Mitral Valvular Disease

Introduction

Transesophageal echocardiography (TEE) has evolved dramatically over the last 3 decades and is now considered an indispensable tool for the practice of cardiac anesthesia. There is arguably no other aspect of cardiac surgery in which perioperative echocardiography plays a more profound role in surgical decision making as in surgery for mitral valve disease. Additionally, surgical philosophy in treating mitral valve disease is constantly changing, making it imperative that the echocardiographer adapt to this very dynamic subspecialty.

Mitral valve disease is now as common as aortic valve disease in the developed world, especially in the aging population, ¹ and mitral valve regurgitation is the primary mechanism of valve disease. In the developing world, however, mitral stenosis (MS) continues to play an important role. ² While contemporary surgical practice recommends mitral valve repair as the standard of care for many regurgitant lesions,^3–5 leading valve surgeons are also advocating repair techniques in the setting of mitral valve stenosis.

The information acquired during a perioperative TEE exam for mitral valve disease has a different focus than that obtained by cardiologists during the preoperative evaluation. Determining the severity of regurgitation or stenosis is no longer the sole focus of the exam. Instead, precise valve analysis in determining mechanisms of dysfunction plays a more important role in planning the correct surgical procedure. Which segments are prolapsing or restricted and whether or not the commissures are calcified in the setting of rheumatic MS are commonly asked questions that must be addressed to determine repair feasibility. Intraoperative TEE provides a roadmap for a successful procedure and controls the quality of the reconstruction.

Although these new demands placed upon the perioperative echocardiographer increase both responsibility and liability, they also solidify the role of the perioperative echocardiographer as an integral member of the surgical team.

Functional Anatomy of the Mitral Valve

The foundation to understanding pathologies of the mitral valve apparatus lies in the echocardiographer’s knowledge of normal valvular anatomy from the surgeon’s perspective. While the surgeon will always examine the mitral valve apparatus once on cardiopulmonary bypass (CPB), many subtle pathologies could be missed. Many nuances of mitral valve disease can only be diagnosed by imaging the moving heart and evaluating the topographic relationship among different aspects of the valve.

Normal Mitral Valve Anatomy

Two-dimensional (2D) echocardiography has become the standard of care in imaging the mitral valve apparatus. However, the mitral valve is a complex three-dimensional (3D) structure. It lies in the left atrioventricular groove, ensuring unidirectional flow of blood from the left atrium (LA) into the left ventricle (LV). From a clinical perspective, five distinct anatomic features are relevant.

Leaflets

The mitral valve is the only cardiac valve to have, under normal circumstances, two leaflets instead of three. Carpentier et al. described a useful nomenclature for segmental anatomy of the mitral valve. ⁶ Other classifications are described in the literature, but the Carpentier classification has been adopted by the American Society of Echocardiography (ASE). It may not matter which classification is used, so long as the echocardiographer and surgeon share a common classification for segmental differentiation of the leaflets.

The posterior leaflet of the mitral valve is quadrangular in shape and is attached to three fifths of the annular circumference in the region of the parietal atrioventricular grove. The posterior leaflet typically has two well-defined indentations that divide the leaflet into three individual scallops. These indentations are believed to aid leaflet opening during diastole. By definition, P1 designates the anterior or lateral scallop, P2 the middle scallop, and P3 the posterior or medial scallop. The three opposing segments of the anterior leaflet are continuous without indentations and designated as A1 (anterior segment), A2 (middle segment), and A3 (posterior segment) ( Fig. 15-1, Video 15-1). The anterior (or aortic mitral) leaflet has a semicircular shape and attaches to approximately two fifths of the annular circumference in the region of the fibrous trigone. A fibrous continuity exists between the anterior leaflet and the aortic valve in the area of the left and noncoronary cusp. This region is referred to as the aortic-mitral curtain. The motion of the anterior leaflet defines an important boundary between the inflow (during diastole) and outflow (during systole) tracts of the LV. The height of the normal posterior leaflet is less than half of the anterior leaflet; however, both leaflets have similar surface areas because of the difference in their circumferences.

Figure 15-1 Surgeon’s view of mitral valve apparatus with segmental division. AC, Anterior commissure; PC, posterior commissure.

A rough zone or coaptation zone (CZ) can be found on the atrial side of the leaflets at the free edge. This rough zone represents the coaptation surface of the valve. The coaptation zone of the valve must provide an adequate surface to maintain valve competency during systole. ⁷ The non-coapting portions of the valve leaflets are relatively smooth and referred to as the smooth zone ( Fig. 15-2).

Figure 15-2 Importance of zone of coaptation to ensure optimal valvular competence during systole. Red line indicates “smooth zone,” blue line marks actual coaptation between leaflets (“rough zone”).

Mitral Valve Dysfunction and Etiology of Disease

The foundation to understanding the types of valvular dysfunction that comprise mitral valve disease was set forth in Carpentier’s landmark paper “The French Correction,” in which the different segments of the mitral valve were defined. ⁶ In addition to defining this segmental approach, Carpentier also proposed a pathophysiologic triad that is a useful adjunct to clearly differentiating the particular causes of mitral regurgitation (MR). The triad consists of: (1) the etiology (underlying disease causing MR [e.g., Barlow disease, ischemic cardiomyopathy]), (2) the lesions (pathologic changes in the valve that result from the disease process [e.g., chordal rupture, chordal elongation, leaflet tethering, annular dilation, calcification]), and (3) the dysfunctions.

It is highly recommended that the echocardiographer take a similar organized approach when evaluating the mitral valve. Frequently, however, the imager will first start by identifying the dysfunction (e.g., presence of leaflet prolapse or restriction). Once a dysfunction has been identified, the cause should be determined. In other words, the imager needs to search for the lesion responsible for the dysfunction (e.g., chordal elongation or rupture, leaflet perforation). Finally, in certain cases, it is possible to propose likely etiologies (e.g., thickened calcified leaflets in type IIIa dysfunction are frequently associated with rheumatic mitral valve disease) ( Table 15-1).

TABLE 15-1

Types of Mitral Valve Dysfunction

Dysfunction	Leaflet Motion in Regard to Annular Plane	Surgical Approach to Repair
Type I	Normal leaflet motion	Annuloplasty ring, closure of clefts or endocarditis lesions
Type II	Excessive leaflet motion	Complex reconstruction
Type IIIa	Restricted leaflet motion in systole and diastole	Complex reconstruction, leaflet augmentation with pericardial patch
Type IIIb	Restricted leaflet motion in systole	Annuloplasty ring, possible chordal release

Preprocedural Valve Analysis by TEE

In 1996, the ASE/Society of Cardiovascular Anesthesiologists (SCA) published guidelines for performing a comprehensive intraoperative multiplane TEE examination. ⁹ In this landmark paper, 20 standard TEE views were defined (see Chapter 1). Once acquired, many of these views can be correctly interpreted even by the non-echocardiographer (e.g., transgastric short-axis view for LV function), but assessment of a complex structure like the mitral valve apparatus requires thorough understanding of both the anatomy and the spatial relationship of the 2D interrogating plane to the valvular apparatus.

Once on CPB, the surgeon views the valve in its entirety in a flaccid state from the LA looking downward toward the LV. In this view, the patient’s left and right side correspond to the surgeon’s left and right side, while the anterior and posterior mitral valve leaflets appear in the appropriate positions. The lateral and medial aspects of the mitral valve are to the left and right, respectively. This is in contrast to the echocardiographer’s view, who sees the images at angles that are by definition 180 degrees reversed from the surgical view. In other words, the left and right sides of the image on the monitor are reversed relative to the surgeon’s perspective ( Figs. 15-5 and 15-6).

Figure 15-5 Transgastric basal short axis view, also known as “fish mouth” view. AC, Anterior commissure; AL, anterior leaflet; PC, posterior commissure; PL, posterior leaflet.

Seven standard views are used to evaluate the mitral valve apparatus. Four views are transesophageal (TE); three are transgastric (TG). The order in which these seven views are obtained is a matter of individual preference so long as the comprehensive exam acquires all of the pertinent information.

Two-dimensional echocardiography provides the examiner with an overview of the anatomy of the mitral valve apparatus and should always be performed prior to additional Doppler-based diagnostic studies (e.g., color flow Doppler [CFD], spectral analysis). Although 2D TEE cannot directly assess the severity of valve regurgitation, its strength lies in its contribution to understanding the mechanisms underlying the regurgitant lesion. Most questions posed by Carpentier’s pathophysiologic triad (etiology, lesion, and dysfunction) can be answered by a comprehensive 2D exam. The following is an overview of the seven views frequently used to examine the mitral valve apparatus.

Four-Chamber View

According to the ASE/SCA guidelines, the two scallops being interrogated with this view are A3 and P1. ⁹ Reiterating Carpentier’s classification of the mitral valve complex, it is apparent that the zone of coaptation cannot be unambiguously defined with this view, since A3 and P1 by definition do not share a common coaptation surface ( Fig. 15-7 and Video 15-3). The inability to properly define which scallop is currently in view at the zone of coaptation is due to this view’s oblique cross-section of the mitral valve. Analysis of the zone of coaptation, rather than the smooth zone of the leaflet, is pivotal to understanding the mechanism(s) of MR. In this view, the anterior leaflet is interrogated posteromedially from the base of A3 to a more anterolateral aspect at its free edge (A2). If the imaging plane is directed more anterolaterally, the left ventricular outflow tract (LVOT) and aortic valve are imaged, and the portion of the coaptation zone imaged would represent A1/P1. Obtaining the more anterolateral view is often facilitated by either by anteflexing the probe or rotating the multiplane transducer to approximately 20 degrees.