Non-invasive impedance analysis: a new non-invasive test for graft surveillance

Detection of failing grafts with early reoperation is clearly associated with better long-term patency than intervention after graft failure. Duplex ultrasonography is more accurate than ankle:brachial index for graft surveillance, but is expensive, time consuming and technically demanding. Non-invasive estimation of graft impedance is now possible. The present study was undertaken to evaluate the utility of non-invasive impedance in detecting the failing vein graft. Sixty-nine grafts in 51 patients were followed over a period of 12 months (April 1992-March 1993). High risk infrainguinal arterial vein bypass patients were entered into a graft surveillance program. Ankle:brachial index, non-invasive impedance and duplex ultrasonography were performed upon discharge, 1 month after surgery and then at 3-monthly intervals. Non-invasive impedance was measured using a mean Doppler flow signal obtained from both upper and lower ends of the graft paired with the mean pulse volume recording obtained from the distal arterial bed. The mean pulse volume recording and flow signals were digitized by discrete Fourier transform and an impedance index generated. An impedance index > or = 0.5 was considered abnormal. Impedance results were compared with ankle:brachial index, duplex ultrasonography and angiography when appropriate, and detected 28 failing and five failed grafts. Non-invasive impedance achieved a sensitivity of 91% and a specificity of 94%. Similarly, duplex ultrasonography was 91% sensitive and 97% specific, while ankle:brachial index was 58% sensitive and 94% specific. Non-invasive impedance is a simple, inexpensive and effective test which detects the failing graft and is an appropriate first-line alternative to duplex ultrasonography for postoperative graft surveillance.