Page 316 - Libro 2

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PART 5 — ABDOMINAL
breathing momentarily to record the Doppler wave- form and determine the correct angle can increase accuracy. The angulation of the proximal SMA is acute, and Doppler angles change quickly in a short distance. If breathing is not suspended, it is possi- ble for the angle of the Doppler sample volume to change significantly from where it was first placed or even to inadvertently slip between the celiac and the SMA. To avoid potential errors due to incorrect Dop- pler angles, particularly in tortuous segments, ask the patient to suspend respiration in order to keep these vessels as still as possible during acquisition of the Doppler spectrum. As with other types of arte- rial spectral Doppler evaluation, angles of 60° or less should always be used.
Use of Color Features for Aliasing, Turbulence, and Flow Direction
A color bruit frequently offers an instant clue to the presence of a significant stenosis. Once observed, in- spect thoroughly with Doppler to find the maximum velocity (Fig. 19-8).
Close attention should also be given to the Dop- pler pulse repetition frequency (PRF) and the orien- tation of the color-scale bar. The PRF scale should be adjusted (usually increasing the frequency range to limit aliasing of the color scale) to more reliably determine flow direction. The color on the top half of the color bar scale is assigned to flow coming to- ward the transducer, whereas the colors on the bot- tom half represent flow away from the transducer. This feature allows the technologist to identify and track antegrade flow in tortuous vessels and more readily identify retrograde flow. Flow direction be- comes particularly important when the celiac artery is occluded or severely stenotic. In this situation, low pressure in the celiac artery induces SMA collaterals to divert blood toward the liver and spleen through
Figure 19-8 Color bruit in celiac artery. Doppler velocities indi- cate significant stenosis, with PSV 5 570 cm/s and EDV 5 277 cm/s.
Common hepatic artery
Gastroduodenal artery
Pancreatico- duodenal artery
Celiac trunk
Left gastric artery
Figure 19-9 When the celiac artery is occluded or severely ste- notic, collaterals from the SMA divert blood through the gas- troduodenal artery toward the liver and spleen. Retrograde flow in the common hepatic artery fills the splenic artery. (Original drawing by F. Elizabeth LaBombard, RVT, Dartmouth-Hitchcock Medical Center Vascular Lab; reproduced with permission.)
the gastroduodenal artery (GDA). The GDA back- fills the common hepatic artery such that retrograde flow in the common hepatic crosses the celiac artery origin to perfuse the splenic artery (Figs. 19-9 and 19-10). The finding of retrograde flow direction in the common hepatic artery is always (100% predic- tive) associated with severe celiac artery stenosis or occlusion.13 Thus, even when the celiac artery can- not be well visualized, the finding of retrograde flow in the hepatic artery is a significant finding, as long as the examiner is confident that flow direction has been determined accurately.
Turn Color Off: Inspect with B-Mode
Color can mask important features. Although there are distinct color cues in arterial dissections, the thin echogenic line seen in the grayscale image is an
Figure 19-10 Color flow image (left) and Doppler spectral waveform (right) illustrating the importance of color-scale and pulse repetition frequency (PRF). The pulsed Doppler sample volume is positioned in the common hepatic artery, and both the spectral waveform and color flow (blue) image indicate flow away from the transducer. This represents retrograde collateral flow in the common hepatic artery and antegrade flow (red) into the splenic artery.
Superior mesenteric artery
Splenic artery