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The
ICAVL Board Of Directors has mandated that contralateral, comparative
venous Doppler signals be recorded in those patients satisfying
internal laboratory algorithms for unilateral duplex examinations.
The physiology of venous blood flow supports the bilateral comparison
of Doppler spectral patterns. By carrying out these comparisons,
misdiagnosis can be avoided and better patient care can be delivered
with a more complete evaluation. The proceeding paragraphs describe
some of the causes for variations in venous flow patterns. While
it is important to answer the question of whether or not a venous
thrombosis is present, it is also important to not overlook
incidental findings. If a hematoma, lymphocele, or Baker’s
cyst is observed during the course of a venous duplex examination,
this information should be reported. The same should be true
for abnormal venous Doppler signals. Only by making bilateral
observations can an examiner try to separate unilateral from
more central or systemic pathology. These requirements are reflected
in the 2000 edition of the ICAVL Essentials and Standards.
Spectral
Doppler analysis obtained during the course of a duplex ultrasound
examination provides a representation of blood flow patterns
within the vessel being insonated. The flow of blood, like any
fluid, is governed by various physical principles. Within the
vascular system, the movement of blood occurs as a result of
changes in resistance and pressure. Pressure forces are comprised
of the dynamic pressure supplied by the heart and hydrostatic
pressure due to the force of gravity. In the venous system,
hydrostatic pressure is a much greater force than the dynamic
pressure.
Veins are high capacitance, low resistance vessels. By changing
shape, they can offer resistance to flow. Consider the vein
with a partial DVT within it. The thrombus has made the vein
less compliant, thus it doesn’t change shape and appears
fairly circular. Venous pressure is also increased due to the
partial blockage "backing up" the venous outflow.
The type of Doppler signal obtained in this situation is continuous
and shows little variation or resistance to flow. Normally,
in the lower extremities phasic venous flow is expected to occur.
With each inspiration, the diaphragm moves down, increasing
intrabdominal pressure. This increased pressure causes the inferior
vena cava to change shape and become partially compressed. This
results in an increased resistance to flow from the legs. Common
femoral Doppler patterns display this normal pattern of no flow
through inspiration followed by flow toward the heart during
expiration.
The
shape of the veins is affected by several factors. These factors
include transmural pressure, which is the difference between
the tissue pressure outside the vein wall and the pressure within
the vein. The IVC example above shows the effects on venous
flow with changes in transmural pressure. Venous volume also
affects the shape of the veins as well as venous pressure. The
overall hydration status of a patient, the presence of valvular
incompetence, arteriovenous fistulae, as well as cardiac status
can all affect blood volume.
Given all these factors which affect venous flow it is clear
that while general patterns in venous flow profiles can be expected,
much variation can and does occur. Normal venous flow can be
quite different between individuals. Even the same individual
examined on different days can display different degrees of
normal. In the absence of pathology, blood flow patterns are
fairly symmetrical when evaluating right and left limbs. By
comparing Doppler signals between limbs, an examiner can evaluate
the venous system as a whole and truly characterize what is
normal or not.
It is well known that the presence of a unilateral pulsatile
venous signal in the lower extremities may indicate an arteriovenous
fistula in that limb or increased inflow to the leg (as in the
case of cellulitis). Bilateral pulsatile signals represent something
completely different. Bilateral pulsatility may be reflective
of systemic venous hypertension as seen in patients with congestive
heart failure, tricuspid insufficiency or pulmonary hypertension.
However, young thin patients often display rather pulsatile
femoral venous signals due to a healthy cardiac status and very
compliant blood vessels.
The presence of a unilateral continuous femoral Doppler signal
might be associated with a partial iliofemoral DVT, as stated
earlier. It can also result from extrinsic compression of the
iliac vein, which will increase the resistance to flow and increase
the pressure within the iliac and femoral venous system. It
can also occur if there was a previous DVT that is recanalized
and has left the vein sclerotic and non-compliant. If bilateral
femoral signals display continuous flow, then IVC pathology
should be expected. The IVC may be partially thrombosed or extrinsically
compressed.
In the upper extremity, venous Doppler signals can be even more
difficult to interpret. Yet, because there are upper extremity
veins that can not be compressed or insonated due to their anatomic
position, Doppler signals are more heavily relied upon to complete
a diagnosis. The increased effect of hydrostatic pressure and
the close proximity of the heart can complicate the upper extremity
venous flow patterns. Pronounced pulsatility should be observed
with the more central veins such as the brachiocephalic and
subclavian. Less pulsatility and more respiratory phasicity
should be observed in the axillary and brachial veins. Again,
there is no magic formula for what one should expect as truly
normal. For example, a partial brachiocephalic vein thrombosis
may reduce but not eliminate pulsatility within the ipsilateral
subclavian vein. Comparison to the contralateral subclavian
vein would reveal the difference in signals and alert the examiner
to the problem.
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