 |
| Fig. 1 |
In mitral stenosis, fusion of the commissures between the mitral
valve leaflets causes obstruction to left ventricular inflow.
The leaflets also become infiltrated with fibrous tissue, which
produces shrinkage, thickening and immobility, particularly at
the tips of the leaflets (Fig.
1). Over a longer period, the fibrotic tissue may calcify
and eventually the valve becomes a funnel- shaped structure that
impedes blood flow as much by its rigidity as by actual stenosis
of the orifice.
Chronic elevation of left atrial pressure in mitral stenosis usually
causes the chamber to dilate, although the extent to which this
occurs depends additionally on atrial wall stiffness. Elevated filling
pressure also results in pulmonary venous hypertension, and hence
a rise in pulmonary artery pressure. If severe, this leads eventually
to right ventricular hypertrophy and the development of tricuspid
regurgitation.
 |
| Fig. 2 |
The pathological changes in the valve apparatus produce dramatic
alterations to the echocardiographic images (Fig.
2). On the M-mode recording, thickening of the leaflets and
reduction in their mobility can be appreciated. Commissural fusion
alters the motion of the posterior leaflet; instead of moving
in the opposite direction to that of the anterior, it is pulled
forward when the valve opens at the beginning of diastole. In
the normal valve, rapid early diastolic filling of the left ventricle
means that the valve can partially close in mid-diastole, and
it then reopens during atrial systole.
This sequence produces the characteristic "M" shape of the recording,
with the down slope of the "M" designated as the "E-F slope". With
mitral stenosis, however rapid filling is not possible, so the valve
leaflets have to remain as widely separated as they can throughout
diastole, and the "M" disappears. Another reason for a different
pattern of leaflet motion is that atrial fibrillation is frequently
present, and this eliminates the late diastolic reopening.
 |
| Fig. 3 |
These changes are also apparent on two-dimensional recordings.
(Fig.
3) shows commissural fusion causing the valve leaflets to
assume a dome shape in diastole, with the anterior leaflet forming
a characteristic "elbow" due to its tethering to the posterior
leaflet. The hallmark of rheumatic mitral valve disease is the
restriction of motion of the anterior mitral valve leaflet tip.
Abrupt halting of the early diastolic opening motion coincides
with the opening snap evident on auscultation.
 |
| Fig. 4 |
Echocardiography is a very sensitive method for detecting mitral
stenosis and simple inspection of the recordings
(Fig. 4) can often differentiate between mild and very severe
stenosis. Quantification of severity has, however, proved more
difficult by M-mode. Early attempts measured the "E-F slope",
which indicates the difference between early and mid-diastolic
leaflet position, but these proved unsatisfactory because the
E-F slope is influenced by factors such as stroke output and ventricular
stiffness.
Traditional approaches for the calculation of mitral valve area
involve cardiac catheterization. The value of cardiac catheterization
methods for assessing a patient with mitral stenosis is questionable;
the end-diastolic valve gradient is influenced by heart rate, stroke
volume, and the presence of mitral aortic regurgitation, which also
invalidate estimation of valve orifice area by the "Gorlin" formula.
 |
| Fig. 5 |
Two-dimensional echocardiography is able to visualize the mitral
orifice directly in diastole, thus providing information previously
available only to the surgeon. Using the parasternal short-axis
view, it is possible to differentiate between various orifice
shapes that are indistinguishable by M-mode (Fig.
5).
 |
| Fig. 6 |
Planimetry of a still-frame image (Fig.
6) allows the mitral valve orifice area to be measured. Great
care is needed to ensure that the image selected is accurate.
If the scan plane transects the valve obliquely, above or below
the region of maximum constriction, the orifice will appear too
large; if the wrong part of the cardiac cycle is used, it will
appear too small. Other technical difficulties arise from limitations
of lateral resolution of the instrument, and from multiple reverberations
produced by valve calcification. In spite of these problems, in
the hands of an experienced operator two-dimensional echocardiography
has proved the best method available for preoperative estimation
of mitral valve area. Thoughtful clinical uses of the two-dimensional
echocardiographic data will obviate the need for cardiac catheterization
in most patients.
In the setting of previous mitral valve commissurotomy, however,
caution should be exercised. Such direct planimetric methods have
not been shown to be as reliable in this setting.
Doppler echocardiographic methods have also been shown to be useful
for estimation of mitral valve area using the pressure half-time
method and for the estimation of mean mitral gradient. Such methods
are particularly useful in a patient with previous commissurotomy
and also serve to double-check the direct planimetric approaches.
 |
| Fig. 7 |
Of course, with significant mitral stenosis, the left atrium
will dilate. Such dilatation can be observed in any view of the
left atrium (Fig.
7). Estimation of left atrial size using echocardiography
is very important. The normal left atrial size is below 4 cm in
adults and dilates with chronic mitral stenosis, insufficiency,
or atrial fibrillation. Patients with severe left atrial dilatation
are usually very difficult to convert to normal sinus rhythm with
cardioversion. The echocardiogram helps to select patients in
which cardioversion might or might not be successful
2004
