Which of the following statements is/are true regarding the use of duplex scanning as a means to follow and monitor bypass grafts?

a. Duplex scanning is accurate and cost effective

Duplex scanning is an excellent way to monitor bypass grafts. It can detect graft threatening defects before the patient becomes symptomatic and before the ankle pressure begins to drop. A localized increase in systolic velocity greater than 100% of that of the adjacent graft identifies a diameter reduction of more than 50%. Peak systolic velocities less than 40 cm/sec are an ominous sign of markedly reduced flow. Arterial venous fistulas are regularly recognized by their pattern of localized flow disturbances with increased velocities at the site of the fistula and immediately proximal to the fistula and concomitant decreased velocities just below the fistula.