This concept
has important implications for the clinical detection of prosthetic
valvular regurgitation. These physical properties of prosthetic
valves may significantly alter the ability of Doppler systems to
detect abnormal flow even when present. Such masks also exist in
vivo and evidence for their presence may be found in certain patients.
Considerable clinical caution must, therefore, be exercised when
encountering patients with prosthetic valves. It may be impossible
to detect any flow on the opposite side of a prosthetic valve when
the valve is interposed between the interrogating transducer and
the area being examined.
 |
| Fig.2.40 |
The best clinical example of this problem is when an operator is
examining a patient with a prosthetic mitral valve for mitral regurgitation
from the apical view (Fig. 2.40, left panel). From this approach,
almost the whole of the left atrium is masked by the prosthesis
and the operator could incorrectly conclude that no mitral regurgitation
existed. In this case, a very high parasternal view or a subcostal
view of the left atrium must be selected for detection of the mitral
insufficiency.
The problem is made worse (Fig. 2.40, right panel) in patients with
both aortic and mitral prosthetic valves. The left atrium is inaccessible
from the apical views due to the presence of the mitral prosthesis.
Both prostheses obscure the left atrium from the parasternal views.
In this setting, only the subcostal view is available for viewing
the left atrium and, in our experience, this approach is rarely
rewarding. Prosthetic aortic regurgitation may, of course, be detected
from the apical view because the prosthesis is not between the transducer
and the area of interest in the left ventricular outflow tract.
Because of these observations concerning the difficulty in detecting
flow on the far side of a prosthetic valve, we always adjust our
examination methods to interrogate these valves from all possible
views, so that the prosthetic valve does not lie between the transducer
and the chamber being examined. This requires considerable operator
skill and is true for all Doppler methods. In many instances there
is no view available in which the beam can be properly directed.
Thus, we strongly suggest that operators of ultrasound equipment
do all that is possible to detect flow properly. When none is detected
on the far side of a prosthetic valve, it should not be assumed
that none is present. We have seen cases of severe valvular regurgitation
where the Doppler examination was rendered artifactually negative
due to the masking effect. There are, however, some exceptions to
this rule. In cases where the valve ball or disc is not allowed
to seat correctly due to thrombus or vegetation, some sound may
be transmitted through the partially open area if it is correctly
oriented to the sound beam.
2004