The continuous increase in the incidence of end stage renal disease and hemodialysis patients, has raised the interest of the vascular access scientific community and many researchers are currently investigating the potential mechanisms of arteriovenous fistulas (AVF) dysfunction. Roy-Chaudhury et al. were the first to suggest neointimal hyperplasia (NIH) as the main reason of AVF dysfunction and vessel negative remodeling as another important factor. First, Yamamoto et al. described 3 stenotic lesions categories of arteriovenous grafts (AVG) observed by ultrasonography: 1. vascular constriction type, 2. neointimal hyperplasia type, 3. mixed type (constriction and neointimal hyperplastic type). The main goals of this prospective observetional study are: * The characterization of AVF stenotic lesions by ultrasonography * The correlation of the stenotic lesion type with the outcomes of percutaneous transluminal angioplasty and the rate of reintervention.