Echo: A Leader in TAVR Planning
Aortic valve stenosis is a burgeoning, degenerative disease, for which transcatheter aortic valve replacement (TAVR) has entered U.S. clinical practice for some clinically indicated patients. While planning for the treatment is a multifaceted process including physicians and caregivers from varied specialties, the imaging work-up from diagnosis to post-procedure relies heavily on echocardiography. However, CT and fluoroscopy also play a routine role in pre- and intra-TAVR planning.

First-line assessment

Aortic stenosis (AS) is the most common valvular heart disease in the elderly, for which the numbers are growing due to the aging population in developed nations. In older adults, mild thickening, calcification or both of a trileaflet aortic valve without restricted leaflet motion (i.e., aortic sclerosis) affects approximately 25 percent of the population older than 65 years, while calcific aortic stenosis affects approximately 2 percent to 3 percent of those older than 75 years (J Am Coll Cardiol 2006;48:e1-e148).

Over the past decade, Doppler echocardiogram has replaced cardiac catheterization as the gold standard diagnostic technique for identifying AS. This was solidified by the 2012 multi-societal expert consensus document. "Although invasive cardiac catheterization has historically been the standard for quantification of AS, this function has been largely replaced by echocardiography," partly due to the "convenience and wide-spread applicability TTE [transthoracic echocardiogram]" (J Am Coll Cardiol 2012;59;1200-1254).

The real-time, 3D images produced by contemporary TTE systems provide a unique perspective of heart valves. "Most physicians today would start with a TTE to help define the severity of the aortic stenosis and its effects on the heart," says William J. Stewart, MD, of the section of cardiovascular imaging at the Heart and Vascular Institute at the Cleveland Clinic.

Using echocardiography in their initial assessment, Linda D. Gillam, MD, MPH, medical director of the cardiac valve program at Columbia University Medical Center in New York City, says that physicians are able to assess three things:
  1. The anatomy of the valve and the anatomic basis for the stenosis, which is "critically important";  
  2. A quantitative assessment of the severity of the stenosis to measure the gradients across the valve and calculate the size of the opening, called the aortic valve area; and
  3. Any associated leakage of the valve.
Also, the 2009 American Society of Echocardiography/European Association of Echocardiography consensus document stated that TTE "usually is adequate" for anatomic evaluation of the aortic valve because the combination of imaging and Doppler allows for the determination of the level of obstruction (whether subvalvular, valvular or supravalvular)—although transesophageal echocardiography (TEE) also may be helpful when image quality is "suboptimal" (J Am Soc Echocardiogr 2009;22[1]:1-23).  

While TTE has an external probe that is placed on the surface of the patient's chest, TEE contains an ultrasound transducer at its tip that is passed into the patient's esophagus, allowing the image and Doppler evaluation to be registered and recorded.

Pre-TAVR planning

In clinical practice, at least 30 percent of patients with severe AS do not undergo surgery for replacement of the aortic valve, due to advanced age, left ventricular dysfunction or the presence of multiple coexisting conditions, according to the PARTNER researchers (N Engl J Med 2010;363:1597-1607). Due to the elderly nature and comorbidities of this population, providers need to meticulously assess which patients are appropriate for TAVR. Once a patient is diagnosed with AS, there is a complete diagnostic work-up of these typically elderly patients to determine whether or not he or she is a candidate for TAVR, which includes echo screening.

"We need to precisely plan everything about a TAVR procedure with echo, including the size of the patient's aortic annulus, which is where the valve will be implanted, and the leakiness of the valve, which could present a problem during the valve implantation," says Stewart.  

"The [preoperative] aortic valve morphology is assessed; bicuspid valves may or may not be less amenable to percutaneous valve replacement," wrote Stewart in a 2008 editorial (J Am Coll Cardiol Img 2008;1:25-28). "Both TTE and TEE visualize the distribution of calcium on and adjacent to the valve, the distance from the valve to the left main coronary ostium, the angle of the left ventricular outflow tract to the aorta and the baseline severity of left ventricular size, function and hypertrophy."

However, CT also has a role in pre-procedural planning. "We use CT to examine the iliac and femoral arteries for access site assessment; to provide additional measurements of the annulus," explains Jonathon A. Leipsic, MD, chairman of the department of medical imaging and co-director of advanced cardiac imaging at Providence Health Care Heart Center at St. Paul's Hospital in Vancouver, British Columbia, Canada. "Enabled by its multiplanar capabilities, CT enables a 3D assessment of the annulus allowing for reproducible and accurate area and circumference measurements of the annulus."

Gillam suggests that the most important role for a pre-procedure CT is to assess the arteries through which the percutaneous valve has to pass. "The percutaneous valve is a fairly large device, which most often is introduced through the femoral artery, making for a long course to the heart," she says. "CT can best determine vessel tortuosity or protruding plaque, which can inform artery selection." In addition, Stewart says the pre-procedural CT aids in the selection of the angiographic planes that are important to coaxial positioning of the valve.

Lastly, CT can assess the takeoff of the coronary arteries, particularly the left coronary artery, because when the heart team deploys the percutaneous valve, the thickened valve gets pushed off to the side, and the team needs to be cautious not to block the coronary arteries. "This takeoff assessment also can be obtained with 3D TEE, but CT is probably the most widely used modality to make that measurement," Gillam says.

Rarely, MRI is used to pre-procedurally assess patients with AS. "For instance, if you had a patient with truly abnormal renal function, making him or her ineligible to undergo a CT, then you could use MR to assess the annulus," says Leipsic.

Even before TAVR procedures begin, the process for patient  and approach selection are complicated, necessitating a heart team to evaluate and manage the patient carefully, which the Centers for Medicare & Medicaid Services (CMS) reinforced through its Coverage with Evidence Development issued in May. For healthcare providers to qualify for reimbursement, the heart team must include a cardiovascular surgeon, an interventionalist and "additional members of the heart team such as echocardiographers, imaging specialists, heart failure specialists, cardiac anesthesiologists, intensivists, nurses and social workers."  

However, CMS did not designate whether the echocardiographers or the imaging specialists should be radiologists or cardiologists. Stewart and Gillam suggest that the in-house expertise needs to determine who reads the images, not the job title. Leipsic, as a trained radiologist with an appointment in a cardiology department, concurs that the "most expert specialist at an institution should be reading the images. If a site chooses to perform these procedures, then they must simultaneously recognize the importance of working together as a team to guide the procedure." Yet, Leipsic adds that a radiologist may need to be consulted if extracardiac findings are evident on a CT.

Intra- & post-procedure

According to the 2012 consensus document, "the mainstay of intraprocedural imaging is fluoroscopy and angiography for device placement. TEE is an important adjunct to this and is used at the operator's discretion" (J Am Coll Cardiol 2012;59;1200-1254).

With that choice, operators seem to be leaning toward TEE with more frequency. "TEE is used, more or less constantly, to achieve numerous goals, analogous to the constant ultrasound guidance of transcatheter treatment of mitral stenosis, atrial-septal defect, hypertrophic cardiomyopathy and other lesions," wrote Stewart in his 2008 editorial. "During balloon valvotomy of the stenotic aortic valve, but before implantation of the AVR, the location of the indentation on the balloon is noted by TEE and fluoroscopy, in comparison to adjacent landmarks. Immediately after balloon dilation, TEE rechecks the severity of aortic stenosis and regurgitation."

Recent research has supported the utilization of TEE. Moss et al, for instance, found that 86 percent of patients in their study had successful implantation, of which 77 percent had TEE (J Am Coll Cardiol Img 2008;1:15-24). In the study, TEE was used to determine annular dimension and was "useful" in guiding correct device sizing. In addition, TEE "successfully" guided device implantation in 97 percent of patients in whom the native valve was crossed with the percutaneous valve, according to the researchers. Finally, Moss and his colleagues wrote that TEE was used for the early detection of paravalvular aortic regurgitation and complemented fluoroscopy in the detection of complications.

However, fluoroscopy still has a role during the procedure, and Leipsic suggests its strength lies in its assessment of wires and catheters and in patients with more calcified lesions.

Also, using the CT scans that were taken prior to the procedure can inform a TAVR, according to Stewart. The team is able "to register some information about what imaging planes are the best ones to line up for the particular anatomy of that patient, including the angulation of heart, the aorta, the plane of the annulus and the direction of flow in the aortic valve," he says.

In addition to the clinical benefits of understanding the patient's anatomy, some data have indicated that CT also may reduce the length of the procedure. One study, presented at ACC.12 by Fleishmann et al, showed that pre-procedural CT accurately predicts angiographic projections for device deployment in the majority of TAVR patients and appears to decrease overall procedure time (104.6 minutes vs. 123.1 minutes), fluoroscopy time (12.5 minutes vs. 17.2 minutes) and contrast volume (104 ml vs. 133 ml) during TAVR, compared with patients who had TAVR without CT prediction.

While the transfemoral approach is the most common to deliver percutaneous valves, transapical, transaortic or subclavian are additional routes. "The imaging options are pretty much the same" for all approaches, according to Gillam, but the heart team is assessing different site areas and functions. For instance, with the transapical approach, there is a slightly increased risk of entangling parts of the mitral valve, so that possibility would have to be evaluated through imaging, she explains.

Following AVR deployment, the positioning of the prosthesis within the native valve is confirmed with fluoroscopy, aortography and TEE, wrote Leipsic et al (Am J Roentgen 2009;193[3]:W214-W219).

After the percutaneous valve is expanded into place, "TEE is used to assess the severity and location of aortic regurgitation. Most patients have mild perivalvular regurgitation, but if TEE shows it to be more severe, repeat balloon dilation of the AVR can be helpful" (J Am Coll Cardiol Img 2008;1:25-28).

For post-procedure assessment, the 2012 consensus document recommends that patients should be followed closely by the implanting physician team for the first 30 days after TAVR to diagnose and follow any procedural complications. Evaluations should include a post-TAVR baseline echocardiogram and ECG to document any conduction abnormalities (J Am Coll Cardiol 2012;59;1200-1254).

Post-TAVR patients typically undergo a CT prior to discharge, Leipsic says. After that, they have a follow-up appointment at six months with an echocardiogram.

While the expert team of the consensus document suggested the reliance on echo in an intra- and post-procedure may dissipate as experience widens, physicians currently implanting percutaneous valves continue to rely heavily on the modality. "Echocardiography will likely contribute to better outcomes of TAVR, provided it is used optimally. But the field is new, and there is much to learn," wrote Stewart in his 2008 editorial.