Graft Thrombosis

B-Mode Ultrasonography

B-mode ultrasonography has been used intraoperatively to obtain anatomic images noninvasively, although it is more commonly used in conjunction with duplex ultrasonography. Initial experimental studies established that its ability to detect small defects in patients was comparable to that of arteriography. In an evaluation of arterial defects created in dogs, both arteriography and B-mode ultrasonography were nearly 100% specific in excluding arterial defects. However, ultrasonography has significantly greater sensitivity in detecting defects, 92% overall, than serial biplanar arteriography at 70% and portable arteriography at 50%. These techniques have comparable accuracy in detecting stenoses.

B-mode ultrasonography has utility in assessing lower extremity arterial reconstructions. Kresowik et al. reported that in 106 patients, intraoperative B-mode ultrasonography detected defects in 20% of patients, and that half of these defects were deemed important enough to warrant correction. In follow-up, there were no early graft occlusions in the B-mode group, and no residual defects were discovered with duplex scanning follow-up in the postoperative period.

Intraoperative use of B-mode ultrasonography alone, however, is not without its problems. Because this modality does not evaluate blood flow, it cannot differentiate fresh thrombus from flowing blood, which has the same echogenicity. Compared with Doppler pencil probes, B-mode ultrasound probes are larger and cannot be sterilized, requiring a sterile covering containing a gel to maintain an appropriate acoustic interface. Significant operator experience is needed to obtain optimal images and make accurate interpretations.

Duplex Ultrasonography

With the addition of flow-measuring capability to B-mode ultrasonography, duplex scanning brings a more powerful tool to the operating room. Like B-mode ultrasound probes, duplex scanning probes are large, cannot be sterilized, and require considerable operator skill so that accurate velocity and imaging data can be obtained. Duplex color-flow technology provides continuous Doppler signals along the graft and artery at multiple points. Color imaging facilitates identification of areas of higher velocity.

Duplex scanning provides an alternative mechanism for identifying defects in proximal arterial anastomoses in situations where placing a catheter for arteriography proximal to the proximal anastomosis is cumbersome or difficult. In addition, duplex scanning can identify low graft velocities that are only indirectly measured by arteriography and depend on the observations of the surgeon. Intraoperative experience with this imaging modality has shown greater sensitivity for detecting technical defects than other methods. Early results with intraoperative duplex scanning have demonstrated an association between these defects and suboptimal results in the postoperative period. ^, A study by Rzucidlo et al. reported intraoperative completion duplex scanning to be a useful tool after the completion of infrageniculate arterial reconstruction. Specifically, the authors documented that a 10-MHz, low-profile transducer could be used successfully to identify compromised grafts with a predilection for early failure. Moreover, it was determined that low end-diastolic velocity (EDV) was both associated with, and predictive of, early graft failure. Further, Scali et al. validated the utility of intraoperative completion duplex scanning following distal bypass, documenting that EDV measurements less than 5 cm/s predicted early graft failure ( Fig. 48.1 ). Johnson et al. found that duplex use was associated with a 15% intraoperative revision rate that resulted in a significant reduction in early graft failure/revision. However, it is important to note that duplex ultrasound has some important drawbacks. It is challenging and sometimes not possible to assess newly placed polytetrafluoroethylene (PTFE) and polyester (Dacron, Hemashield) grafts because the graft walls contain air, which prevents penetration of the ultrasound waves. Furthermore, it can be difficult to assess the outflow in the foot following bypass creation due to the presence of vasoconstriction or outflow disease.

Arteriography

Since its introduction, intraoperative completion arteriography has been the gold standard for anatomic evaluation of the technical adequacy of arterial reconstructions. One appealing feature of arteriography is its ability to assess anatomic arterial outflow – the “runoff.” This is particularly important in the clinical context of preoperative studies that fail to reveal adequate target vessels in diffusely diseased vascular systems. Although completion arteriography is an invasive procedure associated with potential complications because of arterial puncture (intimal injury, dissection), injection (air embolism), use of radiographic contrast agents (renal failure, anaphylaxis), and radiation exposure, the actual observed complication rate has been negligible in large series. ^, Arteriography has become our method of choice for intraoperative graft assessment because like most vascular surgeons we are comfortable performing this technique and at interpreting arteriographic images. Additionally, an increasing number of vascular surgeries are performed in operating rooms with fluoroscopic capability which makes it quick and easy to perform arteriography.

The technique varies according to individual application but generally involves insertion of an 18- to 20-gauge plastic angiocatheter or 4- to 5-Fr sheath into the arterial graft to allow subsequent injection of 10 to 30 mL of radiographic contrast agent. Temporary occlusion of arterial inflow maximizes the concentration of contrast agent without the need for excessively rapid injection. Digital subtraction angiography, rather than simple fluoroscopy with concomitant contrast injection, will then provide clearer images and allow for visualization of vessel patency and runoff ( Fig. 48.2 ).

Indirect information obtained with angiography includes the observed flow rate (emptying) of the conduit which provides an estimate of conduit patency and outflow. When evaluating autogenous conduits, the proximal anastomosis and the entire conduit should be included in the evaluation. This allows for the detection of proximal anatomic defects as well as the presence of any intrinsic defects or twists within the conduit itself. In the setting of an in situ reconstruction, the presence and location of arteriovenous fistulas can be identified and treated. For prosthetic grafts, the graft itself may be punctured and the evaluation begins with the distal anastomosis. Care should be taken during injections to ensure that there are no air bubbles or overlying structures that may lead to false-positive interpretations. Recalling that angiography is a two-dimensional modality this method can result in underestimation of the stenosis from a small defect, such as an intimal flap or platelet aggregate. Contrast density may be reduced in a focal area suggesting such an occurrence and an oblique projection may more definitively reveal an underlying defect.