Assessment of infrainguinal bypass grafts at operation for technical adequacy can be performed using continuous-wave (CW) Doppler flow analysis, arteriography, angioscopy and CW Doppler after flow is restored, or color duplex ultrasonography [22–24]. Color duplex ultrasound is the preferred method because it provides both anatomic and hemodynamic assessment of graft function and limb perfusion. When used at operation, approximately 15% of procedures have an unrecognized abnormality with velocity spectra of a stenosis identified. Repair of these problems and documentation of normal graft hemodynamics are associated with a low (<1%) incidence of graft thrombosis. In a series of 626 consecutive infrainguinal vein bypasses, duplex scanning prompted revision of 104 lesions in 96 bypass grafts, which included 82 vein/anastomotic stenoses, 17 vein segments with platelet thrombus, and 5 low-flow grafts. The revision rate was highest (p < 0.01) for alternative vein bypass grafts (27%) compared with the other grafting methods (reversed vein bypass grafts, 10%; non-reversed translocated, 13%; in situ, 16%). A normal intraoperative scan on initial imaging (n = 464 scans) or after revision (n = 67 scans) is associated with a 30-day thrombosis rate of 0.2% and a revision rate of 0.8% for duplex-detected stenosis (PSV > 300 cm/s). By comparison, 20 (21%) of 95 bypass grafts with a residual (n = 29 grafts) or unrepaired duplex stenosis (n = 53 grafts) or low flow (n = 13 grafts) had a corrective procedure for graft thrombosis (n = 8) or stenosis (n = 12); p < 0.001. Correction of residual graft defects and rescanning to document absence of residual stenosis reduced the incidence of graft problems identified on postoperative duplex surveillance [25].

The algorithm for intraoperative duplex ultrasound assessment of an infrainguinal vein bypass involves imaging the entire arterial reconstruction and classification of findings into one of four categories (Fig. 25.6 and Table 25.2). Scanning is performed after completion of the bypass graft, and patency is verified by clinical inspection and pulse palpation. A 10–15 MHz linear array transducer, placed in a sterile plastic sleeve containing acoustic gel, is used to scan exposed vessels and graft segments beneath the skin. Imaging of the vein graft, anastomoses, and adjacent native arteries is performed in a longitudinal plane along the vessels, beginning at the distal graft anastomotic segment and then proceeding proximally to include the entire venous conduit and the inflow artery-proximal anastomosis graft segment. Papaverine-HCl (30–60 mg) is injected into the distal vein bypass via a 27-gauge needle to augment flow, thereby increasing the diagnostic sensitivity for stenosis detection. Graft PSV is measured (60° or less Doppler angle) at the proximal and distal anastomosis, at selected sites along the graft length (high-thigh, HT; above-knee, AK; below-knee, BK; and distal graft), and from inflow and outflow arteries. Following in situ saphenous bypass, imaging for patent side branches is performed, and the absence of a side-branch flow is confirmed by the presence of a staccato waveform with distal graft occlusion.

Vein conduit stenosis is the most frequent abnormality found with intraoperative duplex scanning. Other abnormalities, in decreasing frequency, include anastomotic stenosis, platelet thrombus, and low graft flow as a result of outflow disease. Immediate repair is indicated for vein valve/anastomotic sites with PSV > 180 cm/s and a velocity ratio (Vr = PSV at lesion/PSV proximal) of >2.5 (Fig. 25.7). Velocity spectra of a high-grade stenosis (PSV > 300 cm/s) at the site of the imaging graft abnormality may represent formation of platelet thrombus. This graft segment should be replaced and a thrombolytic agent infused downstream to lyse thrombus embolized to the distal graft and runoff arteries. In vein grafts of normal (3–5 mm) caliber, a low PSV was measured in only 13 of the 626 grafts, of which 6 were to blind segments. Five of the 13 grafts failed within 90 days. Rzucidlo et al. [24] also reported a high rate of early graft thrombosis when duplex scanning demonstrated no or low (<8 cm/s) flow in diastole in the distal graft. When a high resistance, low graft flow is identified, a careful search for a technical error should be performed, and if none is identified, perioperative heparinization or a procedure to increase graft flow (sequential bypass, distal arteriovenous fistula) should be considered.

Duplex ultrasound is used to confirm graft patency, identify stenotic lesions, assess their risk for producing graft thrombosis, and, if not repaired, monitor stenosis progression.

Testing should include a clinical evaluation for symptoms or signs of limb ischemia, measurement of ABI, and color duplex imaging of the entire bypass graft including anastomosis and inflow/outflow arteries. A “predischarge” scan is recommended to confirm normal functional patency, identify the presence of technical problems and the progression of a residual stenosis detected at operation, and provide baseline graft flow velocities. Upon discharge, patients should be counseled and their primary care physician notified regarding the importance and necessity of graft surveillance. Graft failure has been associated with lack of a surveillance examination within 3 months of the procedure [26, 27]. At each graft surveillance study, patients should also be queried regarding current use of tobacco products and whether they are taking antiplatelet medications. Modifying these two factors associated with graft failure can improve long-term patency.

Factors affecting the frequency of surveillance include the type of bypass and results of the predischarge duplex scan (Fig. 25.8). Grafts at higher risk include those with residual stenosis, those modified during surgery, and those with low graft velocities. If the predischarge scan is normal, the first outpatient graft surveillance is performed 1 month after discharge. At this time, complete graft imaging should be possible, and if no abnormalities are identified, the next surveillance study is scheduled for 3 months later and every 6 months thereafter if normal surveillance studies are obtained. Bypasses with low-flow, residual graft lesions or constructed of arm veins should be evaluated every 3 months for the first year.

Repair of duplex-detected graft stenosis with a PSV > 300 cm/s and Vr > 3.5 is recommended, especially if the lesion has demonstrated progression on serial examinations and MGV has decreased to <45 cm/s [28]. Duplex imaging should demonstrate an anatomic stenosis on power Doppler, and other features of lesion should be characterized including stenosis length, site (graft vs. anastomosis vs. native artery), and the diameter of the graft proximal to the lesion (Fig. 25.8). Other vascular groups also have published similar threshold velocity criteria:

The risk of graft thrombosis is predicted by using the combination of high- and low-velocity duplex criteria discussed above and the ABI values (Tables 25.3 and 25.4).