There is clear
justification for concern over accurate recording of very high velocity
jets within the heart. As will be discussed in more detail later,
the presence of an obstruction to flow, such as aortic stenosis,
will result in a significant increase in velocity across the aortic
valve in systole. In practice, these jets attain speeds of up to
7 m/sec.
 |
| Fig.1.31 |
The Bernoulli equation is a complex formula that relates the pressure
drop (or gradient) across an obstruction to many factors, as is
seen in Figure 1.31. For practical use in Doppler echocardiography
this formula has been simplified to: p1-p2=4V2
As we shall see later, Doppler recordings of velocity may, in certain
situations, be used to estimate pressure gradients within the heart.
When used for this purpose, it is important to keep in mind that
the angle the Doppler beam is incident to any given jet may not
be known since these examinations are frequently done blindly by
CW. In these cases the operator always tries to orient a beam as
parallel to flow as possible so that the full velocity recording
is obtained (this assumes cosine Ø =1).
 |
| Fig.1.32 |
Note that the full Bernoulli equation requires velocity data from
below (V1) and above (V2) any given obstruction. Since V1 is normally
much smaller than V2 (Fig. 1.32) it can usually be ignored in the
calculation of a pressure gradient.
In the example cited, the peak velocity is approximately 3.5 m/sec
and this would correspond to an aortic gradient of 48 mmHg by the
simplified Bernoulli equation. Obviously, faithful recording of
abnormal velocities has great importance, not only for clear identification
and recognition of abnormal profiles but also for quantitative purposes.
2004