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The rhythm can be sinus or atrial fibrillation. A notched P wave or “P mitrale” in leads II and III and/or a biphasic P wave in lead V1 can be present, which reflect left atrial enlargement. If there is right ventricular enlargement and right ventricular failure, they manifest as right axis deviation, incomplete right bundle branch block, and tall R wave in V2 or deep S wave in V6. Right atrial enlargement is represented by high amplitude (≥ 2.5 mm) of the P wave in lead II (Figure 19–5).
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Left atrial appendage enlargement on radiography results in straightening of the left superior cardiac border on the posteroanterior view, which is seen more commonly than the double density, where both the left and right atrial borders are seen when the left atrium enlarges to the right. Occasionally, the left main bronchus is also elevated due to left atrial enlargement. If there is enlargement of the pulmonary veins, an antler configuration is visible. In the frontal projection, when pulmonary hypertension coexists with mitral stenosis, the main pulmonary artery is enlarged and extends beyond the tangent line drawn from the aortic knob to the left ventricular apex (Figure 19–6). Chronically elevated pressure in the pulmonary veins can result in redistribution of blood flow to the upper lobes, seen as cephalization of the pulmonary vessels, and interlobular edema at the bases appearing as Kerley B lines. When pulmonary arterial hypertension ensues, pruning of the peripheral vessels is present. The left ventricular contour is usually normal, and if there is right heart failure, the right ventricle fills the retrosternal space in the lateral projection.
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M-Mode, Two-Dimensional, and Three-Dimensional Echo Imaging
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Echocardiography is the imaging modality of choice to investigate the severity of mitral stenosis because it offers both anatomic and functional information. Due to commissural fusion, the anterior and posterior leaflets move anteriorly, and the E-F slope is reduced on M-mode images (Figure 19–7). Two-dimensional echo images show fusion of the commissures, thickened and calcified leaflets, and chordae tendineae. During diastole, the posterior leaflet is fixed and the anterior leaflet opens and appears like a hockey stick in the parasternal long axis view. In the parasternal short axis view, the mitral valve has a “fish-mouth” appearance in the open position (Figure 19–8). The left atrium is enlarged and could have visible thrombus. The Wilkins echocardiography score is used to determine suitability for successful balloon valvotomy, where 1 to 4 points are assigned in each of four categories: leaflet thickening, leaflet calcification, leaflet mobility, and subvalvular apparatus thickening. A score of ≤ 8 predicts a favorable outcome.
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Over the past decade, the availability of real-time, three-dimensional (3D) echocardiographic imaging allows better visualization of the stenotic valve in 3D space and provides simultaneous acquisition of orthogonal views of the mitral valve, which improves the accuracy of planimetry. 3D imaging also provides additional information on the orientation of the subvalvular apparatus, which permits detailed planning prior to interventional procedures (Figure 19–9).
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Quantification of Mitral Stenosis
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The parameters used to diagnose and quantify mitral stenosis are depicted in Table 19–2. Continuous wave Doppler flow is used to measure the mean mitral gradient based on the Bernoulli equation where velocities across the mitral valve are measured and the mean pressure gradient is the Σ4v2/n, where v is the velocity and n is the number of individual velocities measured. The mean gradient is determined by tracing the velocity time integral (VTI), which then is automatically calculated by ultrasound software packages. This measurement is straightforward to obtain but is dependent on heart rate, heart rhythm, and global left ventricular systolic function. The mean gradient can be low in bradycardia or in low cardiac output states (Figure 19–10).
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The mitral valve area (MVA) can be measured by planimetry of the mitral orifice in the parasternal short axis view. The area can be overestimated if the imaging plane is not at the level where the orifice is the smallest or underestimated when there is significant calcification. The availability of 3D echo images has improved the accuracy and reproducibility of this technique. The advantage of this method is that it is easy to obtain if image quality is excellent (Figure 19–11A).
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Using Doppler measurements, estimating the MVA by pressure half-time (PHT) is the simplest Doppler method. The PHT is the time during diastole when the transmitral gradient falls to 50% of the initial peak gradient.
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This method is invalid immediately after balloon valvotomy and in patients with severe aortic regurgitation or in conditions with high filling pressures or decreased left ventricular compliance. In atrial fibrillation, at least five measurements should be recorded and averaged (Figure 19–11B).
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The second Doppler method used to calculate MVA is based on the principle of energy conservation. The continuity equation used to calculate the cross-sectional area of the mitral valve is:
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where LVOT D is the diameter of the left ventricular outflow tract measured in the parasternal long axis view and VTI is the time velocity integral of the LVOT and mitral valve (MV), respectively. This method assumes that flow across the mitral valve is equivalent to flow across the aortic valve. It is inaccurate when there is significant concomitant mitral or aortic regurgitation. Mitral regurgitation can overestimate the severity of mitral stenosis, and aortic regurgitation can underestimate the severity mitral stenosis. In addition, poor visualization of the LVOT on two-dimensional imaging can significantly affect the MVA (Figure 19–11C).
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The third Doppler method used in MVA calculation is the proximal isovelocity surface area (PISA) method, where
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This method is the most complex and technically challenging because it requires measuring the radius of PISA in the left atrium and the angle between the two mitral leaflets in the left atrium. However, since it is uses only velocity measurements, it is unaffected by factors that alter flow conditions (Figure 19–11D).
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Exercise-Stress Echocardiography
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When symptoms are discordant with two-dimensional and Doppler findings, exercise-stress echocardiography can be used to assess mitral stenosis severity since exercise increases cardiac output and gradient without changing the MVA. The mean mitral gradient and pulmonary artery systolic pressure are measured during each stage of bicycle exercise and are correlated with the perception of dyspnea. Dobutamine stress can also be employed, and a dobutamine-induced mean transmitral gradient of ≥ 18 mm Hg has a 90% accuracy rate for predicting a high-risk subpopulation. The tricuspid annulus S-wave velocity reflects right ventricular function, and its response during exercise is an independent predictor of functional capacity. A reduced S-wave velocity is a marker of poor prognosis.
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Cardiac Computed Tomography and Magnetic Resonance Imaging
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Currently, computed tomography and magnetic resonance imaging are not routinely used to assess mitral valve morphology. However, due to high image resolution of anatomic features, the role of both imaging techniques is being investigated.
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Cardiac Catheterization
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When echocardiographic data are ambiguous and/or suboptimal, often in situations when left atrial and left ventricular compliance are altered, cardiac catheterization is used to simultaneously and directly measure the left atrial and left ventricular pressures during diastole. On the left atrial pressure tracing, the “a” wave is accentuated, due to elevated pressure during atrial contraction. The left atrial pressure tracing also records an enlarged V wave, but it is not exclusively seen in mitral stenosis (Figure 19–12). If direct left atrial access is not performed, then the pulmonary capillary wedge (PCW) pressure is used as the surrogate for left atrial pressure. When PCW pressure is used, the operator must be absolutely certain that the balloon right heart catheter is in the “wedged” position by evaluating the hemodynamic waveform and, ideally, by measuring the oxygen saturation distal to the catheter, which originates from the oxygenated blood of a pulmonary vein. The diagnostic finding of mitral stenosis is the presence of a diastolic gradient when the PCW pressure or left atrial pressure is measured simultaneously with the left ventricular pressure. The magnitude of difference in gradients at end-diastole reflects the severity of obstruction. Tachycardia will increase and bradycardia will decrease the transmitral gradients. The Gorlin formula is used to calculate the MVA, where
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This formula accurately determines MVA only in patients with isolated mitral stenosis. In those who also have mitral regurgitation, the Gorlin formula overestimates the degree of mitral stenosis because it does not account for the increased regurgitant flow across the valve. Pulmonary arterial pressure is also measured and is usually elevated proportionally with left atrial hypertension in the absence of intrinsic pulmonary disease. Similar to noninvasive measurements, in the setting of atrial fibrillation, a minimum of five measurements should be collected and averaged since the mean gradient is dependent on the R-R interval.