Echocardiography has been instrumental in providing significant mechanistic insights into the complex pathophysiology of SMR that involves the myocardium, the mitral subvalvular apparatus, and mitral annulus. Therefore, the information derived from a pre-operative assessment of secondary MR using echocardiography can provide the surgeon and cardiologist with a more directed approach to the management of these patients and help tailor the operative strategy in individual patients. Clear determination of the mechanism for secondary MR is particularly important in a pre-operative setting as it can influence the approach and course of clinical management in the pre-operative setting.

The value of transthoracic (TTE) and intra-operative transesophageal (TEE) echocardiography has been established in terms of assessing the severity of MR and predicting postoperative outcomes [40–44]. However, in the case of functional MR the dynamic nature of the lesion warrants that the severity of MR is assessed prior to surgery rather than intra-operatively (Fig. 8.2). Due to the influence of various hemodynamic factors arising from general anesthesia and load conditions, there is clear evidence indicating that severity of functional MR assessed by TEE intra- operatively frequently results in a significant under-estimation of the severity of functional MR [45, 46].

Echocardiographic assessment of SMR should include an assessment of LV function and quantitation of the degree of MR. The assessment of LV function should be relatively thorough and include ejection fraction, LV dimensions, assessment of wall-motion abnormalities, and structural abnormalities of the mitral valve chordae and papillary muscles. There is preliminary evidence to indicate that the use of more novel echocardiographic techniques such as strain and strain rate by speckle tracking in a pre-operative setting may also have benefit in terms of providing prognostic information. A prospective study of 38 consecutive patients with chronic severe MR scheduled for mitral valve replacement revealed that the pre-operative 2D speckle- tracking analysis at the level of the inter-ventricular septum detected early abnormalities in left ventricular contractile function which was a powerful predictor of early postoperative left ventricular ejection fraction decreases in patients with chronic severe MR [47].

An integrative approach to the quantitation of MR should also be performed including Doppler techniques for direct quantitation as well as supportive echo data (left atrial size, LV chamber size, and pattern of pulmonary vein flow). Particular focus should be given to the assessment of the mitral valve leaflets. Concomitant valvular pathology such as flail leaflet, endocarditis, and/or severe calcific mitral valve disease should be clearly distinguished since mitral valve leaflets in secondary MR should be normal. In ischemic MR, the leaflets are typically apically displaced, tethered, and may have restricted mobility, especially the posterior leaflet. The presence of a bend in the body of the anterior leaflet also suggests tethering of the basal or strut chordae, which can contribute to impaired coaptation (Fig. 8.3). The incomplete mitral leaflet closure pattern is more difficult to assess in the long axis views, in which the continuity between the anterior mitral leaflet and aortic valve is demonstrated, as the mitral annular plane is not defined in this view. This pattern is best seen in the apical 4- chamber view. The leaflets are apically displaced, tethered, and may have restricted mobility, especially the posterior leaflet. There are several methods to quantify the degree of tethering. The most common is a simple area measurement from the leaflets to the annular plane (Fig. 8.4). Another measure is leaflet coaptation height or depth, which simply measures the maximal distance from the leaflet tips to the annular plane and appears to correlate with the presence and severity of ischemic MR [48]. More recently, 3-dimensional echocardiography has been applied to quantify leaflet tethering by measuring the tethering distance from papillary muscle tip to the mitral annulus, and measurement of tenting volume (volume from leaflets to annular plane) and may provide additional data to assess tethering [21, 49].

Standard quantitative measures of the severity of MR such as the vena contracta (narrowest part of the regurgitant jet as it passes through the orifice), the effective regurgitant orifice area (EROA; area of the vena contracta) and regurgitant volume (RVol) are important, as these have been shown to have important long term prognostic implications. There is evidence correlating an EROA greater than 20 mm² or an RVol greater than 30 ml with a doubling of mortality compared to patients without MR. These thresholds are also significantly different to patients with degenerative valve disease where only an EROA greater than 40 mm² predict a poor outcome [17, 50].

Obtaining accurate quantitative measurements of the severity of SMR is particularly challenging due to the dynamic nature of secondary MR. Significant geometric distortions of the mitral valve annulus result in highly variable failure of coaptation along the closure line. Furthermore the complexity can be further compounded by the elliptical shape of the regurgitant orifice or the presence of several separate regurgitant orifices along the closure line which has significant implications particularly for the 2D assessment of the vena contracta and the proximal isovelocity surface area (PISA). In the case of a circular orifice the vena contracta, which is conventionally assessed in the parasternal long axis view or apical three- chamber view where the color jet is perpendicular to the ultrasound beam, provides a reasonable estimate of severity of regurgitation. However in the case of an elliptical orifice the extent of regurgitation will be underestimated if measured in the conventional views but possibly overestimated if assessed in the apical two chamber view along the long axis of the closure line [51]. One way to overcome this limitation is to measure the vena contracta area using 3D assessment [52]. Direct planimetry of the vena contracta in 3D greatly improves accuracy of the assessment of EROA with excellent correlation between RVol derived from EROA measured by 3D planimetry compared to measures obtained using velocity-encoded cardiac magnetic resonance imaging (CMR). Comparisons of EROA and RVol measured obtained by 2D and 3D echo have revealed that 2D echo consistently underestimated the EROA and RVol when compared to 3D echo [53], although there is some evidence to suggest that the mean of the vena contracta measured in the apical 4-chamber and 2-chamber views may correlate well with real-time 3D echo assessment [54]. Measurement of PISA in functional MR by 2D echo is also limited by the array of different morphologies of the PISA shell based on the underlying geometry of the valve. The array of different morphologies of PISA include those that are located away from the midpoint of the closure line, those that are dominant in both the medial and lateral aspects of the closure line and those that are relatively small in the center [6, 53].

Traditionally, PISA measurements are made in mid-systole where it is assumed that the PISA is maximal at this point, therefore EROA would be largest and MR the worst. Furthermore the mid-systolic point is also believed to be coincident with the point where the regurgitant volume is at its peak, hence the best point to estimate the peak regurgitant flow rate, which in turn is used to derive the maximal EROA and regurgitant stroke volume. However, in the case of functional MR, the dynamic nature of this lesion results in significant variation in the EROA and regurgitant flow rates throughout the cardiac cycle, with maximum regurgitation occurring in early and late systole and minimum regurgitation in mid-systole when there is improved coaptation and a smaller EROA due to maximal closing forces [31]. Hence an isolated PISA measurement made in mid-systole, as conventionally performed, would in fact result be measuring the minimum regurgitation in the cardiac cycle and thus underestimating the true hemodynamic load of regurgitation. On the other hand, utilizing a PISA measurement from early or late systole where the EROA and regurgitant volume is likely to be the largest, would not be coincident with the mid- systolic peak regurgitant velocity, thus overestimating the EROA and regurgitant volume [6]. One alternative method for the assessment of regurgitant volume is the volumetric method where aortic forward stroke volume is subtracted from the total stroke volume through the mitral annulus which provides an estimate of mean EROA throughout systole and therefore independent of the dynamic variations in the EROA throughout the cardiac cycle. However this time-consuming method is also subject to a relatively high degree of variability based on a number of factors including dependence on accurate measurement of the left ventricular outflow tract and the mitral annulus and good image quality [6, 54, 55].

Stress echocardiography has a well-established role in the pre-operative assessment of patients with CAD and LV dysfunction. Dobutamine stress echocardiography (DSE) is the most widely used method for assessing viable myocardium in patients with LV dysfunction and who may benefit from coronary revascularization [56]. However, DSE should not be used for the assessment functional MR due to the potential effects of dobutamine in reducing the preload and afterload hence severity of mitral regurgitation [57, 58]. The dynamic characteristics of ischemic MR can be effectively assessed and quantified using exercise stress Doppler echocardiography [59].

Furthermore, the use of a dedicated exercise table in the semi supine position allows the continuous monitoring of Doppler and echo parameters during exercise, including exercise-induced changes in the EROA, systolic pulmonary artery pressure and LVEF.

Generally, a decrease in ischemic MR severity during exercise is observed although there have been studies which demonstrate that in the absence of acute ischemia, exercise can potentially also increase functional MR (Fig. 8.5) [60]. The degree of exercise induced increase or decrease in MR appear to relate to changes in LV synchronicity, in mitral valve geometry (tenting height and area) and in mitral valve apparatus (papillary muscle displacement) but not to changes in global LV function [61]. In patients who demonstrate an increase in the amount of MR with exercise, the increase is generally small particularly in patients with an inferior infarction, since these patients have recruitable contractile reserve in the basal segments [62]. The information obtained from exercise stress echocardiography is further highlighted in determining surgical treatment strategies for patients with ischemic MR. Patients with minimal to mild MR (ERO >10 mm² and <20 mm²) at rest with no significant changes or exercise induced changes in MR as a result of the presence of functional and recruitable myocardium in the infero-basal territory since these patients may only require bypass grafting alone, as opposed to patients with who demonstrate a dynamic increase in MR (change in ERO ≥ 13 mm²) during exercise where combined revascularization and valve repair surgery might be a more favorable strategy.

Even though exercise stress echocardiography provides crucial information on myocardial viability, inducible ischemia and the dynamic nature of the MR and the use of exercise stratification in the pre-operative setting seem intuitive; its value to predict the results of surgery still remains to be demonstrated [61]. The proximal isovelocity area and the Doppler methods have been identified as the most accurate measures for defining the dynamic changes of MR [59], but the evolution of MR during exercise can vary dramatically with each individual case and does not appear to correlate with the degree of MR at rest [63]. Nonetheless, an exercise induced increase in EROA ≥13 mm² has been identified to have independent prognostic value [63, 64].

Pre-operative echocardiographic assessment of SMR should include a comprehensive and systematic analysis of all the components of the mitral valve apparatus and the underlying myocardium.