 |
| Fig.3.16 |
Use of Doppler ultrasound to estimated the severity of a valve
stenosis is based principally on the fact that such obstructions
result in an increase in the velocity of flow. In clinically significant
mitral stenosis, the diastolic velocity of mitral flow usually
exceeds 1.7 m/s. Systolic velocity of aortic flow in clinically
significant aortic stenosis may reach 5-6 m/s
(Fig.
3.16). Thus, CW Doppler is required for the detection of these
increased velocities and for recording the full spectral profiles.
We have already noted that there is a relationship between the
pressure increase (or gradient) across a valve and the velocity
of blood flow across the valve. For any given pressure gradient
there is a corresponding increase in velocity, as predicted by
the simplified Bernoulli equation:
p1-p2 = 4V2
where p1 = pressure distal to obstruction p2 = peak velocity
of blood flow across the obstruction.
As the stenosis becomes more severe, the valve orifice area will
become smaller, and the velocity of flow across the orifice will
increase as a function of the increased pressure gradient. Thus,
by measuring the peak velocity in a systolic aortic jet with Doppler
echocardiography, it is possible to estimate the pressure gradient
that produced it using the above simple algebraic expression.
The peak aortic velocity of the spectral recording in Figure
3.16 is approximately 5.8 m/s. Using the previous formula
p1-p = 4(5.8)2
the pressure gradient is therefore 135 mmHg.
There are, however, three major technical requirements that must
be satisfied if Doppler is to be used for this purpose. First,
an adequate "window" into the chest for ultrasound propagation
and reception must be found so that well formed Doppler profiles
can be recorded. Second, as emphasized in Units 1 and 2, for the
velocity measurement to be accurate, this window must allow orientation
of the ultrasound beam so that it is as parallel as possible to
flow through the valve. Third, the high velocities present in
the disturbed jet often exceed the Nyquist limit of PW Doppler,
so that CW or high pulse repetition frequency Doppler must be
used.
2004