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Department
of Podiatry - Vascular Assessment
Doppler
Assessment
The
Doppler ultrasonic flow meter works on the principle that sound waves
reflected from moving objects undergo a shift in frequency relative to
the speed of the moving object from which they are reflected (Bowker,
2001). In this case the incident sound wave is reflected from the erythrocytes
moving within the vessel in question. The Doppler device consists of two
piezoelectric crystals mounted in a hand-held probe. An electrical charge
is then passed through one of the crystals stimulating the emission of
sound waves of various frequencies. When the reflected wave is received
by the second crystal a voltage difference is recorded, which is converted
into either sound or analog waveforms, or both. The depth of penetration
of the emitted beam is indirectly proportional to the frequency of the
beam. An operating frequency of 5MHz or lower gives deep penetration and
a comparatively broad beam suited to monitoring deep venous return in
the peripheral venous circuit. An operating frequency of 10MHz is less
penetrating and permits a sharper focus ideal for blood velocity detection
in the superficial veins and arteries of the lower extremity and digits
(Bowker, 2001).
Doppler
evaluation should be performed with the patient supine and rested for
five minutes. The angular direction the probe should be held in varies
with the manufacturer and the use of a large amount of aquasonic coupling
gel allows the practitioner to vary the angle of intonation to find the
point at which the signal is clearest. A beam of ultrasound travels to
the underlying vessels where it is reflected from the blood cells and
shifted in frequency in an amount proportional to the flow velocity of
the erythrocytes in that vessel (Bowker, 2001). As such the pitch of the
audible signal produced by the second crystal is proportional to the average
velocity of the blood flow of the vessel under examination (Bowker, 2001).
In general terms, the greater the velocity of the blood in the vessel
and therefore the greater the frequency of the reflected sound wave, the
higher the pitch of the audio signal. The converse also being true. The
amplitude of the audio signal works on a similar principle. In cases where
the sound wave is reflected from a large number of blood cells, the phase
shift in the reflected frequency will be greater and proportionately,
the audible signal produced will be louder, and vice-versa.
Initial Doppler study should note the presence, absence and quality of
the femoral, popliteal, posterior tibial, dorsalis pedis and perforating
peroneal arteries noting pitch, monophasic or bi-pahasic. As with the
ABPI, the Doppler study does not reveal information on the patency of
the blood flow distal to the site of examination. If the arterial supply
to the forefoot is questionable or if surgery is being contemplated, auscultation
of the deep plantar and digital arteries is recommended.
Using
the doppler unit on the posterior tibial artery.
Using
the doppler unit on the dorsalis pedis artery.
INTERPRETATION
OF NORMAL SOUNDS & WAVEFORMS
The audible signal produced by a normal healthy individual will be brief,
clear and multiphasic. A minimum of two and a maximum of four sounds will
be heard. The first sound heard will be the loudest, occurs during left
ventricular systole and represents a period of forward flow. A second
sound is heard during early diastole and represents a period of reverse
flow corresponding to vessel wall recoil. The third sound is heard during
late diastole and represents a return to forward flow. With the limbs
elevated and the heart rate below 60 beats per minute a fourth sound may
be heard and represents a second return to reverse flow resulting from
diastolic pressure being overcome by the hydrostatic pressure (Kinden,
1998).
A
sample doppler printout: note the triphasic pattern of the waveform. The
signal has been rectified such that it is all positive.
The normal vessel waveform will be multiphasic, pulsatile and have regular
amplitude. The pulse width will be narrow and there will be a period of
reverse flow at the end of systole and into early diastole. When a bi-directional
Doppler system is used forward flow will be graphed above the baseline
and reverse flow below the baseline.
INTERPRETATION
OF ABNORMAL SOUNDS & WAVEFORMS.
The degree of pathology and the placement of the Doppler probe will both
have a direct result on the alteration of the audible signal. Diseased
vessels will demonstrate alterations in flow characteristics and therefore
the audible signal will change in accordance with the degree of occlusion.
Loss of vessel wall elasticity will result in a monophasic signal, as
recoil does not occur. If the probe is held directly above an atheromatous
plaque a ‘knocking’ quality will be heard. Vessel occlusions
will result in disruptions in volume and velocity of blood flow at pre
and post stenotic sites. Distal to an occlusion there will be turbulent
flow and a rough sound as the lumen increases in size. Holding the probe
proximal to an occlusion will result in a louder audible signal due to
the collection of blood cells though the pitch will be diminished, as
the blood cells are moving at a decreased velocity. PAD will alter the
characteristics of normal wave morphology depending on the degree of disease
present. An abnormal waveform may lack any or all of the normal characteristics.
Moderate vessel occlusion will lead to diminished amplitude, with progression
of the disease blunting of peaks and an increased pulse width will develop.
Diminished amplitude corresponds to a decrease in blood volume; loss of
rapid upstroke and the prevalence of broad based waves indicate a decrease
in blood flow velocity (Kinden, 1998).
The Doppler assessment is by no means a quantitative measure, at best
it may be considered semi-quantitative when used in conjunction with ankle
and toe pressure measurements. The results are therefore open to clinical
interpretation and can be affected by the depth of the artery, by the
probe pressure that actually compresses the patient’s artery, or
even by the consistency of the vessel under investigation (Kinden, 1998).
Comparison of waveforms and the audible signal to pressure measurements
and other non-invasive methods of evaluation can help to avoid errors
in interpretation, comparison to the contra-lateral extremity when reliability
of data is questionable is recommended.
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