 |
| Fig. 39 |
Causes of left ventricular outflow obstruction include aortic valve
stenosis and subvalvular and supravalvular obstructions. Subvalvular
obstruction can be further divided into the discrete diaphragmatic
or fibromuscular ring types, and obstruction secondary to septal
hypertrophy caused by hypertrophic cardiomyopathy. Only primary
valve leaflet pathology will be discussed here and the others dealt
with in subsequent units.
Aortic valve stenosis can be present from birth due to malformation
of the valve, or can develop in later life as a result of calcification
either of a congenitally bicuspid valve (Fig.
39) or of a valve inflamed by rheumatic disease.
 |
| Fig. 40 |
A congenitally stenotic valve may be grossly dysplastic, and in
such cases its echocardiographic appearance is usually abnormal.
(Fig.
40) demonstrates a short-axis of a bicuspid aortic valve in
systole with a slit-like orifice.
Alternatively, the valve cusps may be thin, either with one
commissure imperfectly formed, or consisting essentially of a
diaphragm with a small hole through which the blood must pass.
During ventricular ejection, such a valve is pushed upward to
form a dome shape; while this may be apparent on a long-axis two-dimensional
view, it cannot be appreciated by M-mode echocardiography, which
frequently gives normal recordings in cases of severe stenosis.
Even two-dimensional echocardiography can fail to detect any abnormality.
The absence of echocardiographic evidence cannot, therefore, rule
out the presence of congenital aortic stenosis.
 |
| Fig. 41 |
Mild congenital defects of the aortic valve are common, occurring
in about 1% of the population. In most cases, the valve is "bicuspid",
either with two equal-sized cusps or with three cusps, two of
which are fused together. The patient is usually asymptomatic
and there may be little or no murmur. Typically, the M-mode diastolic
closure line is eccentrically placed within the aortic lumen (Fig.
41), and there are often multiple echoes in diastole, arising
from corrugations on the cusp edges or from the raphe.
 |
| Fig. 42 |
There is, however, an aortic ejection sound, generated as the
valve is suddenly halted as it opens and forms the "dome" shape
described above. High-speed M-mode echocardiograms recorded simultaneously
with a phonocardiogram show the coincidence of the ejection sound
with maximal valve opening and provide a reliable method for detecting
bicuspid valves (Fig.
42. In many cases, the valve itself appears abnormal.
 |
| Fig. 43 |
The doming action of the valve during systole can sometimes be
seen on two-dimensional recordings, but it is not easy to obtain
technically adequate images from the thin cusps
(Fig. 43).
In adult life, though rarely before the age of forty, some bicuspid
valves calcify and become progressively more stenotic. Although
histological examination can usually distinguish such cases from
rheumatic aortic stenosis, the echocardiographic appearances of
the valves are identical due to the severe fibrosis with calcification
in both entities. In mild cases, fibrous thickening of the cusps
is apparent, and their separation in systole is restricted. Here
it may be possible to estimate the orifice area using the two-dimensional
short-axis view. Assuming the lumen to be circular, the reduction
in orifice diameter, which increases the gradient from 40 to 100
mmHg, is only 2mm. Bearing in mind the probable irregular shape
of the hole, and the presence of reverberation echoes from the
calcium, visualization of this is, at present, beyond the capability
of current techniques.
Nevertheless, if two-dimensional echocardiography shows that
a substantial orifice exists, there is unlikely to be severe stenosis.
This can be very helpful, for example in eliminating aortic stenosis
as the primary pathology in a patient who presents in a moribund
state with a systolic murmur.
 |
| Fig. 44 |
Two-dimensional echocardiography is also useful for detecting
changes associated with chronic rheumatic or degenerative aortic
stenosis. In such cases, the aortic cusps are immobile, or only
partially mobile and are markedly thickened (Fig.
44).
A rough correlation exists between the severity of calcification
and the pressure gradient across the valve. Echocardiographic
estimations of calcification are, however, not very reliable as
reverberations from small regions of calcium tend to obscure the
entire aorta, giving a misleading impression of the overall severity.
 |
| Fig. 45 |
Echocardiography is a very precise method for assessing left
ventricular hypertrophy, and this means that some helpful information
about the effect of aortic stenosis on the myocardium can be obtained
(Fig.
45). M-mode recordings permit measurement of ventricular wall
thickness and indication of their stiffness is obtained from the
rate of diastolic filling. The relative dimensions of the ventricular
cavity and wall thickness can also be used to estimate wall stress
using the formula:
Wall Stress (S) = LVP x (r/h),
where LVP = peak LV pressure; r = cavity radius; h = mean wall
thickness.
Thus, assuming the ventricle hypertrophies to maintain constant
wall stress:
LVP - (constant) x (h/r)
 |
| Fig. 46 |
If peak pressure can thus be estimated and systemic arterial
pressure measured by a cuff around the brachial artery, it ought
to be possible to determine the valve gradient. Several reports
have suggested that this is the case, but not all researchers
can produce adequate correlations to permit useful prediction
in individual cases. Left ventricular hypertrophy may also be
detected with two-dimensional echocardiography (Fig.
46).
Doppler echocardiography provides the most reliable noninvasive
means for determining aortic valve peak and mean gradients, as
does estimating valve orifice area by the continuity equation.
 |
| Fig. 47 |
The primary echocardiographic signs of aortic regurgitation are
volume overload of the left ventricle combined with a rapid fluttering,
either of the anterior mitral valve leaflet, or the septal endocardial,
in the region of the left ventricular outflow tract, or both
(Fig. 47). Rarely, and only in patients with dilated ventricles,
it may not be possible to detect this fluttering. Some detectable
abnormality of the aortic root or valve is normally found combined
with these signs.
 |
| Fig. 48 |
In cases of acute, severe regurgitation, premature closure of
the mitral valve may be seen
(Fig.
48) and is usually an ominous prognostic sign.
This is caused by extremely high end-diastolic pressures in an
uncompensated ventricle, and is associated with a loud mid-diastolic
(Austin Flint) murmur.
2004
