Role of Radiographic Imaging in Transcatheter Aortic Valve Implantation
Kengo Z. Soghoyan UCD School of Medicine and Medical Science, University College Dublin, Belfield, Dublin 4, Ireland
With increasing rates of cardiovascular disease worldwide, it is imperative to provide a means of definitive treatment for stenotic and regurgitant valves. Transcatheter aortic valve implantation (TAVI) has become a definite treatment for end-stage valvular disease. The role of radiological imaging in the transcatheter aortic valve implantation allows for vital pre-procedural planning, guidance during the procedure, and postoperative evaluation. Transthoracic echocardiogram is the first-line imaging modality for pre-procedural planning. Although multidetector computed tomography is reserved as the optimal modality, there are some drawbacks associated with its use. Fluoroscopic angiography is the primary method used during the procedure. Postoperative evaluation imaging is not standardized but transthoracic echocardiogram is often the modality of choice. In addition to imaging, it is crucial to understand the natural history of aortic valve disease and the details regarding the transcatheter aortic valve implantation procedure itself.
Cardiovascular disease is the leading cause of death in Europe and around the world.1 Amongst the various types of cardiovascular disease, valvular diseases comprise a particular niche. With advancing technologies and diagnostic techniques, valvular diseases such as aortic stenosis are being diagnosed more rapidly and are being treated with increased precision. Aortic valve stenosis (stiffening of the tissue), is the most common form of vascular disease in the Western world . Aortic valvular stenosis is now a more manageable disease with the advent of procedures such as transcatheter aortic valve implantation (TAVI). This has become the standard procedure for patients with severe disease, and has outstanding short and long term results . TAVI is at the forefront of multidisciplinary treatment with the coordinated effort of radiologists, anaesthetists, interventional cardiologists, and support staff. By accessing a patient's faulty aortic valve through a distal vascular location (ilio-femoral, brachial, subclavian, or even directly via the aorta) the team guides a frame and a bioprosthetic valve to replace the native .
This procedure is not possible without the help of radiological imaging. Various imaging modalities—from multidetector computed tomography (MDCT) to transthoracic echocardiography (TTE)—are used for diagnosis, pre-procedural planning, the procedure itself, and for postoperative follow up. Imaging allows the physicians to assess the extent of the disease, quantify the patient's heart function and guide the treating physicians during the procedure itself. This paper will discuss how radiological imaging makes TAVI possible.
Pathology and natural history of aortic stenosis
Cardiac valves act as vital alternating barriers and conduits for the circulation of blood through the heart and systemic circulation. The aortic valve allows for the outflow of oxygenated blood from the left ventricle of the heart into the systemic circulation. The morphology of the valve contributes to its function: it has three leaflets that normally close and meet at a central point; they open inwards into the left ventricle with each leaflet attached to tendinous cords and muscles that help prevent prolapse. Of the four valves in the heart, the aortic valve is exposed to the highest pressure. This increases the likelihood of sustaining damage over time, which may require treatment and possibly replacement. Various factors can lead to stenosis of the leaflets and of the valve as a whole. These include congenital abnormalities, chronic hyperlipidemia, hypertension, rheumatic autoimmune disease, increased age, infection, and calcification [2,5]
The most common natural history of aortic valve stenosis is increased calcifications and/or scarring due to damage or inflammation with age. These processes narrow the valve and lumen of the aorta [5,6]. These processes reduce the valve and aortic distensibility and ability to conduct blood causing increased pressures in the left ventricle. The myocardium of the left ventricle begins hypertrophying by adding concentric layers of muscle throughout the ventricle. This allows the heart to eject the blood at elevated pressures. Chronically elevated and sustained pressures cause the heart to continue remodelling. However, the thickened left ventricular wall is not viable as blood vessels in the wall fail to provide it with nourishment, causing the muscle to begin to atrophy. In turn, the left ventricle can no longer eject the cardiac output at elevated pressures. This leads to the symptoms commonly seen in aortic valve stenosis, including: murmur, exertional dyspnea, angina pectoris, syncope, valvular regurgitation, and palpitations [5,6]. If left untreated, this can lead to congestive heart failure, possible stroke and death . By replacing the valve, many of these symptoms can be eliminated and the disease process halted.
TAVI is reserved for high risk patients with symptomatic aortic valvular stenosis and can be indicated for those with asymptomatic aortic valvular stenosis (AVS) with rapid disease progression . The patient’s eligibility for TAVI is assessed through guidelines outlined by European and American Cardiology Associations (ESC/EACTS and AHA/ACC, respectively) [7,8] and the degree of severity of a patient’s AVS must be determined. The patient is then classified based on their valve anatomy, hemodynamics and symptoms [7,8]. These values are obtained by a series of examinations that are reviewed by the multidisciplinary team. Imaging plays a vital role in the pre-procedural planning phase. It is vital for assessment of AVS disease state, estimating the survival score, measuring the aortic root, and for evaluating the sites for peripheral access.
There are several different imaging modalities used during pre-procedural planning. These include transthoracic echocardiogram (TTE), multidetector computed tomography (MDCT), and magnetic resonance imaging (MRI) . Imaging studies demonstrate the morphology, severity of aortic valve disease, absence of other valvular diseases and other contraindications. Given the modalities speed, lack of ionizing radiation and availability, TTE is considered first-line for imaging. It is used to identify any mitral regurgitation and to assess left and right ventricular function. Doppler is used to measure both the systolic and diastolic peak velocity in both ventricles and the peak velocity across the aortic valve . The tendency for Doppler to underestimate flow rates must also be taken into consideration.8 Additionally, transesophageal echocardiogram (TEE) can be used to better visualize the aortic root; it is usually reserved for cases in which TTE is inadequate.9 Two separate studies conducted by Altiok et al and Jilaihawi et al compared 2D and 3D imaging studies for pre-procedural planning, and both found that 3D imaging is superior to 2D for measuring the aortic root [10, 11].
Once AVS is confirmed and evaluated, the patient’s risk and survival are estimated using the European System for Cardiac Operative Risk Evaluation in Europe (EuroSCORE) and the Society of Thoracic Surgeons Predicted Risk of Mortality in the U.S. [12,13]. A EuroSCORE of ≥20% and an STS PRM score of ≥4 is considered the minimum for eligibility by the respective associations [7,8]. With a satisfactory survival prediction, the patient’s aortic anatomy and surgical access site are evaluated to determine whether the proposed device can be safely and sustainably implanted .
Multiple Detector Computed Tomography (MDCT) allows the cardiac team to obtain precise measurements of the aortic root, including the aortic annulus and aortic sinuses. Measurements are obtained using coronal and oblique sagittal slices in mid-systole . The obtained images can be reconstructed using various software packages (e.g. INSIGHT) in order to generate a 3D image of the aortic valve, aortic annulus and ascending aorta.  Precise measurements with the use of MDCT allows the cardiac team to estimate the implantable size, as an effort to prevent paravalvular regurgitation, which is the passage of blood along the sides of the implanted valve . MDCT generally overestimates the annular size by 1-1.5mm . Willson et al found that overestimation of the size of the implantable TAVI significantly reduced the major postoperative complication of atrial regurgitation . Thus, the precision provided by MDCT allows the cardiac team to recommend and utilise the correct valve for the best fit. Afterwards, the peripheral access sites are evaluated.
MDCT is also used to determine the eligibility of a peripheral access site . The two main approaches are either anterograde, which is either transapical via the subclavian artery or retrograde via ilio-femoral artery . This route varies based on the cardiac team’s evaluation of the patient’s condition. For example, in a patient with severe calcifications throughout his peripheral vasculature, transaxillary via the brachial artery or direct aortic routes may be considered. Van der Boon et al evaluated the morbidity and mortality rates related to transfemoral versus transapical TAVI. They found a significant increase in mortality related to transapical procedures when compared to transfemoral . Such findings are considered by the team when designing the procedure plan for each patient. With a single MDCT scan, all of the potential locations can be evaluated . Previously, conventional angiography was used to evaluate peripheral access sites and vascular flow, however, this is now reserved as an adjunct imaging study .
significant increase in mortality related to transapical procedures when compared to transfemoral
If TTE and MDCT do not provide sufficient information, MRI is indicated to assess the severity of aortic valve stenosis and allow the team to plan the optimal strategy for the sizing and placement of the valve. Friedrich et al demonstrate that MRI is equally as effective as TTE and cardiac catheterization . However, due to cost and other limitations, it is reserved as a later stage imaging modality [7, 8].
The procedural plan involves selecting the proper bioprosthetic valve by considering the patient’s valve size, peripheral access site and hemodynamic state. Once the valve and peripheral access site and route are selected, potential contraindications and complications are considered. A medication regimen is designed and then the procedure is scheduled [7, 8].
The goals of imaging during the procedure is to ensure correct heart valve selection, assessing TAV placement and function and identifying any complications. Fluoroscopic angiography is the recommended method for all TAVI procedures and the native valve is viewed in the coplanar or perpendicular view . This modality allows for real time observation of the catheter and valve route and accurate assessment of flow. In addition, images from pre-procedural MDCT or angiography can be superimposed onto the fluoroscopy screen for more precise visualisation .
The standard procedure involves a balloon angioplasty while the patient is under general anaesthetics. If the retrograde path is chosen, an 18-French (Fr) catheter is inserted transilio femorally (most common), or if the anterograde path is chosen, a 25-Fr catheter is placed transapically . Once again, the path is determined by the size of the valve and the patient’s condition. The bioprosthetic valve is then advanced across the native valve in a series of concurrent steps. As the right ventricle pace is increased, the balloon at the end of the catheter is inflated to crimp the heart valve and overlay the frame while simultaneously deploying the prosthetic valve. This expands the frame and secures the new valve to the underlying annulus and leaflets. With the valve securely in place, angiography and TEE are used to evaluate the placement and blood flow through the valve .
There are two major balloon expandable bioprosthetic valves that are commercially available. They are the Edwards’ SAPIEN and Medtronic’s CoreValve. Both have seen significant commercial success and are very similar. However, the CoreValve can only be deployed retrograde in a transiliofemoral approach .
Another class of prosthetic valves are the self-expandable valves (Medtronic CoreValve R, Edwards’ CENTERA). They do not require a balloon for expansion and use a self-expanding nitinol frame . In a multicenter randomized control study, Adams et al proved the self-expandable valves to be equally as effective as the standard balloon TAVIs .
In the near future, 3D printed heart valves will be designed and created per specifications for each patient. Duan et al have created a proof-of-concept, anatomically correct human heart valve using human cells and hydrogels . This new technology will likely be refined and utilized in the future.
outcomes and complications
TAVI is considered a last resort treatment. However, the procedure significantly improves the quality of life and the life expectancy, with long term survival rates close to those of the age-matched general population . Following the procedure, the patient must be placed on antithrombotic regimen dual antiplatelet therapy for six months and must remain on a lifelong daily aspirin regimen . The patient will receive a short course of prophylactic antibiotics to prevent endocarditis. The gold standard is a penicillin, but in cases of allergy or insensitivity, a cephalosporin is used .
The complexity of the procedure and morbidities associated with the patient population can lead to significant complications. Many factors including older age, female gender, advanced disease state, emergency operation, left ventricular dysfunction, pulmonary hypertension, co-existing coronary artery disease, and previous bypass or valve surgery have been shown to increase the risk of postoperative mortality . Additionally, the need for postoperative transfusion, major vascular surgery, pacemaker insertion, stroke, renal failure, and pneumonia are common . Valve embolization and the need for a second replacement valve are uncommon .
Imaging is essential to evaluate the device’s efficacy and to assess for annular rupture, atrial regurgitation, valvular migration, and heart block . TTE is the modality of choice for these studies [9,22]. However, MDCT can also be used to further examine any detected valvular problems .
Thus far, there are no guidelines for acceptable postprocedural assessment values from neither the European nor the American associations . Jayasuriya et al use changes in the aortic valve area and pressure gradient across the aortic valve as their markers for efficacy . Both of these parameters are measured using TTE, and can be obtained using MDCT .
The critical role of imaging cannot be understated in the TAVI procedure. From diagnosis to the postoperative follow up, imaging is a vital tool for the cardiac team during the entire process. With the advancement of technology, conventional imaging modalities have given way to more precise visualization of the aortic root with MDCT and the ability to 3D render the structure for accurate planning. Once the patient’s aorta is evaluated and the procedural plan constructed, fluoroscopic angiography is utilized to properly visualize the route of the tools, the placement of the device, and its function. After the new valve is in place, TTE is used to assess the valve’s activity. It is clear that imaging is vital to every step of the TAVI process. Just as technology has improved the implantable devices, the future will most likely bring new imaging modalities that reduces the patient and staff radiation exposure.
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