 |
| Fig.3.4 |
Doppler echocardiography
is useful for the determination of cardiac output; this is the volume
of blood pumped by the left ventricle every minute and is expressed
in liters per minute. The volume of blood ejected every systolic
beat is called the stroke volume (Fig. 3.4) and is the basis for
calculation of cardiac output according to the following equation:
Cardiac output =
stroke volume x heart rate
 |
| Fig.3.5 |
Doppler calculation of cardiac output is based on the assumption
that the aorta is a cylinder during every systolic beat. This cylindrical
flow volume may be determined if its area and its length are known.
The area is determined from the two-dimensional echocardiographic
image while the length is derived from the Doppler spectral recording
(Fig. 3.5).
 |
| Fig.3.6 |
These relationships between area and flow velocity are shown schematically
in Figure 3.6. Given identical volume of flow through a large cylinder
(Fig. 3.6, left panel) and small cylinder (right panel), the velocities
recorded by Doppler will vary considerably. The same volume through
the larger cylinder will render a lower peak velocity (and small
flow velocity integral) in comparison with the flow recorded through
the smaller orifice. The flow velocity integral reflects the average
velocity of the red cells during systole. Because the red cells
are moving faster through the smaller cylinder, they travel farther.
Thus, the Doppler recording of velocity relates to distance traveled.
Conceptually, derivation of cardiac output begins with the recognition
that the volume of blood ejected every time the heart beats is first
limited by the area of the aortic root (the area of the cylinder).
While there remains some argument as to the precise point where
this area is best determined from the two-dimensional echocardiographic
image, most Doppler users measure the narrowest diameter in systole
at the bases of the aortic valve cusps. This is most reliably accomplished
using the parasternal long-axis view. Dividing the diameter in half
then results in the radius of the open aortic valve. This area is
assumed to be a circle and is determined by the standard plane geometric
equation: area = r2.
Secondly, the distance the ejected blood travels may be calculated
from the Doppler spectral recording. Since this is a measure of
velocity over time, the flow velocity integral will result in the
average velocity during systole.
 |
| Fig.3.7 |
As a result of knowing the average velocity, this may be normalized
for one second and is an index of how far the blood has traveled.
The method for calculation of cardiac output is demonstrated in
Figure 3.7.
It is important to recognize that for proper calculation of cardiac
output using this approach the beam must be as parallel to flow
as possible. Alterations of a few degrees from parallel will result
in lower Doppler velocity recordings and underestimation of cardiac
output. Therefore, aortic outflow is best obtained from the apical
or suprasternal approaches where the beam is nearly parallel to
normal flow
It is also important to remember that the Doppler estimate of cardiac
output is based on the square of the measured radius of the aorta.
Any error in this measurement will be multiplied and may profoundly
affect the resulting calculation.
 |
| Fig.3.8 |
Doppler estimates of cardiac output compare quite favorably with
those obtained by other methods. Comparisons have been made with
cardiac output estimated by the Fick principle at catheterization,
with thermodiultion, as well as with a host of other approaches.
In general, these studies show very good correlations, being within
±10% of the other method (Fig. 3.8). Cardiac output may also be
determined from flow and diameter measurements through any of the
other cardiac valves.
2004