Adaptation and evolution of transcatheter aortic valve replacement (TAVR) technologies has led to approval of TAVR for consideration in patients at intermediate risk for surgical aortic valve intervention. As TAVR becomes more mainstream, attention is shifting toward percutaneous mitral valve (MV) repair and transcatheter MV replacement (TMVR) techniques. Transcatheter heart valves (both purpose-built and off-label-use TAVR valves) are being implanted during TMVR procedures to treat clinically significant MV disease (native disease, degenerated bioprosthetic valves, and dysfunctional surgical MV annuloplasty repairs) when the risk of open heart MV surgery is prohibitive. The success of these high-risk procedures is directly related to accurate periprocedural imaging with echocardiography and other modalities. Although a multidisciplinary heart valve team approach is necessary for optimal patient selection, a multimodality team-based imaging approach and comprehensive understanding of the MV are required for safe procedural planning. Collaboration between noninvasive cardiac imagers and the intraprocedural interventional imaging team and translation of the periprocedural imaging to the implanting team are crucial to the success of TMVR technology. Currently, the TMVR procedures discussed here are conducted either as part of clinical research or off label. The US Food and Drug Administration–approved mitral valve-in-valve procedures for the treatment of degenerated mitral bioprosthetic valves are not discussed here.
Highlights
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Echocardiography along with other imaging modalities is essential to assess suitability for TMVR.
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Image guidance with 2D and 3D echocardiography is key to the success of transcatheter MV replacements.
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The advent of novel TMVR devices will result in high demand for interventional echocardiographers.
The widespread success of transcatheter aortic valve replacement (TAVR) has had profound implications on the treatment of aortic stenosis, and TAVR is now considered a noninferior modality for intermediate-risk patients. As TAVR becomes more mainstream, attention is shifting toward percutaneous mitral valve (MV) repair and transcatheter MV replacement (TMVR) techniques. Specifically, transcatheter stentless and stented valves (both purpose-built transcatheter MV and off-label-use TAVR valves) are being implanted during TMVR procedures to treat clinically significant MV disease (native disease, degenerated bioprosthetic valves, and dysfunctional surgical MV annuloplasty repairs) when the risk of open heart MV surgery is prohibitive. Although these therapies are nascent, the global learning curve is exponential, with a rapidly expanding pool of early feasibility studies, clinical trials, and registry data.
With the rise of transcatheter-based mitral interventions, the success of these high-risk procedures is directly related to a multidisciplinary heart valve team approach for optimal patient selection, a multimodality imaging approach for comprehensive understanding of the MV pathology, and accurate periprocedural imaging. Collaboration between noninvasive cardiac imagers and the intraprocedural interventional imaging team and translation of the periprocedural imaging to the implanting team are crucial to the success of TMVR technology. Given the complexity of transcatheter MV technology and imaging demands for a successful multidisciplinary heart valve team, the aim of this document is to describe key imaging considerations, discussions, and investments that must be applied for the development of a successful transcatheter MV program. For this purpose, this document is focused specifically on the pre-, intra-, and postprocedural echocardiographic imaging associated with the use of TMVR in native MV (valve–in–native valve [VinV]) and mitral annuloplasty rings (valve-in-ring [ViR]). Where appropriate, complementary imaging modalities are discussed as well. In-depth discussion of percutaneous MV repair (PMVR) techniques and of mitral valve-in-valve procedures for the treatment of degenerated mitral bioprosthetic valves (now a US Food and Drug Administration–approved procedure) is beyond the scope of this document.
Role of Echocardiography in Patient Selection
Diagnosis
Adequate patient selection for TMVR depends on an accurate initial diagnosis of the underlying MV pathology. Cardiovascular ultrasound, both transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE), are critical to the planing and guidance of these procedures. Because of its wide availability and low cost, TTE is the foundation for initial diagnosis as well as patient selection and preprocedural planning. TTE is particularly valuable because of its high temporal resolution, availability of multiple acquisition windows, and integration of Doppler echocardiography, which allows a robust integrative characterization of hemodynamically significant MV disease. TTE provides considerable information regardless of whether the pathology is mitral stenosis (MS), mitral regurgitation (MR), or some combination of mitral and other valvular pathologies. Because MV pathology is highly susceptible to hemodynamic perturbations such as heart rate, cardiac output, blood pressure, and the degree of MR, performance of a standard transthoracic echocardiographic examination, without anesthetic agents or sedatives, provides the single best hemodynamic assessment. In centers facile in advanced echocardiographic techniques such as three-dimensional (3D) echocardiography and speckle-tracking, TTE can provide a unique and nuanced understanding of the severity and impact of MS and/or MR, as well as the mechanics of the surrounding structures. In-depth discussion of the diagnosis and grading of MS and MR can be found in recent guideline documents.
In addition to TTE, preprocedural TEE including 3D imaging is considered a key component of procedural planning. As a more recent imaging modality, 3D TEE has rapidly grown in the past decade with several technological breakthroughs in scanner design, beam formation, image acquisition, and display and quantification.
Electrocardiographically gated multidetector computed tomography (MDCT) has evolved as an important imaging modality for anatomic characterization of the MV before TMVR procedures. The ongoing investigator-initiated, prospective, multicenter Mitral Implantation of Transcatheter Valves trial ( ClinicalTrials.gov identifier NCT02370511 ) was designed to achieve most of the preprocedural imaging using MDCT and echocardiography and was supported by dedicated MDCT and echocardiography core laboratories. With patient enrollment now complete, new evidence on the utility of this imaging modality and its specific role in the planning of TMVR procedures will soon emerge.
Risk Stratification
Once the initial diagnosis of severe MV disease has been confirmed, the heart valve team discusses patient selection for TMVR rather than a surgical or palliative approach. Stratification of surgical risk is often weighed via one of several quantitative databases, such as the Society of Thoracic Surgeons risk calculator and the European System for Cardiac Operative Risk Evaluation II, which assess the predicted risk for mortality and morbidity. These calculators are robust but do not take into account more qualitative metrics such as frailty, which is increasingly recognized to have a large impact on patient outcomes. Some attempts to quantify frailty by tests such as the 6-minute walk test or grip strength are routinely used, but must be interpreted in the entire clinical context. As transcatheter therapies develop, large registries such as the Society of Thoracic Surgeons/American College of Cardiology Transcatheter Valve Therapy Registry, the Valve-in-Valve International Data registry, or other multicenter global registries may become helpful in understanding risk and providing accurate data for informed consent. Ultimately, the cardiac surgeon is required to determine the surgical risk profile of a given patient and declare the patient at prohibitive risk for surgery (in native MV disease) or repeat surgery (in patients with previous MV annuloplasty repair).
Valve Types for Transcatheter Mitral Replacement
TMVR is a dynamic space with rapidly changing technology. The initial tests of feasibility for TMVR began with patients who were deemed to be at too high risk for surgery but in addition had anatomy that was felt to be unfavorable for a percutaneous edge-to-edge repair technique. Other early efforts focused on valve-in-valve placement because of the preexisting landing zone provided by a prior prosthetic MV. Experience with TMVR in native MVs came later and was first reported for use in native MS in 2014. The early attempts at TMVR used existing valve technology and most commonly used second-generation balloon-expandable technology developed for TAVR. However, it quickly became evident that TMVR had unique challenges and potential complications and that the adaptation of valves meant for the aortic and pulmonic position was not ideal for the uniquely shaped mitral annulus. In addition to the challenging complex anatomy of the MV, other challenges of TMVR therapies include the prosthetic anchoring and annular retention, TMVR delivery systems, long-term durability, valve thrombogenicity from slower atrial flow, and proper patient selection. The ideal purpose-built transcatheter heart valve (THV) for TMVR permits a percutaneous transseptal delivery via the left atrium into the MV position, provides sufficient anchoring and retention mechanisms that stabilize the valve within the dynamic MV annulus, results in optimal sealing to prevent paravalvular leak (PVL), and avoids significant left ventricular outflow tract (LVOT) obstruction. To this end, current THV designs include ventricular or annular anchors, annular winglets or flanges, valve stents with radial forces, annular docking rings or systems, or other designs that permit native leaflet engagement or mitral annular clamping. With the intent to reduce the risk for paravalvular MR, some devices come with larger sealing skirts. Many of these new valves have also eschewed the cylindrical shape used in TAVR valves in favor of D-shaped or flexible devices to reduce protrusion into the LVOT. Other innovations include the use of an apical tether to secure an optimal distal position of the TMVR valve away from the LVOT.
Preprocedural Planning
Advances in echocardiographic imaging allow characterization of a number of details that are critical for establishing feasibility to plan the approach to therapy ( Table 1 ). Many patients with MR who are at prohibitive risk for open heart surgery will initially undergo consideration for PMVR. Currently, the only Food and Drug Administration–approved commercial device is the MitraClip (Abbott, Abbott Park, Illinois), but several new PMVR technologies are on the horizon. These include technologies to perform percutaneous direct MV annuloplasties, procedures to cinch the subvalvular annulus through a retrograde aortic approach, or the transapical implantation of artificial chords. Similar to edge-to-edge repair with the MitraClip, all of these techniques are applied under echocardiographic guidance on a beating heart. If there are no PMVR options available or feasible given anatomic constraints, TMVR may be considered. The role of imaging in helping with this decision process includes (1) elucidation of valve leaflet morphologic characteristics, (2) evaluation of the mitral annulus inclusive of sizing and calcification, (3) assessment of the mitral subvalvular apparatus, (4) geometric characterization of the left ventricle and left atrium, and (5) assessment of structural and hemodynamic features that could lead to significant complications, most notably dynamic LVOT obstruction ( Figure 1 ). A comprehensive cardiovascular evaluation will typically require multimodality imaging that uses the unique strengths of each imaging modality ( Tables 1-3 ).
| Parameters and structures assessed by preprocedural echocardiographic imaging in TMVR |
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| Modality | Long anterior leaflet length | Extensive mitral leaflet and annular calcification burden | Prominent thickness or sigmoid contour | Large mitral annulus | Aortomitral angle | Presence of atrial thrombus |
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| TTE | ++ | + | ++ | + | + | + |
| TEE | +++ | ++ | ++ | +++ | +++ | +++ |
| MDCT | ++ | +++ | +++ | +++ | +++ | +++ |
| CMR imaging | +++ | + | +++ | + | ++ | +++ |
Imaging of MV Leaflet Characteristics
Evaluation of the MV leaflets begins with characterization of their mobility, thickness, and calcification and the presence of prolapsing or flail segments, leaflet tears or clefts, and the presence of systolic anterior motion of the anterior mitral leaflet (AML). An integrated approach starting with TTE should be used, incorporating all available imaging planes and, when feasible, use of 3D imaging techniques ( Figure 2 ). Leaflet characteristics are generally well seen by two-dimensional (2D) TTE, but particularly when transthoracic windows are suboptimal, TEE is often required to precisely delineate the anatomy. Real-time 3D TEE provides excellent anatomic and functional characterization of mitral leaflet pathology, and this is augmented by semiautomated software that allows a quantitative evaluation of annular and valve leaflet excursion. If poorly seen with echocardiography, electrocardiographically gated MDCT can also evaluate valve leaflets. However, because of inferior temporal resolution, MDCT can be less adept than TEE at visualizing the subtleties of thin, rapidly moving leaflets. Furthermore, MDCT lacks an equivalent to Doppler echocardiography and thus cannot assess hemodynamics or visualize regurgitant jets. Cardiovascular magnetic resonance (CMR) can be used to interrogate specific valve morphology and physiology. At centers of excellence, it can provide unique and valuable insights without the need for the sedation required for TEE ( Figure 3 ). However, CMR is time consuming, image quality can be operator dependent, and despite increasing adoption, expertise in CMR at the level required for structural heart disease planning remains limited.
MV Hemodynamics
Valvular hemodynamics and flow dynamics should be fully characterized before TMVR. Systemic blood pressure and heart rate are important considerations, as MV pathology may be substantially altered by filling status, diastolic dysfunction, certain multimorbid conditions (anemia, thyroid disease, etc.), and the effects of sedation or general anesthesia. In challenging cases, stress echocardiography can be used to characterize the hemodynamic effects of regurgitation and stenosis during exertion and evaluate for changes in pulmonary artery systemic pressure. When evaluating valve hemodynamics with echocardiography, it is important to recall the limitations and pitfalls that may confound analysis. Issues such as the effect of shadowing from significant calcification or annuloplasty rings may mischaracterize regurgitant jets. In particular, it is important to apply a meticulous approach to the correct alignment of the Doppler interrogation beam to avoid underestimation of lesion significance.
Three-dimensional TEE has been shown to provide accurate sizing of transcatheter valves, identify and describe specific MV pathology more accurately than 2D TEE, and provide invaluable spatial information. Three-dimensional TEE with or without 3D color Doppler permits the direct assessment of the MV area, which has been shown to correlate well with MV area estimated using the continuity equation. Three-dimensional color Doppler is also very useful when assessing vena contracta area or anatomic regurgitant orifice of the MR jet and permits the assessment of the true 3D proximal isovelocity surface area shell. Kahlert et al. revealed a significant asymmetry of vena contracta area in functional MR compared with degenerative primary MR, resulting in poor estimation of the effective regurgitant orifice area by single measurements of vena contracta width. However, although 3D color Doppler is a valuable emerging tool, additional prospective validation is warranted.
Two-dimensional and 3D TEE also facilitate the assessment of the extent of annular, subannular, and leaflet calcification, the evaluation of mobility and motion of the MV leaflets, and the description of specific valve pathologies. Although MDCT is the primary imaging modality to quantitatively assess the MV annulus for TMVR sizing (see below), 3D TEE offers the opportunity to statically or dynamically evaluate the MV annulus throughout the cardiac cycle to obtain very detailed quantitative information.
Imaging of the Mitral Annulus
Heavy mitral annular calcification (MAC) has long provided technical challenges for the surgical repair or replacement of the MV in high-risk patients. Although posterior MAC prevents surgical reconstruction and reduction of the posterior leaflet, the surgeon also depends on an intact mitral annulus to securely anchor a prosthesis with sutures. The surgical debridement of the calcified annulus is also associated with high risk, as this may cause atrioventricular separation.
To circumvent this, valves developed for transcatheter therapies have been increasingly used intraoperatively for challenging situations in which calcification cannot be easily debrided. This adaptation highlights the importance of understanding the calcification pattern and burden with imaging before any MV intervention and may help guide the heart valve team’s decision to select TMVR in the setting of heavy annular calcification ( Figure 4 A, Videos 1-6 available at www.onlinejase.com ). For TMVR, MAC and surgical rings offer both an anchoring solution for new THV technologies and a source of paravalvular leaks. The presence of large posterior eccentric calcifications, in the absence of significant anterior calcifications, may cause anterior displacement of the transcatheter valve during deployment and increase the risk for LVOT obstruction. TTE can give a sense of the extent of calcification of the mitral annulus. Care must be taken that quantification of MAC severity and localization of MAC be performed in the parasternal short-axis view only, to avoid overcalling of MAC severity. Because of the proximity of the MV to the esophagus, TEE provides superior visualization of the atrial surface of the MV annulus. Three-dimensional TEE easily defines protruding and irregular concretions of MAC on the proximal surface, but distal structures may be obscured by shadowing from heavy calcifications ( Figures 4E and 4 F, Video 3 , available at www.onlinejase.com ). By 2D TEE, the subvalvular apparatus can be visualized from transgastric short- and long-axis views. As with leaflet calcification, a complete sense of the extent and degree of calcification is often best confirmed by MDCT ( Figures 4 C and 4D).
Accurate sizing of the mitral annulus is critical for successful TMVR. Bioprosthetic valves may not exist in sizes adequate for deployment in an excessively large annulus. A valve that is significantly undersized risks immediate embolization, whereas a valve that is mildly undersized is still at risk for late migration and increased amounts of subsequent paravalvular regurgitation. Delineation of mitral annular size as well as an understanding of the complexity of adjacent structures is most easily performed using 3D software tools and 3D printing ( Figure 5 ).
Geometric Characterization of the Left Ventricle and Left Atrium
Mitral valve function is highly dependent on the complex interplay of its neighboring structures. Enlargement of the left ventricle and the left atrium may lead to annular dilation and tethering of the MV leaflets. Wall motion abnormalities and left ventricular (LV) chamber enlargement due to ischemic heart disease may also cause secondary MR because of pronounced tethering of the papillary muscles, with secondary effects on one or both leaflets. Assessment of leaflet tethering is best performed by TTE and TEE. The mitral chordal apparatus itself is a complex network of primary, secondary, and tertiary chords that are often not well visualized using any imaging modality. However, transgastric 3D TEE permits visualization of the subvalvular apparatus of the MV and has been demonstrated to provide accurate measurement of chordae tendineae lengths. Abnormal chordal attachments and accessory papillary muscles may make PMVR techniques more difficult and may inform the need for a TMVR rather than an attempt at PMVR.
The papillary muscles are also an important component of the mitral apparatus to be thoroughly assessed before TMVR. Papillary muscle number, location, morphology, and function are all important determinants of MV function and associated pathology. For example, when the papillary muscles are malpositioned anteriorly, they may increase the risk for LVOT obstruction, when reduced in number may lead to MS, and when displaced apically may lead to MV leaflet tethering and functional MR. The importance of an appropriately sized apical tether has even been incorporated into one TMVR valve design.
Anticipation of Complications
The primary goal of preprocedural planning is to mitigate complications. Compared with TAVR as a more established procedure, TMVR carries a much higher risk for life-threatening adverse outcomes. Catastrophic complications include LVOT obstruction, mitral annular rupture, mitral prosthesis dislodgement, and pericardial tamponade. Particularly if a transseptal puncture is required, a close evaluation of the interatrial septum is prudent. Morphologic aspects such as a surgically placed patch, lipomatous interatrial septum, the presence of a patent foramen ovale, and/or interatrial septal aneurysm may make the procedure more challenging. Other important complications include paravalvular regurgitation, vascular access complications, and cerebrovascular events. The presence of left atrial or left atrial appendage thrombus may inform whether a percutaneous approach should even be considered.
As with TAVR, the use of MDCT is helpful in anticipating these complications and for planning for potential emergent bailout surgical approaches. One of the most important is the identification of a narrow LVOT due to disproportionate upper septal thickness or asymmetric hypertrophic cardiomyopathy with elevated LVOT gradients. Alcohol septal ablation may be necessary, either before implantation or as a bailout maneuver during the TMVR procedure. The efficacy of alcohol septal ablation can be increased by using echocardiography during contrast injection into septal branches of coronary arteries to ensure that the branch of the septal perforator coronary artery selected for alcohol infusion perfuses the correct region of interest in the septum. Much of the benefit of alcohol septal ablation occurs because of reduction in contractility of the basal septum. Thus, its use can be effective at the time of TMVR even in a normal-sized septum or as a bailout for unacceptably high LVOT gradients or significant LVOT obstruction after MV deployment.
In patients with prohibitive risk for LVOT obstruction before TMVR, periprocedural laceration of the AML to prevent outflow obstruction is being clinically trialed ( ClinicalTrials.gov identifier NCT03015194 ). This is a novel percutaneous method aimed at preventing LVOT obstruction in TMVR-eligible patients. The technique resembles a percutaneous maneuver to lacerate the AML with chordal sparing so that when the THV is placed, blood continues to flow unobstructed through the sliced AML and the open cells of the THV ( Figures 5-7 , Videos 7-17 , available at www.onlinejase.com ).
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