard therapy or AVR in inoperable or extremely high-risk patients with severe aortic stenosis (Leon et al., 2010, Smith et al., 2011). One arm of the study, Cohort B, randomized 358 inoperable patients at 21 centers with severe symptomatic aortic stenosis to TAVR with the Edwards SAPIEN valve or standard therapy. Severe AS was defined as an AVA less
Complication rates with TAVR
Although TAVR has been remarkably successful in improving the symptoms and survival associated with severe aortic stenosis in high-risk and inoperable patients, it is not without a cost. All available TAVR systems are associated with procedural complications, most notably vascular injury, bleeding and stroke (Table 1). In order to minimize the lack of uniformity in definitions and data collection seen in early TAVR studies, the Valve Academic Research Consortium (VARC) recommended a set of
Procedural process: Patient selection and valve deployment
Evaluation of patients for TAVR requires an assessment of aortic valve disease severity, surgical risk, frailty, coronary and peripheral vascular disease and aortic valve annular size. In order to be considered for TAVR, severe aortic stenosis should be documented with an AVA less than 0.8 cm2 and a peak velocity of more than 4 m/s or a mean gradient of more than 40 mm Hg. In the case of low-gradient AS, failure to augment stroke volume by 20% indicates lack of contractile reserve and increased
Long-term outcomes
Although long-term outcomes are not yet available, registry data suggests a favorable outcome up to three years. Follow-up of 70 very high-risk patients for an average of 3.7 years showed a survival at 1, 2 and 3 years of 81%, 74% and 61%, respectively (Gurvitch et al., 2010). As described above, similar outcomes have been reported at two years for extreme and high-risk patients enrolled in the PARTNER trial (Kodali et al., 2012, Makkar et al., 2012). Clearly, longer term follow-up is needed to
Expanding indications for TAVR
Although FDA approval in the US is currently limited to inoperable and high-risk patients, investigation is ongoing for a rapidly expanding number of indications. The role of TAVR is being studied in intermediate-risk patients as compared to surgical AVR in the PARTNER 2 (Edward SAPIEN XT) and SURTAVI (Medtronic CoreValve) trials. Several groups have reported successful deployment of percutaneous aortic valves in high-risk surgical patients for severe bioprosthetic stenosis or regurgitation
Conclusion
The introduction of transcatheter aortic valve replacement over the past 10 years in nonoperative and high-risk patients with severe aortic stenosis marks a new and exciting era in the treatment of valvular disease. Advances in technology and new devices under investigation should lead to further improvements in outcomes. Whether the technology will provide durable results that are comparable or superior to surgical management in lower risk AS patients or those with varied types of valve
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