Surgical aortic valve replacement has been the standard treatment for patients with severe symptomatic aortic stenosis (AS). Previously, patients determined to be at high risk for surgery could only be offered diuretics and balloon valvuloplasty which served as palliative treatment and had no effect on long-term outcomes. The development of transcatheter aortic valve replacement (TAVR) has emerged as a lifeline for patients considered to be inoperable providing both improvement in symptoms and statistically significant mortality benefit.
The concept of transcatheter balloon expandable valves was first introduced in the 1980s by a Danish researcher by the name of H. R. Anderson who began testing this idea on pigs. In 2002, Dr. Alain Cribier performed the first successful percutaneous aortic valve replacement on an inoperable patient. The first approval of TAVR for the indication of severe AS in prohibitive risk patients came in 2011. In 2012, the FDA approved TAVR in patients at high surgical risk. In 2015 the indication was expanded to include “valve-in-valve” procedure for failed surgical bioprosthetic valves. Most recently, in 2016 the FDA approved the SAPIEN valve for use in patients with severe AS at intermediate risk.
The normal aortic valve is comprised of three leaflets attached to an aortic annulus normally opening to approximately 3 cm^2 to 5 cm^2. Echocardiography best evaluates the aortic valve. An aortic valve is considered severely stenotic when it opens to an area of less than or equal to 1.0 cm^2. High gradient AS is the most common form of severe AS as is defined by the mean gradient of greater than or equal to 40 mmHg and an aortic jet velocity of greater than 4 m/s across the valve. Low flow, low gradient severe AS is a less common form of severe AS. This subset of patients has low flow rate across the valve either due to systolic dysfunction with reduced ejection fraction or small ventricular volumes secondary to left ventricular hypertrophy (with normal LVEF). When LVEF is normal, low flow is defined as a stroke volume index ≤35 mL/m^2. Patients with low flow and depressed LVEF undergo dobutamine stress echocardiogram to differentiate true severe stenosis from "pseudostenosis."
Indications for aortic valve replacement (surgical or transcatheter) are as follows:
Severe high-gradient AS with symptoms (class I recommendation, level B evidence)
Asymptomatic patients with severe AS and LVEF < 50 (class I recommendation, level B evidence)
Severe AS when undergoing other cardiac surgery (class I recommendation, level B evidence)
Asymptomatic severe AS and low surgical risk (class IIa recommendation, level B evidence)
Symptomatic with low-flow/low-gradient severe AS (class IIa recommendation, level B evidence)
Moderate AS and undergoing other cardiac surgery (class IIa recommendation, level C evidence)
TAVR is approved for the following:
Intermediate to prohibitive surgical risk patients with severe AS
Life expectancy less than 12 months owing to a noncardiac cause, myocardial infarction within the last thirty days, congenital unicuspid, bicuspid or noncalcified valve, hypertrophic cardiomyopathy, need for emergency surgery, left ventricular ejection fraction less than 20%, severe pulmonary hypertension with right ventricular dysfunction, echocardiographic evidence of intracardiac mass, thrombus or vegetation, native aortic annulus smaller than 18 or larger than 25 mm, severe mitral regurgitation, MRI confirmed CVA or TIA within last six months, end-stage renal disease, mixed aortic valve disease (concomitant aortic regurgitation), or significant aortic disease.
A multidisciplinary heart team evaluates patients and puts them through extensive evaluation and pre-procedural testing to determine candidacy before undergoing a TAVR. The heart team consists of cardiothoracic surgeons, anesthesiologists, and cardiologists. In addition to transthoracic echocardiography, transesophageal echocardiography is often utilized for better visualization of aortic valve anatomy. Computed Tomography angiography of the chest, abdomen, and pelvis are also performed for accurate measurement of the aortic annulus for determination of valve size. Additionally, this study is essential for visualization of anatomy and determination of the vascular procedure approach to be taken. Left heart catheterization is also done before TAVR to rule out any coexisting coronary artery disease that may be contributing to symptoms or may need revascularization before determination of either surgical or transcatheter approach. Factors that may influence the decision to pursue surgical aortic valve replacement include the need for concomitant coronary artery bypass grafting.
Multiple transcatheter aortic valves are available on the market. However, the only two currently FDA-approved for use in the United States are the SAPIEN valves (Edwards Lifesciences, Irvine, CA) and the CORE valves (Medtronic Fridley, MN). The SAPIEN valves are composed of bovine pericardial tissue and a chromium cobalt alloy frame. These valves are balloon expandable. The newest generation Medtronic valve is the EVOLUT-R. It is composed of porcine tissue and a nitinol frame. This valve has the advantage of being self-expandable (not requiring balloon expansion) and has the ability for repositioning after deployment. To date, there have not been any head-to-head trials comparing these two TAVR valves.
The procedure is typically done in a hybrid room with both operating room and cath lab capabilities. The team consists of interventional cardiologist, cardiac surgeon, and anesthesiologist. The procedure is done under direct visualization with fluoroscopy and occasionally transesophageal echocardiogram (TEE) guidance. The most preferred and least invasive approach is the transfemoral approach. If not feasible, a more invasive method may need to be used.
Possible complications to TAVR include conduction disturbances and the need for a permanent pacemaker, stroke, paravalvular leak, vascular site complications, bleeding, annular rupture, left ventricular perforation, cardiac tamponade, need for surgery, acute myocardial infarction, acute kidney injury, infection, hypotension, and death.
A recent meta-analysis revealed statistically significant evidence of lower rates of both acute kidney injury and major bleeding, and a non-statistically significant trend favoring TAVR over SAVR on overall mortality and stroke. Additionally, there was a statistically significant reduction in length of stay. To the contrary, TAVR was associated with higher rates of vascular injury, paravalvular regurgitation and the need for permanent pacemaker placement. One caveat, being that high-risk patients in the TAVR cohort of the PARTNER 1-A trial had higher rates of stroke.
Transcatheter aortic valve replacement is well established in the treatment of severe symptomatic aortic stenosis (AS) for patients considered to be at prohibitive risk for surgery. It is also an alternative to surgical correction in intermediate and high-risk patients.