Special Imaging Considerations for Transcatheter Mitral Therapy

INTRODUCTION

Imaging plays an essential role in structural transcatheter interventions revealing intracardiac landscape and events throughout the diagnostic work-up, procedural planning, intraprocedural guidance, and follow-up. Unlike transcatheter aortic valve replacement (TAVR), mitral interventions cannot be performed under sole fluoroscopic guidance because fluoroscopy cannot adequately visualize soft tissues such as the endocardial surface and mitral leaflets. Cardiac computed tomography angiography (CTA) enables precise anatomic measurements and assessment of calcification to plan TMVR, but is stationary, does not provide real-time imaging including hemodynamic data, and has suboptimal temporal resolution compared to echo. Echocardiography has been the primary tool in the structural imager’s armamentarium. Portable and safe, it provides illuminating anatomic and hemodynamic information about the heart and circulation in general - all in real-time. Current transcatheter mitral interventions rely heavily on echocardiography for procedural guidance.

Complexities of intracardiac navigation and maneuvering safely around the delicate structures of the heart require specific skillset and imaging expertise. Structural imaging has emerged as a new subspecialty within the neighboring fields of cardiology and anesthesiology. ^, Specific training requirements for structural imagers emphasize expertise in interpreting cardiac hemodynamics, functioning in a fast-paced environment of the operating room, and applying advanced echocardiographic skills, including 3D reconstructive techniques.

Three-dimensional (3D) echocardiography is now omnipresent in structural imaging. In addition to providing spectacular volume-rendered images of intracardiac landscapes, 3D reconstruction generates any pre-defined 2D imaging plane, thus ensuring complete visualization of intracardiac structures and allowing for more accurate and precise measurements.

In this chapter, we describe and illustrate special echocardiographic methods and techniques on the example of transcatheter edge-to-edge repair (TEER)—currently the only Food and Drug Administration (FDA)-approved device-based transcatheter mitral intervention. In addition, we review special imaging considerations for some of the approved TMVR (CE mark only) and transcatheter annuloplasty platforms. The chapter’s structure maps onto the clinical course of events in transcatheter valve therapy: screening, pre-procedural planning, the procedure itself, and the follow-up.

SCREENING

The goal of screening is to determine MR mechanism, MR severity, and the feasibility of transcatheter repair. In TEER, screening is accomplished using both transthoracic echocardiogram (TTE) and transesophageal echo (TEE). The two modalities complement each other by their opposite vantage points: anterior for TTE and posterior for TEE. Thus, while TEE provides superior visualization of the basal (posterior) cardiac structures, including the mitral valve and left atrium (LA), TTE is best to evaluate ventricular size and function and to assess right-sided structures in most cases. Data from both modalities are integrated in the final anatomic and physiologic assessment.

The Mitral Axis

The surgical view of the mitral valve has been used as the frame of reference for transcatheter mitral interventions ( Fig. 7.1 ). The mitral valve in the surgical view is seen en-face from a hovering perspective in LA. The aortic valve positioned at the top (12 o’clock position) defines the antero-posterior axis, and the two commissures where the bases of the anterior and posterior leaflets meet define the medial-lateral axis. The echocardiographic views that map onto the mitral axes are (1) the commissural view (medial-lateral axis), (2) the long-axis view (antero-posterior axis), and (3) the en-face view —the 3D (surgical) or the 2D short-axis view (see Fig. 7.1 ). Achieving the precise “on-axis” orientation of echocardiographic views is fundamental to locating pathology and procedural guidance.

The Commissural View

In the commissural view, also known as bicommissural or commissure-to-commissure (CC) view, the mitral scallops are completely separated: the P1 and P3 are seen on opposite sides, with the A2 “flying in the air” in the center ( Fig. 7.2 ). Given that the commissures are aligned in this view, the heads of both papillary muscles, postero-medial and antero-lateral, are normally seen. The CC view lays out the entire line of coaptation, from lateral to medial commissure. In this view, the individual MR jets can be located and compared to each other—an essential task in selecting targets for intervention. Since the coaptation line is curved, slight rotation of the TEE probe is often needed to arrive at jet origins: clockwise to reach anteriorly, counterclockwise to focus on the posterior portions of the coaptation line.

The Long-axis View

The long-axis (LAX) view is a single orthogonal cut across the medial-lateral axis at a particular location on the coaptation line (see Fig. 7.2 ). The midline (on-axis) LAX view will show A2 and P2 scallops as well as the fully opened left ventricular outflow tract (LVOT). To reach medially, the TEE probe is rotated clockwise, while counterclockwise rotation will shift the viewing plane laterally. The LAX view demonstrates specific segments of both leaflets and the profile of the MR jet at that location (see Fig. 7.2 ).

The En-Face View

The surgical view reveals the entire circumference of the mitral annulus, a full view of the coaptation line, including both commissures, and the atrial surfaces of both leaflets (see Fig. 7.2 ). The volume-rendered image is rotated to bring the aortic valve (AV) to the 12 o’clock position. Once aligned, the volume may need to be tilted slightly to fully expose both leaflets and commissures. In addition to the surgeon’s view, a 2D short-axis view at the level of mitral leaflet tips provides axial reference, particularly when 3D imaging is not available or suboptimal.

Additional Views

Mid-esophageal 5-, 4-, and 2-chamber views and the corresponding transthoracic views complement the key mitral views and are part of the comprehensive echo exam. ^, Occasionally, non-standard views are required to elucidate pathology not adequately captured in conventional views. Ultimately, the best view is the one that provides an unequivocal answer to the specific clinical question.

The Role of the Three-dimensional Reconstructive Techniques

The current 3D technology provides lower spatial and temporal resolution compared to 2D and does not replace a dedicated 2D echo assessment. In conjunction with 2D and Doppler echocardiography, 3D echo is a highly valuable tool for clarifying MR mechanism, severity, and feasibility for transcatheter therapy. 3D reconstructive techniques allow multiplanar reconstruction of any specific 2D plane from the acquired 3D volume ( Fig. 7.3 ). If the key views were not captured adequately by conventional 2D imaging, they can be reconstructed from the 3D volume. For offline reconstruction, the 3D data must be in the “raw” or native format, which includes the entire pyramidal 3D volume of data, as opposed to the flat unmanipulable volume-rendered reference view. The 3D volume should include the entire mitral annulus, leaflets, and the subvalvular apparatus. For 3D color Doppler acquisitions, the entire origin of the target regurgitant jet should be included in the volume for adequate analysis. In the end, quality 3D color Doppler imaging depends on a narrowly tailored compromise between the spatial and temporal resolution (optimized settings, multibeat acquisition), state-of-the-art ultrasound equipment, and the echocardiographer’s experience.

Mitral Regurgitation Mechanism

The Carpentier classification is based on abnormalities of leaflet motion and covers a wide range of mital regurgitation (MR) etiologies—both common and rare ( Fig 7.4 ). Mitral regurgitation can also be classified broadly as primary or secondary , based on whether the MR is due to primary abnormalities of the valve itself or the MR is secondary to extrinsic distortion of the mitral apparatus due to abnormal left ventricle and/or the LA. The most common subtype of primary MR seen in current US clinical practice is degenerative MR (DMR) due to prolapsed or flail leaflets (Carpentier type II). The most common secondary MR is functional (FMR ) due to (left ventricle (LV) dilatation and systolic dysfunction (Carpentier type IIIb) (see Fig. 7.4 ).

Another relatively common type of secondary MR—but distinct from FMR—is the atrial functional MR (AFMR) or pure annular dilatation (Carpentier type I) (see Fig. 7.4 ). AFMR is distinct from FMR because the left ventricular ejection fraction (LVEF) is normal, and the pathologic substrate is dilatated LA and the mitral annulus. Annular dilatation is said to be present when the maximum antero-posterior annular diameter measured in the parasternal long-axis view is greater than 35 mm or the ratio of antero-posterior annular diameter to the length of the anterior leaflet is greater than 1.3. As the annulus dilates, it flattens from its normal saddle-shape configuration which results in systolic straightening of the mitral leaflets without prolapse (<2 mm billowing above the annulus). The regurgitant jet has a “functional” appearance with a wide central jet due to diffuse malcoaptation. AFMR is commonly seen in the setting of LV diastolic dysfunction or atrial fibrillation. Other, less common etiologies such as MR due to rheumatic disease, mitral annular calcification, drug-induced valvulopathies, Libman-Sacks endocarditis, and infective endocarditis are less common and rarely suitable for TEER.

MR Severity

Integrative Approach

Reliable grading of mitral regurgitation severity is complex because the reference “gold standard” is yet to be discovered. Although several specific criteria for MR severity have been established, none can be used as a sole determinant of MR grade because of multiple technical and hemodynamic caveats. Qualitative assessment of the jet size on color Doppler, a widely used method in clinical practice, has proven to be unreliable outside of clear-cut cases where MR is very mild or very severe. ^, As a result, multiple datapoints obtained by different methods should be collected and included in the final analysis ( Table 7.1 and Fig. 7.5 ). The methods include quantitative, semiquantitative, and qualitative Doppler assessments (color, spectral, quantitative, 3D color), as well as indirect clues to MR severity such as LV and LA chamber size, pulmonary vein flow profile, pulmonary artery systolic pressures, and right-sided function. The final integrative assessment of MR severity also takes into account data quality and applicability in a particular case. The ultimate goal is to determine whether MR answer the practical clinical question: whether MR is clinically severe MR or nonsevere MR, because only the severe MR, coupled with symptoms and/or its detrimental effects on LV or upstream, is an indication for invasive treatment. ^, This distinction is clear in the presence of multiple specific criteria but less so when none or only a few are present (see Fig 7.5 ).

TABLE 7.1

Grading the Severity of Chronic MR by Echocardiography

		MR SEVERITY a
	Mild	Moderate		Severe
Structural
MV morphology	None or mild leaflet abnormality (e.g., mild thickening, calcifications or prolapse, mild tenting)	Moderate leaflet abnormality or moderate tenting		Severe valve lesions (primary: flail leaflet, ruptured papillary muscle, severe retraction, large perforation; secondary: severe tenting, poor leaflet coaptation)
LV and LA size b	Usually normal	Normal or mild dilated		Dilated c
Qualitative Doppler
Color flow jet area d	Small, central, narrow, often brief	Variable		Large central jet (>50% of LA) or eccentric wall-impinging jet of variable size
Flow convergence e	Not visible, transient, or small	Intermediate in size and duration		Large throughout systole
CWD jet	Faint/partial/parabolic	Dense but partial or parabolic		Holosystolic/dense/triangular
Semiquantitative
VCW (cm)	<0.3	Intermediate		≥0.7 (>0.8 for biplane) f
Pulmonary vein flow g	Systolic dominance (may be blunted in LV dysfunction or AF)	Normal or systolic blunting g		Minimal to no systolic flow/systolic flow reversal
Mitral inflow h	A-wave dominant	Variable		E-wave dominant (>1.2 m/s)
Quantitativett. i , j
EROA, 2D PISA (cm 2 )	<0.20	0.20–0.29	0.30–0.39	≥0.40
				(may be lower in secondary MR with elliptical ROA)
RVol (mL)	<30	30–44	40–49	≥60
(may be lower in low-flow conditions)
RF (%)	<30	30–39	45–59	≥50

a All parameters have limitations, and an integrated approach must be used that weighs the strength of each echocardiographic measurement. All signs and measures should be interpreted in an individualized manner that accounts for body size, sex, and all other patient characteristics

b This pertains mostly to patients with primary MR.

c LV and LA can be within the "normal" range for patients with acute severe MR or with chronic severe MR who have small body size, particularly women, or with small LV size preceding the occurrence of MR.

d With Nyquist limit 50 to 70 cm/s.

e Small flow convergence is usually less than 0.3 cm, and large is ≥1cm at a Nyquist limit of 30 to 40 cm/s.

f For average between apical 2- and 4-chamber views.

g Influenced by many other factors (LV diastolic function, atrial fibrillation, LA pressure).

h Most valid in patients greater than 50 years old and is influenced by other causes of elevated LA pressure.

i Discrepancies among EROA, RF, and RVol may arise in the setting of low or high flow states.

j Quantitative parameters can help subclassify the moderate regurgitation group. From Zoghbi WA, Adams D, Bonow RO, et al. Recommendations for noninvasive evaluation of native valvular regurgitation: a report from the American Society of Echocardiography developed in collaboration with the society for cardiovascular magnetic resonance. J Am Soc Echocardiogr . 2017;30(4):303–371. Bolded qualitative and semi-quantitative signs are considered specific for their MR grade. LA, left atrium; ROA , regurgitant orifice area.

Specific Criteria

Specific criteria for mild MR include small, narrow centrally directed jet, vena contracta width (VCW) ≤3 mm, proximal isovelocity surface area (PISA) radius less than 3 mm at a Nyquist limit of 30 to 40 cm/s, A-wave-dominant mitral inflow, and normal LV and LA size (see Fig 7.5 ). These features are not consistent with chronic severe mitral regurgitation. Conversely, VCW ≥7 mm, PISA radius ≥10 mm at Nyquist 30 to 40 cm/s, very large central jet filling greater than 50% of the LA area in a standard view, and dilated LV with normal systolic function in the absence of significant aortic regurgitation. Flail leaflet is another specific criterion for severe MR, although small flails may sometimes be associated with less than 3+ MR.

Quantitative Criteria

The three quantitative parameters used clinically to describe valvular regurgitation are the effective regurgitant orifice area (EROA) , the regurgitant volume , and the regurgitant fraction (see Table 7.1 and Fig 7.5 ). The regurgitant volume can be calculated from EROA and vice versa using MR velocity-time integral (VTI) obtained by continuous-wave Doppler. The EROA can be obtained by various Doppler techniques: PISA, direct planimetry of the 3D vena contracta area (3D VCA), and quantitative Doppler. The first two methods use color Doppler to capture the EROA at a single moment with the assumption that the EROA remains relatively constant throughout systole. However, if there is a significant systolic variation in the size of the regurgitant orifice or if MR is not holosystolic, then using unadjusted instantaneous EROA to calculate the regurgitant volume will result in error. In contrast, quantitative Doppler estimates the regurgitant volume as the difference between the forward LV stroke volume and the total volume entering the LV in diastole. The regurgitant volume can then be divided by the MR VTI to obtain the mean EROA. In addition, the regurgitant volume can be divided by the total LV stroke volume to estimate the regurgitant fraction. Although well suited to handle systolic variation in EROA, as well as multiple and eccentric jets, quantitative Doppler is limited by multiple geometric assumptions and requires expertise for reliable results.

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