IMPEDANCE ANALYSIS TO IDENTIFY THE AT RISK FEMORODISTAL GRAFT

Computer-assisted impedance analysis is a newly developed technique to identify femoropopliteal and distal vein graft stenoses before failure. Pulsatile flow is measured from the proximal and distal graft by use of an 8Mhz Doppler velocimeter. A pulse volume recorder measures pulsatile pressure within the thigh and calf. Fourier transfer analysis is performed on paired Doppler pulse volume recorder waveforms and an impedance score derived for the thigh and calf, respectively. In a retrospective review of 50 nonreversed femoropopliteal/distal grafts performed for limb salvage, postoperative biplanar intraarterial digital subtraction arteriography was compared with impedance analysis. Arteriography showed graft or runoff stenoses in 22 grafts (at risk) and 28 normal grafts (controls). Impedance scores were significantly higher in the at risk group (0.58 + [0.43 to 0.72]*), when compared with the controls (0.34 + [0.30 to 0.38], p < 0.001*). A thigh or calf impedance score of > 0.45 was able to detect 20 of 22 stenoses, including 6 lesions in grafts with normal resting and postexercise ankle pressures. This score was then applied prospectively and compared with serial biplanar digital subtraction arteriography in a further 56 femoropopliteal/distal bypasses for limb salvage. Thirty-three of 34 lesions were successfully predicted and impedance scores were significantly higher in the at risk limbs (0.56 + [0.44 to 0.68]*) when compared with the controls (0.38 + [0.35 to 0.41], p < 0.001*). In this series impedance analysis proved more sensitive than resting or stressed ankle pressures and, unlike Duplex scanning, was able to detect runoff as well as graft stenoses. The technique is noninvasive, repeatable, and has a role in graft surveillance to identify those at risk bypasses which require secondary procedures.