Diagnostic ultrasound ( PDFDrive )

954 PART III Small Parts, Carotid Artery, and Peripheral Vessel Sonography
A
B
FIG. 26.53 Incomplete Subclavian Steal and Provocative Maneuver. (A) Presteal left vertebral artery waveform. Flow decelerates in peak
systole but does not reverse. (B) After provocative maneuver, there is reversal of low in peak systole in response to a decrease in peripheral
arterial pressure.
he presteal (“bunny”) waveform shows antegrade low but
with a striking deceleration of velocity in peak systole to a level
less than EDV. his is seen in patients with proximal subclavian
stenosis, which is usually less severe than in those with partial
steal waveform. 239 he bunny waveform can be converted into
a partial steal or complete steal waveform by provocative maneuvers
(Fig. 26.53). A tardus-parvus waveform (also called a
dampened waveform) can be seen in patients with high-grade
proximal vertebral stenosis. 238,239
With a subclavian steal, color Doppler may show two similarly
color-encoded vessels between the transverse processes, representing
the vertebral artery and vein. 129 Transverse images of the
vertebral artery with color Doppler show reversed low compared
with those of the CCA. A Doppler spectral waveform must be
produced in all such cases to avoid mistaking low reversal within
an artery for low in a pulsatile vertebral vein. 129,237
Stenosis and Occlusion
Diagnosis of vertebral artery stenosis is more diicult than
diagnosis of low reversal. Most hemodynamically signiicant
stenoses occur at the vertebral artery origin, situated deep in the
upper thorax and seen in only 60% to 70% of patients. 234,243,241 Even
if the vertebral artery origin is visualized, optimal adjustments
of the Doppler angle for accurate velocity measurements may
be diicult because of the deep location and vessel tortuosity.
No accurate reproducible criteria for evaluating vertebral artery
stenosis exist. Because low is normally turbulent within the
vertebral artery, spectral broadening cannot be used as an
indicator of stenosis. Velocity measurements are not reliable as
criteria for stenosis because of the wide normal variation in
vertebral artery diameter. Although velocities greater than 100 cm/
sec oten indicate stenosis, they can occur in angiographically
normal vessels. For example, high-low velocity may be present
in a vertebral artery that is serving as a major collateral pathway
for cerebral circulation in cases of carotid occlusion 34,188,247
(Fig. 26.54). hus only a focal increase in velocity of at least
50%, visible stenosis on gray-scale or color Doppler, or a
striking tardus-parvus vertebral artery waveform is likely
to indicate signiicant vertebral stenosis. he variability of
resistive indices in normal and abnormal vertebral arteries
precludes the use of this parameter as an indicator of vertebral
disease. 244
Diagnosis of vertebral artery occlusion is also diicult. Oten,
the inability to detect arterial low results from a small or
congenitally absent vertebral artery or a technically diicult
examination. he diferentiation of severe stenosis from occlusion
is diicult for the same reasons. Extremely dampened blood
low velocity in high-grade stenoses may result in a Doppler