The advantages of the AV fistula use are lower infection rates, because no foreign material is involved in their formation, higher blood flow rates (which translates to more effective dialysis), and a lower incidence of thrombosis. The complications are fewer than with other access methods. If a fistula has a very high blood flow and the vasculature that supplies the rest of the limb is poor, a steal syndrome can occur, where blood entering the limb is drawn into the fistula and returned to the general circulation without entering the limb's capillaries. This results in cold extremities of that limb, cramping pains, and, if severe, tissue damage. One long-term complication of an AV fistula can be the development of an aneurysm, a bulging in the wall of the vein where it is weakened by the repeated insertion of needles over time. To a large extent the risk of developing an aneurysm can be reduced by carefully rotating needle sites over the entire fistula, or using the "buttonhole" (constant site) technique. Aneurysms may necessitate corrective surgery and may shorten the useful life of a fistula. Fistulas can also become blocked due to blood clotting or infected if sterile precautions are not followed during needle insertion at the start of dialysis. Because of the high volume of blood flowing through the fistula, excessive bleeding can also occur. This is most common soon after a dialysis treatment. Pressure must be applied to the needle holes to induce clotting. If that pressure is removed prematurely or a patient engages in physical activity too soon after dialysis, the needle holes can open up.