The Doppler principle was defined by J. Christian Doppler. It states that an
apparent shift in transmitted frequency occurs as a result of motion of either
the source or the target. An example is the increase in frequency of sound produced
by a motorbike travelling towards you, and then the decrease in frequency as
it moves away.
In Doppler echocardiography, ultrasound
is transmitted towards the area of interest in the heart, and when it is reflected
by a moving structure, e.g. red blood cells, the reflected ultrasound has a change
in frequency which can be used to calculate the direction of movement and speed
of the red blood cells.
There are different forms of Doppler
ultrasound used in echocardiography. Continuous wave (CW) Doppler transmits
a continuous wave of ultrasound along a line of interest (which is indicated
on the monitor by a cursor line). Any reflected ultrasound along the beam will
then be received by the transducer and interrogated to calculate the velocity
and direction of any moving structure, mainly red blood cells. The direction
of flow is indicated on the monitor as being positive (above the baseline) when
flow is towards the transducer; and negative, when flow is away from the transducer.
CW Doppler is able to measure very high velocities, but it is not possible to
determine where, along its path, any ultrasound was reflected (this is termed
Pulsed Doppler transmits a pulse
of ultrasound at a low frequency, low pulsed repetition frequency (LPRF), such
that one small area along the ultrasound beam is interrogated (this is termed
range gating; and the area encompassed
within the range gating is termed the sample volume). When
reflected ultra-sound is received by the transducer and interrogated by the computer,
the velocity, direction and depth at which the reflection occurred can be calculated.
However, the maximum velocity which can be recorded is severely limited (this
is defined by the Nyquist limit),
hence abnormally elevated velocities cannot be measured (high velocities appear
to 'wrap around' the baseline on the display monitor; this is termed aliasing).
Pulsed Doppler transmitted at a higher
frequency (high pulsed repetition frequency, HPRF) offers
a compromise between CW and LPRF Doppler. HPRF allows measurement of velocities
as high as CW, but because there is more than one pulse of ultrasound within
the body at any time, there is some range ambiguity. However, sample volumes
are usually displayed on the monitor so that the echocardiographer can see whether
more than one sample volume actually falls within the heart (creating more than
one source of velocity measurement) or not.
Colour flow Doppler is an adaptation
of HPRF. Numerous sample volumes are overlaid on a 2-D echocardiogram and the
direction of flow is recorded on the screen as either red or blue, depending
on whether the flow direction was towards or away from the transducer. Some indication
of the velocity is given by the brightness of the colour. Flow which is turbulent,
having no definite direction, is displayed as a third colour, e.g. green or yellow.
The optimal echocardiographic views from
which to record Doppler velocities and normal values have been described (Table
4.2) (Brown et al(1991); Yuill &
O'Grady, (1991); Darke et al., 1993).
To record blood flow velocities, the
Doppler beam or sample volume must be in line (parallel) with the direction of
flow, and not exceeding 20 degrees out of line
Regurgitation from the pulmonic valve has been found to occur in up to 70% of
normal dogs, and from the tricuspid valve in 50% of normal dogs (Yuill
& O'Grady, 1991).
Small and brief regurgitation found in the normal animal is often referred
to as backflow.
Regurgitation from the aortic or mitral valves has not been found in normal
dogs and therefore should be considered pathological.
Blood flow within the heart is usually laminar and the Doppler velocity display
(termed the spectral velocity display) will show that the majority of red
cells accelerate together to a similar peak velocity and decelerate at a similar
rate. This gives a spectral velocity display that is referred to as a 'clean
envelope' or is described as having minimal 'spectral dispersion'.
Abnormal or turbulent flow, such as occurs distal to an obstruction, will produce
a spectral velocity display that has widespread spectral dispersion.
Abnormalities in blood flow (disturbed flow) may be due to valvular regurgitation,
stenosis or cardiac shunts. Thus in the normal examination the velocities should
be recorded proximal and distal to every valve, and in areas where shunts are
likely to be found, e.g. VSD.
The pressure gradient between any two chambers, or the ventricles and the great
vessels, can be estimated from the peak velocity of blood flow between the
two. For example, if the velocity is within the normal range proximal to a
stenosis and grossly elevated distal to it, the pressure gradient that is required
to produce the peak velocity can be estimated from the modified Bernoulli equation
(Goldberg et al (1988) ):
Pressure gradient = 4(V22 -
Where V2 is the peak velocity distal to the obstruction and V1 is the peak
velocity proximal to the obstruction.
Peak velocities through the aorta or pulmonary artery may be reduced when there
is poor ventricular function or contractility, e.g. dilated cardiomyopathy.
Other Doppler measurements of systolic ventricular function include peak and
flow acceleration, stroke volume and cardiac output determination (Goldberg
Ventricular inflow velocities normally result in a passive filling phase (E wave)
and contraction phase (A wave), where the E wave is greater than the A wave.
When the ventricle becomes non-compliant, e.g. hypertrophic cardiomyopathy, then
the E wave is reduced and the A wave increased. Thus the A/E wave ratio may become
Other parameters to quantify diastolic dysfunction include changes in the time-velocity
integral, prolongation of early diastolic acceleration and deceleration times,
and half-times. Accurate recording of inflow velocities is particularly important
in the interpretation of absolute parameters, but less important on relative
flow parameters such as A/E ratio.
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