Towards an Integrated Echocardiographic Assessment of Valvular Mechanics by Three-Dimensional Volumetric Imaging

Echocardiography is the most useful imaging modality for the noninvasive assessment of patients with valvular diseases, by integrating a thorough morphologic characterization with real-time dynamics in the beating heart. The routine use of conventional two-dimensional (2D) and Doppler echocardiography has greatly expanded our understanding of cardiac physiology and pathophysiology, and has improved our ability to assess the optimal timing for interventions and the prognosis of our patients. However, conventional 2D echocardiography is a tomographic imaging technique, basically providing the visualization of only one thin slice of the heart at a time. The assessment of the size and function of a particular cardiac structure taken as a whole can only be performed by making geometric assumptions about its shape and by applying specific formulas to calculate global indexes from measurements performed in multiple predefined tomographic views. In addition, the displacement and cyclic deformation of the moving cardiac structures displayed in these fixed predefined views can be fully captured only if occurring within the corresponding scan planes. Yet, cardiac valves lie in different planes, move in three-dimensional (3D) space and change in orientation and reciprocal spatial relationship continuously during the cardiac cycle; therefore there is no single 2D plane able to image en face two valves simultaneously. Consequently, the cardiac chambers and valves have been studied individually by 2D echocardiography, as if the function of one were independent from the other, and so clinicians have almost lost the ability to consider the heart as a whole and to assess the fascinating functional interplay among its components.

Indeed, the heart and the great vessels represent a highly integrated system in which the function or malfunction of one structure/organ influences the others, and vice versa. Right and left ventricular interdependence, ventriculo-arterial coupling and atrio-ventricular coupling are a few examples of clinically meaningful interplay between different cardiovascular structures, and one must take this interplay into account when trying to understand cardiac physiology. This complex interaction becomes critically important when surgery or device interventions deemed to correct the malfunction of a certain structure are performed. For instance, consider performing a conventional valve replacement for mitral regurgitation, without taking into account the positive effect on left ventricular function of maintaining the continuity between the subvalvular apparatus and mitral leaflets. While valve replacement would restore mitral valve competence, it would be detrimental to left ventricular performance. After applying this consideration, the value of preserving the subvalvular apparatus when replacing the valve has become apparent, and the technique of mitral valve replacement has been modified accordingly.

Mitral regurgitation is the most common heart valve disease in the western world and surgical valve repair is the current gold standard treatment in severe cases. The surgical repair of significant mitral regurgitation usually involves annular remodeling with a surgical ring. In recent years, different annular rings (complete or partial, rigid or deformable, flat or saddle-like) have been developed in an attempt to restore normal annular shape and function from its dilated forms in degenerative disease or from its altered geometry in functional mitral regurgitation secondary to ischemic or dilated cardiomyopathy. Three-dimensional echocardiography (3DE) has been pivotal for our detailed understanding of mitral annulus geometry and kinematics in different mitral valve diseases. In particular, transesophageal 3DE plays a major role in assessing the suitability of various repair strategies aimed at restoring or maintaining the saddle shape of mitral annulus, and can actually evaluate how effective these strategies are immediately after surgery, while still in the operating room, in order to improve repair durability. However, to date the effectiveness of mitral annulus remodeling has been assessed mainly by evaluating the presence and degree of residual regurgitation at the end of the procedure, and the incidence of mitral repair failure (i.e., need for reoperation) over time. Few studies have addressed the possible impact of a prosthetic ring on the geometry and function of the surrounding structures, namely the aortic root complex. This is in part due to cultural reasons (i.e., the current belief that the correction of mitral regurgitation without replacing the valve should be beneficial by definition), and also due to the capabilities of current imaging techniques (all tomographic or radiographic) which are not adequate to assess the complex geometric and functional interactions between neighboring structures.

3DE is the only imaging technique capable of volumetric acquisition of large pyramidal data sets in the beating heart that can encompass almost the whole heart. Significant advances in computing technology, transducer design, miniaturization and beam-forming have allowed the advent of transesophageal 3DE with full-volume acquisitions, providing unparalleled, realistic images with high spatial and temporal resolution of the heart in motion ( Figure 1 ). This major technological advancement, coupled with sophisticated quantitative analysis software, heralds a new era in the assessment of the pathophysiologic interaction of the mitral apparatus with the aortic root complex.

Figure 1

Volume rendering of the base of the heart in a transversal cut-plane visualized from the atrium ( A ; see Video 1 [view video clip online]) and in a longitudinal cut-plane passing through the center of the aortic valve ( B ; see Video 2 [view video clip online]) showing the spatial relationship between the mitral and the aortic valve and the mitral-aortic continuity. LAA , Left atrial appendage; TV , tricuspid valve.

In this issue of the Journal , Veronesi et al. have used transesophageal 3DE to assess the effects of mitral annuloplasty rings used to stabilize the annulus during surgical repair of the regurgitant mitral valve on aortic valve function. They showed that, after mitral annuloplasty, not only the mitral valve area was significantly decreased compared to controls, but also functional parameters of the mitral valve, such as the longitudinal displacement and pulsatility (i.e., the ability of mitral annulus to change in size throughout the cardiac cycle) of the mitral annulus, were significantly reduced. The novelty of the study by Veronesi et al. is that they were able to document the coexistence of significant changes of the untreated aortic valve. While aortic valve area and aortic annulus area did not change significantly compared to preoperative data, the aortic valve pulsatility, and its longitudinal excursion were significantly reduced after mitral annuloplasty.

It has been demonstrated that the dynamic changes in aortic valve area and annular longitudinal displacement during left ventricular systole facilitate blood ejection, reduce aortic cusp stress, and minimize transvalvular turbulence. Therefore, it may be presumed that the insertion of a prosthetic ring in a native mitral annulus can negatively affect blood ejection from the left ventricle. Whether all types of prosthetic rings (i.e., complete or partial, rigid or deformable, flat or 3D-shaped) exert the same effect or not, and how this effect may impact on postoperative left ventricular remodeling and long term outcome, remain to be clarified.

The study by Veronesi et al. is important for several reasons: (1) it clearly shows the importance of assessing not only the effect of our interventions on the affected structure, but also of their possible impact on the function of neighboring structures; (2) it demonstrates the potential of the most recent 3DE technology, which not only improves our understanding of the anatomy and pathophysiology, and the quantitative assessment of heart valve diseases, but also allows the echocardiographer to assess the reciprocal functional relationships between adjacent structures comprehensively; and (3) their observations will open the door for further developments in the design of prosthetic rings, aimed to provide new, more flexible annuloplasty rings capable of shape changes during cardiac cycle, and of reproducing physiological mitral valve kinematics.

During the last decades, the evolution of cardiovascular imaging techniques has broadened our insight into mitral and aortic valve physiology. However, these valves have been studied in isolation in humans and their anatomic and functional relationships have been substantially ignored. Mitral and aortic valves are coupled through fibrous aorto-mitral continuity. Lansac et al. first reported the complementary synchronicity of aortic and mitral valve annular dynamics throughout the cardiac cycle mediated by the aorto-mitral continuity, thus proposing a functional significance of this anatomic region between the two valves and challenging the static physiology of the fibrous annulus of both valves. Unfortunately, the interpretation of these results was hampered by the fact that the data about aortic and mitral valve dynamics were obtained from two separate groups of animals under different hemodynamic conditions. Now, using 3DE, simultaneous assessment of mitral and aortic annulus motion in humans is possible. This possibility opens the door for additional clinically-relevant investigations in the expanding world of percutaneous interventions of heart valves. For example, the different design of aortic bioprostheses (e.g., self-expandable CoreValve system [Medtronic CV, Luxembourg, Belgium] has a larger part extending into the left ventricular outflow tract in comparison with the Edwards SAPIEN bioprosthesis [Edwards Lifesciences LLC, Irvine, CA]) may variably affect mitral valve dynamics, and this should be considered before proposing these procedures to low-risk patients as an alternative to conventional open surgery.

Modern 3DE technology has an adequate spatial and temporal resolution for an accurate and integrated assessment of valve mechanics, but nevertheless this technology also suffers from several shortcomings. In order to obtain large 3D volumes of data with sufficient temporal and spatial resolution, ECG-gated multi-beat full-volume acquisitions are required, which are subject to stitching artifacts stemming from respiration, patient or transducer motion, arrhythmias, and ECG-gating disturbances. In addition, image quality may be impaired by inappropriate gain settings and drop-out artifacts. The acquisition of good-quality 3D full-volumes requires experience and may be challenging at times (e.g., arrhythmias). Although the recent introduction of single-beat acquisition allows for 3D data sets to be acquired without stitching artifacts also in difficult settings, this technology is limited by low temporal resolution. Future advances in 3D ultrasound technology will allow wider angle acquisition and color flow imaging to be completed in a single cardiac cycle with higher spatial and temporal resolution.

Furthermore, analysis software often requires the transfer of 3D data onto dedicated workstations, and the quantification requires a learning period for proper use and is time consuming. However, more advanced scanners provide dedicated software for valve geometry analysis on-board, being faster, more automated, and less reliant on operator expertise. These improvements will allow their routine use to support clinical decision-making and not only for research purposes. This will be of particular importance during interventional procedures in the cardiac catheterization laboratory and during surgical valve repair in the operating room, where immediate qualitative, and quantitative feed-back is important.

In summary, we may anticipate that transthoracic and transesophageal 3DE will become in the coming years the main echocardiographic imaging modality routinely used not only to plan, monitor, and evaluate the outcome of valve procedures, but also to provide an integrated assessment of the local effects of implanted devices on the neighboring structures.

Supplementary Data

Video 1

Three-dimensional transesophageal volume rendering of the base of the heart in a transversal cut-plane visualized from the left atrium.

Video 2

Three-dimensional transesophageal longitudinal cut-plane passing through the center of the aortic valve to display the spatial relationships between the mitral and aortic valves and the mitral-aortic continuity using the volume rendering modality.

L.P.B. and D.M. received research support from GE Healthcare . D.M. was supported by a Research Grant programme awarded by the European Association of Echocardiography .

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Jun 11, 2018 | Posted by in CARDIOLOGY | Comments Off on Towards an Integrated Echocardiographic Assessment of Valvular Mechanics by Three-Dimensional Volumetric Imaging

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