Page 224 - Libro vascular I

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GRAFT SURVEILLANCE AND PREOPERATIVE VEIN MAPPING FOR BYPASS SURGERY
demonstrates areas of marked flow disturbance and flow reversal at the proximal anastomosis due to the size, geometry and orientation of the graft ori- gin from the native artery. This may also be seen at the level of the distal anastomosis and should not be considered abnormal unless spectral Doppler recordings demonstrate significant velocity changes. Beyond the proximal anastomosis, the color flow image should demonstrate an undisturbed flow pattern. Grafts with well-established biphasic or triphasic flow will display normal reversal of flow (from red to blue or vice versa) during the diastolic phase. New grafts may demonstrate hyperemic flow due to peripheral dilation and the flow require- ments of healing tissue, exhibited as constant for- ward flow throughout the cardiac cycle. If the graft has a large lumen, the flow velocity may be very low, and the PRF may have to be significantly low- ered to demonstrate color filling. Some areas of flow reversal may be seen in areas of vein grafts cor- responding to valve sites. In rare instances in which the vein is found to be bifid for a short segment, it is possible to see two flow lumens. The distal anas- tomosis of a femoral distal graft is usually easier to identify with color flow imaging than with B-mode imaging. It is common to see the graft supplying a patent segment of the native artery above the anas- tomosis as well as distally, and retrograde flow will be seen in the native vessel above the anastomosis, producing a Y-shaped junction (Fig. 14.9). There is often a considerable size discrepancy between the distal end of a vein graft, which can be quite large, and the outflow artery, which may be a smaller tibial vessel. This will cause a natural veloc- ity increase due to the change in vessel diameter, possibly producing color aliasing at the position of the anastomosis and proximal run-off vessel, but this should not be assumed to indicate a significant stenosis without close interrogation with spectral Doppler.
Normal appearance of synthetic grafts
Flow in synthetic grafts can sometimes be difficult to demonstrate using color flow imaging, as the graft material attenuates the Doppler signal, requir- ing an increase in the color gain. Significant flow disturbance can be seen at the origins and ends of synthetic iliofemoral or femorofemoral cross-over
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grafts, as the graft is often joined at a 90° angle to the native artery.
Abnormal appearance of vein grafts
A significant graft stenosis will produce marked flow disturbance, which is usually associated with aliasing on the color flow image (Fig. 14.9; see Fig. 14.12), and there may be considerable flow disturbance beyond the stenosis. Failing grafts may demonstrate very low volume flow, which can sometimes be dif- ficult to demonstrate with color flow imaging, and the graft may be mistakenly reported as occluded. If no flow is detected in the graft, the color PRF and high-pass filter setting should be reduced to confirm the occlusion, which should also be checked with spectral Doppler. Arteriovenous fistulas and aneurysms are other graft abnormalities that are vis- ible with color flow imaging, as discussed below.
SPECTRAL DOPPLER WAVEFORMS
Normal appearance
The waveform shapes in normal vein grafts can vary considerably depending on the age of the graft. New grafts may demonstrate a hyperemic mono- phasic flow profile because of sustained peripheral vasodilation, which can be due to a combination of the previous ischemia and healing tissue (Fig. 14.10A). Over time, the flow pattern should become pulsatile, and biphasic or triphasic waveforms are usually recorded (Fig. 14.10B). It is good practice to take spectral Doppler measurements at regular intervals along a graft, even in the presence of a normal color flow display, as changes in the wave- form shape can indicate an approaching problem. Disturbed flow, including areas of flow reversal, is usually encountered around the proximal anasto- mosis, but there should be no significant increase in systolic velocity. Natural changes in the diameter of the graft will produce changes in the peak sys- tolic velocity (PSV), which should not automati- cally be assumed to represent a stenosis. In this situation, velocities should be compared in adja- cent areas of similar vessel diameter. Perhaps the most difficult assessment to make during graft sur- veillance is the estimation of the degree of narrow- ing at the distal anastomosis, where there is often a