Page 189 - Libro 2

11 — Ultrasound Assessment of Arterial Bypass Grafts
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knee portions), the tibioperoneal trunk, or any of the tibial-level vessels (anterior tibial, posterior tibial, or peroneal arteries). Occasionally, the distal anasto- mosis of a bypass may be to the dorsalis pedis artery. Given the options for conduit type, orientation, and anatomical position, it can save a great deal of time if the sonographer or technologist refers to an opera- tive note when examining a bypass graft.
MECHANISMS OF BYPASS GRAFT FAILURE
When performing ultrasound examinations on by- pass grafts, it is important to have a firm understand- ing of normal bypass findings but equally important is knowing the types of problems that may exist. There are distinct problems that arise at specific pe- riods during the lifetime of a bypass graft.
During the first 30 days following surgery, ul- trasound scanning will identify technical problems that can lead to bypass failure. There may be a re- tained valve or valve leaflet, an intimal flap caused by the surgical instrumentation of the vein con- duit, problems at the anastomotic sites due to su- ture placement, or possible graft entrapment due to improper positioning of a free vein graft. Addi- tionally, a bypass may thrombose due to use of an inadequate venous conduit or limited run-off bed. Occasionally, early thrombosis can also occur in some patients with a hypercoagulable state. Periop- erative bypass graft failure accounts for one-fourth of all failures.9
Between months 1 and 24, myointimal hyperpla- sia can develop, leading to a bypass stenosis. This is not atherosclerotic plaque and has a different ultra- sound appearance. A stenosis can occur at any point within the conduit but often a stenosis develops at a valve site. A stenosis can also occur at either the proximal or distal anastomoses. Eleven percent to 33% of all bypasses will have these types of steno- ses occur and often they appear within the first year, causing 75% of all revisions performed during this postoperative period.10
After 24 months, progression of atherosclerotic disease occurs within the inflow or outflow vessels. It is important to pay close attention to spectral waveform characteristics within the bypass itself as changes in these waveforms may help identify disease in native vessels remote to the bypass graft itself. Loss of diastolic flow is routinely seen within veins bypasses within the first few weeks of surgery particularly when carried out for criti- cal limb-threatening ischemia. However, significant changes in diastolic flow after this early phase, in addition to possible increase in acceleration time
and or decreases in peak systolic velocity, can all be signals of development of pathology elsewhere. Aneurysmal dilation of the venous conduit or at the anastomotic sites can develop during this late time, necessitating graft revision.11 These aneurysms are rare but can result in late bypass graft thrombosis if not corrected.
SONOGRAPHIC EXAMINATION TECHNIQUES
Without any specific indications, duplex ultra- sound scanning is performed at routine intervals. There are instances where duplex ultrasound can be performed, which do not follow the routine sur- veillance schedule. If a patient presents with acute onset of pain, diminished or absent pedal pulses, persistent nonhealing ulcers, or a recent history of loss of limb swelling (normally found in most suc- cessful vein bypass grafts) suggestive of graft failure and ischemia, a duplex ultrasound scan can be per- formed. Poor physiologic testing results, including an ankle–brachial index, which falls by greater than 0.15, would also be an appropriate indication for a duplex scan.
A routine surveillance protocol can consist of an ultrasound performed early in the postoperative pe- riod, usually within the first 3 months. Subsequent ultrasounds can be performed at 3-month intervals for the first year, every 6 months for the second postoperative year, and then annually thereafter. In most laboratories, the direct ultrasound scanning is performed in conjunction with indirect physiologic testing, including ankle pressures and plethysmo- graphic waveforms. Pulse volume recordings can be safely performed over bypass grafts because the pressures are low enough not to occlude the grafts. Many laboratories choose to avoid measuring sys- tolic pressures at levels where cuffs are placed over the grafts. In cases of grafts with distal tibial or pedal outflow vessels, toe pressures and waveforms can be recorded.
There are patients in whom surveillance should be more intense. Patients who have undergone an intraoperative revision, early postoperative throm- bectomy or revision, and patients with limited ve- nous conduits should be scanned more frequently. Many surgeons will choose to follow these patients at 2-month intervals.
The surveillance of prosthetic bypass grafts is less common. Many laboratories may follow these pa- tients with physiologic testing and clinical evaluation. It has been shown that duplex ultrasound is more sensitive in determining failing prosthetic grafts than an ankle–brachial index or clinical examination.12