At each appointment, Ankle Brachial Pressure Index (ABPI) is measured and then Duplex scanning (DS) is performed. In particular, by means of DS the peak systolic flow velocity (PSFV) at multiple sites along the entire graft is measured. A graft at risk of failure was defined as having a PSFV < 45 cm/s or a ratio of V2 (peak velocity at the site of the stenosis) to V1 (peak systolic velocity at any other point within 2 cm at the normal adjacent graft) > 2. Any other irregularities, such as inflow/outflow problems, graft dilatation will be also recorded.