Assessment Of Mitral Valve Stenosis

History

If patients with mitral stenosis become symptomatic, dyspnea is the main complaint. Symptoms may be exacerbated by any condition, that increases blood flow across the stenotic valve, such as emotional or physical stress, infection, fever, pregnancy or atrial fibrillation with rapid ventricular response. Atrial arrhythmias may develop in up to 40% of patients and may cause sudden intense dyspnea7. Hemoptysis is rare and is mostly associated with end-stage mitral stenosis. Chest pain occurs infrequently and has to be differentiated from angina pectoris8 The pain is due to right ventricular hypertrophy and rarely from atherosclerotic vascular disease9. In some patients systemic embolisation is the first symptoms of the disease7, the risk related to age and the presence of atrial fibrillation.

Physical examination

The auscultatory findings in a patient with mitral stenosis are an openingssnap and a diastolic murmur7. The first heart sound is accentuated because the prolonged mitral inflow prevents the leaflets from returning to a normal resting position before left ventricular pressure rises at the onset of systole. The rapid rate of pressure rise of the left ventricle then causes the mitral valve to close abruptly. The first heart sound becomes diminished in intensity if the mitral valve is immobile and heavily calcified.

Electrocardiogram

A widened p-wave in the limb leads or a negative P wave in Vi is a sign of left atrial enlargement. Atrial flutter or atrial fibrillation can be observed frequently.

Chest x-ray

The classical chest X-Ray shows an enlarged left atrium (fig 1 arrows), with a normal left ventricular contour, pulmonary artery enlargement and varying degrees of pulmonary congestion.

Figure 1: The classical chest X-Ray shows an enlarged left atrium (arrows), with a normal left ventricular contour, pulmonary artery enlargement and varying degrees of pulmonary congestion.

Diagnostic testing

Assessment of mitral valve stenosis has changed during the last decades from invasive techniques to non-invasive evaluation with echocardiography and Doppler imaging10,11. The role of the catheterization laboratory has changed from a diagnostic tool to a therapeutic tool with the development of percutaneous mitral balloon valvulotomy12.

Echocardiography

Two dimensional echocardiography is able to identify restricted diastolic opening of the mitral valve leaflets due to "doming" of the anterior leaflet and immobility of the posterior leaflet (fig 2).

Figure 2: Parasternal long axis view of a patient with severe mitral stenosis. Typical is diastolic doming of the mitral valve and the hockey-stick appearance of the anterior leaflet.

Planimetry of the orifice area may be possible from the short-axis view (fig 3)13. 2-D echocardiography can also be used to assess the morphological appearance of the mitral valve apparatus, including leaflet mobility, leaflet thickness, leaflet calcification, subvalvular fusion, and the appearance of commissures.

Doppler echocardiography can be used to assess the hemodynamic severity of the obstruction by measurement of the continuous wave Doppler signal across the mitral valve with the modified Bernoulli equation (AP =4v2 )". The transmitral gradient is highly dependent on the RR interval, especially if the patient has atrial fibrillation14. Therefore, the average of 6 to 10 beats has to be taken if the patient has atrial fibrillation. The mitral valve area can be non-invasively derived from Doppler echocardiography with either the diastolic half-time method15 or the continuity equation16. Measurement of the diastolic pressure half-time can be obtained from Doppler echocardiography using the deceleration time. The deceleration time is measured by extrapolating the deceleration of early diastolic flow to the baseline and measuring the time from peak mitral inflow velocity to the point of intersection of the deceleration of flow at the baseline. The product of the deceleration time multiplied by 0.29 provides a diastolic pressure half-time. An empiric constant of 220 for derivation of a mitral valve area from the diastolic pressure half-time was proposed by Hatle11. The formula for calculating mitral valve area is than: MVA = 220/T|/2 (MVA is the mitral valve area in cm2, T|/2 is the pressure half-time in ms). This Doppler derived valve area has a good correlation with valve area obtained by cardiac catheterisation and is now universally applied in almost all echocardiographic laboratories. The pressure half-time method may be inaccurate in patients with abnormalities of left atrial or LV compliance, those with associated aortic regurgitation, and those who have had mitral valvulotomy17.

Figure 3: Parasternal short axis view of the same patient (diastolic image). The mitral valve area can easily be traced to calculate area.

The continuity equation is based on the concept that flow remains constant through all heart valves in the absence of valve regurgitation or shunts18. Therefore mitral valve area can be calculated by equating flow through the left ventricular outflow tract with flow through the stenotic mitral valve orifice. Volumetric flow through an orifice can be measured by Doppler echocardiography as the product of the valve orifice area and time velocity integral of the Doppler flow through the valve. This gives the following calculation for mitral valve area: MVA =(LVOTarea x LVOTTVi )/ MVTV|. (MVA is mitral valve area, LVOTarca is left ventricular outflow area, LVOTtvi is time velocity integral of left ventricular outflow tract velocity and MVTVi is time velocity integral of the transmittal velocity profile. In the a-symptomatic patient who has documented mild mitral stenosis (valve area >1.5 cm2 and mean gradient <5 mm Hg), no further evaluation is needed. These patients usually remain stable for years. If there is more significant mitral stenosis, a decision to proceed further should be based on the suitability of the patient for mitral valvulotomy. Doppler imaging can also be used to estimate pulmonary artery systolic pressure from the velocity signal of tricuspid regurgitation19. Measurement of the right ventricular to right atrial systolic pressure difference can be obtained from the continuous Doppler interrogation of tricuspid regurgitation , applying the modified Bernoulli equation By adding an assumed right atrial pressure, a noninvasive estimation of the pulmonary artery systolic pressure can be obtained.

In patients who lead a sedentary lifestyle, a hemodynamic exercise test with Doppler echocardiography is useful20. Objective limitation of exercise tolerance with a rise in transmitral gradient >15 mm Hg and in pulmonary artery systolic pressure >60mmHg may be an indication for percutaneous valvulotomy if the mitral valve morphology is suitable. A small subset of patients has significant limiting symptoms and yet resting hemodynamics that do not indicate moderate to severe mitral stenosis. If there is a discrepancy between symptoms and hemodynamic data, formal exercise testing or dobutamine stress may be useful to differentiate symptoms due to mitral stenosis from other causes of symptoms.

Exercise tolerance, heart rate and blood pressure response, transmittal gradient, and pulmonary artery pressure can be obtained at rest and during exercise. This can usually be accomplished with either supine bicycle or upright exercise with Doppler recording of tricuspid regurgitation and transmittal velocities21. Patients who are symptomatic with a significant elevation of pulmonary artery pressure (>60 mm Hg), mean transmittal gradient (>15 mm Hg), or pulmonary artery wedge pressure (>25 mm Hg) on exertion have hemodynamically significant mitral stenosis and should be considered for further intervention. Alternatively, patients who do not manifest elevation in either pulmonary artery, pulmonary artery wedge, or transmitral pressures coincident with development of symptoms during exercise most likely would not benefit from intervention on the mitral valve.

Echo-Doppler imaging can also be used to assess severity of concomitant aortic valve disease and tricuspid valve disease in patients with mitral valve stenosis.

Cardiac catheterization

Invasive measurement of transmittal gradient and right ventricular pressures is only needed in those patients, in whom further information is required after two-dimensional echocardiographic and Doppler assessment. In all other patients, invasive measurements is redundant and may confuse the doctor if discrepancies are present with non-invasive testing. Most laboratories use the indirect method of measuring left atrial pressure from the pulmonary artery wedge pressure22.

The accepted approach is measurement of the wedge pressure with a large-bore end hole catheter, firmly wedged into a distal pulmonary artery with a saturation greater than 95%. Pressure measurement is less reliable if a balloon-tipped catheter is used. A dampened pulmonary artery pressure waveform may simulate a true pulmonary artery wedge pressure and may cause a significant overestimation of the transmitral gradient33. Even with a properly performed pulmonary wedge pressure, a 40% to 70% overestimation of the transmitral gradient may occur due to delay in the transmission of pressure33. The Doppler measurements are consistently more accurate than the gradient obtained by catheterisation23.

Assessing suitability for valvulotomy

Percutaneous balloon dilatation of the mitral valve is competitive to mitral valve surgery for the treatment of mitral valve stenosis24,25. Because not all patients are suitable for percutaneous balloon valvulotomy, patient selection is an important component of successful outcome. Although patient factors (age, NYHA classification and pulmonary artery pressure26,27) are important factors determining outcome, mitral valve morphology is the most important factor determining success of mitral valve valvulotomy28. Wilkins et al28 designed a scoring system for patients eligible for mitral valve annuloplasty on the basis of valve characteristics (table 1).

Table 1. Echocardiographic score of the mitral valve

Grade

Mobility

Sub-valvular thickening

Thickening

Calcification

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