Criteria for identification of the "at-risk" infrainguinal bypass graft.

The criteria for identifying the "at-risk" femorodistal bypass are controversial. Eighty-eight patients were entered into a surveillance programme using ankle-brachial pressure indices (ABPI), colour Duplex and intraarterial digital subtraction angiography (IADSA). Changes in ABPI of more than 0.1 identified 12/22 (51%) grafts thought to be "at-risk". In the 88 grafts, a PMV (peak mean velocity) < 45 cm per second had a sensitivity and specificity of 55 and 85% compared to 91 and 95% if a PMV < 45 cm per second and a V2/V1 ratio of greater than 2 was used to identify the 22 grafts "at-risk". In total 341 examinations were performed, the sensitivity, specificity, positive predictive value, negative predictive value and accuracy for impedance analysis was 70, 90, 48, 97 and 91% respectively, compared to 93, 97, 77, 99 and 97% for colour Duplex in identifying the "at-risk" grafts. Between 6 weeks and 12 months the mean PMV was seen to drop by 29% and the mean impedance score by 19%. The most sensitive mode of non-invasive graft surveillance is colour Duplex providing the assessment involves both a measurement of the velocity ratio and the absolute velocity. However, impedance analysis is a better screening test than ABPI and PMV.