Mitral Regurgitation

Adam M. Dryden

A. Stephane Lambert

INTRODUCTION

Transesophageal echocardiography (TEE) has become a standard of care in the cardiac operating room, allowing the anesthesiologist to play an important part in the surgical decision-making process. In that role, few areas are as challenging as the assessment of intraoperative mitral regurgitation (MR). Yet few applications of intraoperative TEE have as much impact on the course of surgery and patient outcome as the evaluation of MR.

ANATOMY

The mitral valve (MV) is bicuspid and consists of a large anterior leaflet and a smaller posterior leaflet (Fig. 8.1). The anterior leaflet covers about two-thirds of the surface area of the valve. The posterior leaflet wraps around the anterior leaflet accounting for about two-thirds of the circumference of the valve. The leaflets join at the anterolateral and posteromedial commissures. The posterior leaflet is further divided anatomically into three scallops, whereas the anterior leaflet does not have scallops per se. It is important to keep in mind when considering TEE imaging planes of the MV that coaptation of the two leaflets forms a semicircular, not linear, path. The valve is encircled by a dynamic fibromuscular ring, the mitral annulus. It is saddle shaped and plays an important role in proper valve closure by reducing its diameter in systole. In various disease states, the mitral annulus dilates and tends to flatten, causing increased stress on the mitral leaflets that further impairs their function (1). The MV attaches to two papillary muscles, anterolateral and posteromedial, through chordae tendineae. Each papillary muscle sends off chordae tendineae to both mitral leaflets. In systole, the papillary muscles contract to keep the chordae tendineae taut and prevent prolapse of the leaflets into the left atrium (LA) (2).

The blood supply to the papillary muscles is variable, but in most patients, the anterolateral papillary muscle receives blood supply from branches of two major coronary arteries, the left anterior descending (LAD) and the left circumflex (LCx), while the posteromedial papillary muscle receives blood supply from a single coronary artery, the right coronary artery (RCA). This is important clinically because the incidence of posterolateral papillary muscle infarction and ischemia is much higher than that of anterolateral papillary muscle infarction.

There are three types of chordae tendineae. First-order (or primary) chordae attach to the edge of the leaflets, second-order (or secondary) chordae attach to the body of the leaflets, and third-order (or tertiary) chordae attach to the base of the posterior leaflet. The anterior leaflet of the MV shares the same fibrous attachment as the aortic valve, an area sometimes referred to as the fibrous body or crux of the heart. This relationship is important and surgery to one valve can result in impaired function of the other.

NOMENCLATURE

There exist three nomenclatures of the MV in the literature. The classic anatomic nomenclature refers to the three scallops of the posterior leaflet of the MV as anterolateral, middle, and posteromedial, according to their anatomic location (3) and are often shortened to lateral, middle, and medial for brevity of conversation. The anterolateral scallop is the closest to the LA appendage. No specific description is given to any part of the anterior leaflet. Note that the names of the anterolateral and posteromedial commissures are often shortened to lateral (near the LA appendage) and medial (near the septum) commissures, respectively.

The most commonly used nomenclature among echocardiographers is attributed to Carpentier (4) and defines the three scallops of the posterior leaflet as P1, P2, and P3, where P1 is closest to the LA appendage. It also defines three areas of the anterior leaflet as A1, A2, and A3, opposite to the corresponding scallops of the posterior leaflet. This nomenclature has been adopted by the American Society of Echocardiography Council for Intraoperative Echocardiography and the Society of Cardiovascular Anesthesiologists Task Force for Certification in Perioperative Transesophageal Echocardiography, making it the most commonly used system (5).

FIGURE 8.1 A: Mitral valve anatomy. B: 3D en face view of mitral valve with anatomic landmarks. RCC, right coronary cusp; LCC, left coronary cusp; NCC, non-coronary cusp; IAS, inter-atrial septum; LAA, left atrial appendage; Ant, anterior mitral leaflet; Post, posterior mitral leaflet.

A third nomenclature, often called the Duran nomenclature (6), describes the MV segments according to their attachment to the papillary muscles. It refers to the three scallops of the posterior leaflet as P1, PM (middle), and P2, where P1 is closest to the LA appendage. The PM scallop is further subdivided into PM1 laterally and PM2 medially. Duran also divides the anterior leaflet into two areas, A1 and A2, opposite to the corresponding scallops of the posterior leaflet. The two commissural areas of the valve are defined as C1 (between A1 and P1) and C2 (between A2 and P2). The rationale for this nomenclature is that every part of the MV attached to the anterolateral papillary muscle is given the number 1 and every part of the MV attached to the posteromedial papillary muscle is given the number 2. A schematic representation of the three nomenclatures of the MV is shown in Figure 8.2 (7).

Each institution or group of practitioners favors one nomenclature over another and it does not matter which one is used, as long as every member of the team agrees on which terminology is used. The reader is encouraged to have a basic understanding of all of them to avoid confusion. For example, P2 refers to a different area of the valve in the Carpentier and Duran nomenclatures.

ETIOLOGY AND MECHANISM OF MR

MR can be classified according to its etiology (Table 8.1), or more simply, according to the pathophysiologic mechanism leading to the regurgitation. Carpentier proposed the now widely used classification of MR based on leaflet motion (Fig. 8.3) (8,9).

FIGURE 8.2 Nomenclature of the mitral valve. Schematic representation of the various nomenclatures of the mitral valve. The mitral valve is shown with its relationship to the aortic valve, viewed from the left atrium. See text for details. (Adapted from Lambert AS, Miller JP, Merrick SH, et al. Improved evaluation of the location and mechanism of mitral valve regurgitation with a systematic transesophageal echocardiography examination. Anesth Analg 1999;88:1205-1212, with permission.)

TABLE 8.1 Causes of Mitral Regurgitation

Congenital
	Endocardial cushion defect Associated with other pathologies (e.g., corrected transposition)
Myxomatous degeneration
Rheumatic (often accompanied by mitral stenosis)
Endocarditis
	Bacterial, viral, etc.
Cardiomyopathy
	Dilated (ischemic, idiopathic, ethanol, drug related) Hypertrophic
Other
	Systemic lupus Rheumatoid arthritis Ankylosing spondylitis

FIGURE 8.3 Carpentier classification. Carpentier classification of mitral regurgitation (MR) based on leaflet motion. A, B: The leaflet motion is normal and the MR jet tends to be central. C, D: There is excessive leaflet motion and the MR jet is typically directed away from the diseased leaflet. E, F: The leaflet motion is restricted and is further subdivided into type 3a (structural) and type 3b (functional). In type 3 lesions, the regurgitant jet may be directed away from the diseased leaflet if only one leaflet is affected, or it may be central if both mitral leaflets are equally affected. (Courtesy Dr. Gregory M. Hirsch.)

FIGURE 8.4 Excessive leaflet motion. A: Billowing (or scalloping) refers to a situation where part of a mitral leaflet projects above the annulus in systole, but the coaptation point remains below the mitral annulus. B: Prolapse is used to describe the excursion of a leaflet tip above the level of the mitral annulus during systole, causing regurgitation. C: The term flail is reserved for a situation where a leaflet edge (arrow) is flowing freely into the left atrium in systole.

Type 1 lesions are defined by normal leaflet motion.
- Most type 1 lesions are a result of annular dilation typically with central MR.
- Less common mechanisms of type 1 MR include MV clefts, aneurysms, perforation, or destruction, commonly as a result of endocarditis.
In type 2 lesions, there is excessive mitral leaflet motion and the MR jet is typically directed away from the diseased leaflet.
- Billowing (or scalloping) refers to a situation where part of a mitral leaflet projects above the annulus in systole, but the coaptation point remains below the mitral annulus (Fig. 8.4).
- Prolapse is used to describe the excursion of a leaflet tip above the level of the mitral annulus during systole, causing regurgitation.
- Flail is where a leaflet edge is flowing freely into the LA in systole, as a result of one or more ruptured chordae tendineae. The distinction between severe prolapse and flail is sometimes difficult to make because the ruptured chordae may not be visible by echocardiogram. Also, this distinction is often unimportant clinically, since the surgical treatment of both is the same.
Type 3 lesions refer to restricted leaflet motion and the MR jet is typically directed toward the diseased leaflet. When both leaflets are equally affected, the jet may be central.
- In type 3a, the restriction is in both systole and diastole, and the leaflet problem is usually “structural” (most often rheumatic). When structural restriction is present, MR often coexists with some degree of mitral stenosis.
- In type 3b, the restriction is only in systole and leaflet motion is normal in diastole. Here the problem is “functional” and the valve structure is normal, but proper coaptation is prevented by systolic tethering of the mitral leaflets due to left ventricular (LV) dilation and/or papillary muscles displacement. Coronary artery disease is often the etiology of type 3b MR, which is also commonly referred to as ischemic MR. An ischemic (i.e., stiff) papillary muscle may also temporarily restrict leaflet motion, causing failure of coaptation.

APPROACH TO THE TRANSESOPHAGEAL ECHOCARDIOGRAPHY EVALUATION OF MR

In the setting of MV surgery, the intraoperative TEE evaluation of MR requires one to answer four basic questions: (a) Where on the MV is the lesion? (b) What is the mechanism of the MR? (c) How severe is the MR? (d) Can the valve be surgically repaired?

FIGURE 8.5 Transgastric short-axis view Transgastric short-axis view of mitral valve, showing the scallops of the posterior leaflet, with the normal “indentations” between the scallops. Note that the relative size of the scallops and the depth of the indentations are highly variable.

Where on the Mitral Valve Is the MR and What Is the Mechanism of MR?

Localizing the MR With Two-Dimensional Echocardiography

Determining the precise anatomic location of the MR jet or jets is paramount in operative planning. While this may seem at first like a simple task, it is often done with haste resulting in incomplete information. This determination relies on the echocardiographer’s thorough understanding of normal mitral anatomy and ability to achieve optimal TEE views. The echocardiographer must also be able to appreciate variations in the anatomy, which predispose or contribute to MR. Finally, the echocardiographer must develop an eye for temporally brief but important movements of the MV.

The hallmark view for localizing MR jets is a transgastric basal short-axis (TG-SAX) view of the MV. The expert echocardiographer can often localize the MR from only this anatomic view by appreciating leaflet restriction or excessive motion (prolapse or flail) relative to the line of coaptation. Most, however, should at least be able to appreciate anatomic variations in the size of scallops of the posterior leaflets as well as the depth (relative to the annulus) of the indentations separating the scallops. The presence of commissural leaflets should also be noted, as well as any calcification of the mitral annulus or leaflets, because they may affect the surgeons’ ability to repair the valve. An example of this view is shown in Figure 8.5.

The addition of color flow Doppler (CFD) makes this view even more informative. Appreciating the location of CFD signals across the MV provides the base for the remainder of the examination. The exact locations of perforations in the valve (type 1 lesions) can be identified. CFD also helps to differentiate pathologic clefts from deep indentations, which may be a physiologic variant. A more detailed examination of the anatomy and characteristics of the MR jets found in this TG-SAX view can then be undertaken in midesophageal (ME) views (described later this chapter).

Localizing the MR With Three-Dimensional Echocardiography

Historically, the process of generating three-dimensional (3D) echocardiography datasets was complex and time consuming, and the images were of marginal quality. Advances in ultrasound transducers and computer processing (both at the hardware and software levels) have removed such barriers to 3D imaging. Consequently, 3D is no longer an adjunct to two dimensional (2D), but it is an integral part of the comprehensive MV evaluation.

The MV is particularly well suited to 3D imaging because of its proximity to the base of the heart, and because the central axis of the valve is perfectly aligned with the ultrasound beam. The 3D “en face” view of the MV is arguably the prettiest view in all of echocardiography, and of all the applications of 3D TEE, the
intraoperative imaging of structural MV disease is probably one of the most useful. Not surprisingly, the body of literature devoted to 3D echo of the MV has outpaced that of all other cardiac structures.

Three-dimensional echocardiography has several advantages over 2D imaging. In many cases, 3D images are intuitively more obvious than 2D images and they may facilitate communication with the surgical team. Images can also be manipulated on the screen, allowing an infinite number of perspectives. Moreover, 3D can provide information that is unavailable with 2D echo.

As the image acquisition technique (i.e., “knobology”) is quite different between manufacturers and even between models from the same manufacturer, the reader is encouraged to seek out specific training for the devices they use and to consult the operating manual.

The acquisition of a 3D dataset with as much relevant anatomic and temporal information as possible is key to the successful examination of the MV. An idealized ME commissural view is obtained for lateral-medial orientation and a 90° orthogonal plane is added for anteroposterior orientation of the images. Once these two 2D images are optimized with careful TEE probe manipulation, the 3D dataset is acquired as per manufacturer recommendation. Where possible, multibeat image acquisition is recommended for increased resolution, but this requires the patient to be in sinus rhythm and it is facilitated by periods of apnea.

CFD can be applied to certain 3D dataset acquisitions (manufacturer and model dependent) and en face views from the LA perspective readily demonstrate MR, allowing precise 3D determination of the location, size, and direction of a mitral regurgitant jet. Note that the addition of CFD signal to a 3D image can dramatically decrease both temporal and spatial resolution. Multibeat acquisition of images is recommended whenever possible.

The 3D dataset may be examined by simply zooming and rotating the 3D representation of the valve until the origin of the jet is located. To avoid errors in orientation, an understanding of the valve in relation to surrounding internal cardiac landmarks (e.g., LAA, aortic valve, interatrial septum) becomes extremely important (see Fig. 8.1B).

Another 3D tool allows the simultaneous visualization of 2D orthogonal planes of the MV, all of which can be adjusted at will (Fig. 8.6). This is a more structured approach to MR localization than the 3D en face view discussed above. While this modality cannot be displayed in real time, it can still be quickly obtained in the operating room. In effect, it provides a deconstruction of the MV into three adjustable 2D planes, and allows accurate recognition of any part of the valve, by confirming the position of the scanning planes in the other two axes. This makes for very accurate localization of pathology. Note that this does not replace a thorough understanding of 2D MV scanning planes, but rather supplements it. An idealized view of the TG-SAX view of the MV can be recreated and the locations of leaks with additional ME view can be obtained simultaneously. Serial cuts in any plane can be made to fully appreciate the location of the MR jets.

Characterizing MR jets with 2D imaging is often sufficient, but 3D imaging adds a depth of understanding as to their precise location and mechanism.

FIGURE 8.6 Multiple planes view with color Doppler and leaks in TG-SAX. A: Multiple planes view showing the origin of regurgitant jet making precise localization in the idealized transgastric short-axis view (left lower panel). B: A transgastric short-axis reconstruction from a color three-dimensional dataset showing the location as well as the size of regurgitant orifice. The surface area of this orifice is known as the vena contracta area (VCA) or actual regurgitant orifice area (AROA).

FIGURE 8.7 Systematic 2D examination of the mitral valve. Sequential examination of mitral valve from midesophageal windows. Manipulations of the probe and imaging angle enhance the assessment of leaflet anatomy. A: Atrial view is similar to direct intraoperative visualization. B: Anterior view mimics the right-to-left orientation of a clinician facing the patient as well as the echo display from the transverse (0°) plane. ME, mid-esophageal.

Systematic Examination of the Mitral Valve in 2D

Once the location of the MR jet(s) and the general mechanism of MR have been established, each segment of the MV must be examined in detail. This is done in 2D and 3D.

The 2D systematic examination of the MV combines techniques described in several publications (7,10,11) and aims at obtaining multiple redundant views of all parts of the valve and identifying each mitral segment using internal, recognizable cardiac landmarks. Accordingly, the sequence described in the following text and illustrated in Figure 8.7 is suggested:

Begin the examination in the ME four-chamber view at 0° of transducer rotation, with the MV in the center of the screen. The anterior mitral leaflet is medial, adjacent to the aortic valve and the posterior leaflet is lateral. Slight withdrawal (10) or anteflexion (7) of the probe brings the left ventricular outflow tract (LVOT) into the plane of the scan, which demonstrates the anterior segments of the valve (A1/A2, P1/P2). Conversely, with slight insertion (10) or retroflexion (7) of the probe, the LVOT disappears from the scanning plane, allowing the examination of the posterior segments of the valve (A2/A3, P2/P3). The entire MV can therefore be seen at 0° of transducer rotation, by gently anteflexing or retroflexing the probe. A note of caution: Because of anatomic variations, the authors do not believe that the average echocardiographer can consistently discriminate between P1 and P2, or between P2 and P3 using only 0° views.
Obtain an ME mitral commissural view by rotating the imaging array to obtain the best possible cut through the commissures. This is usually achieved between 60° and 90° (11). This cross-section typically demonstrates P1 laterally, P3 medially, and variable amounts of anterior leaflet in the middle. The apparent double orifice stems from the semicircular coaptation between the leaflets of the MV. The presence and severity of disease at the level of the commissures can be evaluated here. Finally, the annular diameter (lateral-medial diameter) should also be measured to determine the contribution of annular dilation as a mechanism of MR.
Next, the ME two-chamber view is obtained by rotating the transducer forward to approximately 80 to 100°. In addition, by turning the shaft of the probe leftward and rightward, three reproducible crosssections can be obtained, allowing further identification of the valve segments (7,10).
Then, the ME long-axis view is obtained by rotating the transducer to approximately 130 to 150°. This provides a cut through the center of each MV leaflet, which allows reliable identification of A2 and P2 (11). As this view cuts across the saddle-shaped annular plane at its most superior aspect, it is a preferred view for assessing MV prolapse because it avoids the false positives which occur while using the ME fourchamber view. The second annular diameter (anterior-posterior diameter) should be measured in this view again to determine the contribution of annular dilation as a mechanism of MR.
Finally, the probe is advanced into the stomach and the TG-SAX view of the MV is obtained (7,11). This cross-section is useful to diagnose clefts and perforations and color Doppler provides additional information on the origin of the regurgitant jet(s) as described in the previous section.

Technically, it is extremely important to remember that the classic imaging planes described in the preceding text are obtained only when the echo scan crosses through the center of the MV.

Indeed, turning of the probe shaft from the ME commissural or ME long-axis views will provide a lot of additional information from transitional images, but it may be misleading to the novice eye as it requires 3D reconstruction in the experienced operator’s mind. For example, gently turning the probe to the right in the commissural view will reveal more of the anterior leaflet, showing not only A2 but also extending toward A1 and A3. Turning the probe to the left will reveal more of P2, not only P1 and P3 on either side as expected. Likewise, in the long-axis view, a well-centered scan usually demonstrates A2 and P2, but slight rotation of the probe to the right will move the scan toward A3/P3 and slight rotation to the left will move the scan toward A1/P1. In addition, 3D echocardiography has revealed that these relationships do not always hold true, especially when there is rotation or distortion of the heart by chronic disease (12).

The whole detailed examination can be done very quickly when one becomes familiar with it. The recommended sequence of views mentioned in the preceding text provides sufficient redundancy in the segment identification that in the author’s experience results in high accuracy and consistent reliability. Whatever the sequence of views, the examination should be consistent and systematic. As in other aspects of echocardiography, repetition is important and learning to recognize the variants of normal and the wide spectrum of pathologies are better appreciated.

Systematic Examination of the Mitral Valve in 3D

A 3D en face view of the MV from the LA (“surgeon’s view”) is obtained and saved to the study for subsequent analysis. Good-quality images can provide even the novice observer with an instantaneous appreciation of several mitral pathologies, especially mitral leaflet prolapse or flail (Carpentier, type 2). However, MR due to leaflet restriction (Carpentier, type 3a and type 3b) tends to be much trickier to diagnose with a single 3D image alone. Note that en face views can be obtained from the LA perspective—the surgeon’s view—or from the LV perspective, a projection analogous to a computerized tomography image. While the LA view shows the surgeon exactly what to expect at the time of atriotomy, the LV perspective provides an opportunity to show the surgeon a vantage point that cannot be obtained with the heart open.

Software packages allowing multiple simultaneous 2D reconstructions from the 3D datasets can then be manipulated to create three optimized views. Often, the most information can be obtained when a commissural view, a long-axis, and a transgastric short-axis view are recreated with the 3D dataset as shown in Figure 8.8. Annular dimensions can be measured from this recreation. Serial scans to assess the movement of both leaflets throughout the cardiac cycle can also be obtained in the long-axis view, by moving the cursor in the short-axis plane. When lesions of note are seen, their exact location can be more easily pinpointed, based on their location in the other orthogonal views. Any lesions of interest can also be recorded as a separate video loop in the final study.

In fact, once well understood, all the views contained in the systematic 2D examination of the MV can be obtained off-line, with a full-volume dataset and off-line manipulation. If a good 2D TEE diagnosis rests in the hands of the person manipulating the probe, a good 3D TEE diagnosis rests in the hands of the person manipulating the mouse!

Quantification and Advanced Assessment of MR Mechanisms

From the images obtained, a wealth of data can be obtained that is highly relevant to the repair of MVs. This is especially important in the modern age of catheter-based repair techniques, because these offer less flexibility in the types of lesions that they can treat, and also because there is no possibility to adjust the diagnosis
at the time of atriotomy. Thus, advanced analysis of the causative lesions must be undertaken. The devicespecific utility of these measurements for various repairs and devices is beyond the scope of this chapter, but the measurements can be obtained from the 2D or 3D datasets described previously. Most rely on measurements performed on an ME LAX view (again, either in 2D or the idealized 3D reconstruction).

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