Secondary failure is defined as failure of a permanent access after it has been successfully used for dialysis, according to some investigators, for six or more dialysis treatments. Secondary failure of access contributes to under-dialysis, increased morbidity, and even mortality. In addition, the cost of care is also quite high, estimated to be in billions of dollars annually. Diagnosing an access problem and taking corrective measures in a timely fashion appear to be effective in reducing the above problems. Routine monitoring and surveillance are necessary in order to predict impending failure so that problems can be corrected in a timely fashion in order to (a) prevent under-dialysis, (b) prevent other complications such as infection, and (c) preserve the access vein. Thus, the importance of monitoring and surveillance cannot be emphasized enough. In the United States, the Centers for Medicare and Medicaid Services (CMMS) mandates that all dialysis providers must have a well-defined protocol of successful surveillance and monitoring of access with “…strategies including device-based methods such as access flow measurements, direct or derived static venous pressure ratios, duplex ultrasound, etc.” [5]. While both monitoring and surveillance have the same goal of evaluation of an access and detection of impending problem and are complimentary, monitoring usually refers to evaluation of vascular access by physical examination, and surveillance is an ongoing access assessment by special tests and diagnostic procedures.

Basic understanding of blood flow and pressure dynamics in the permanent access is necessary for a physician to perform an appropriate examination, properly analyze the data, and establish the correct diagnosis. The flow and pressures are different in AVF from that encountered in AVG. Hemodialysis access is created by connecting a high-flow and high-pressure arterial conduit to a low-flow and low-pressure venous conduit either directly, the AVF, or through a tube of synthetic or biological material, the AVG. In an AVF, blood flows rapidly and under high pressure into the vein, pressure being very high adjacent to the anastomosis. Veins are capable of expanding, unlike grafts, and the dilatation leads to reduced resistance and the rapid dissipation of pressure further downstream (Fig. 20.3). However, the synthetic tube for AVG is not distensible, and resistance inside remains high, declining very slowly to the venous pressure at the venous anastomosis. AVG require higher blood flow rates, such as 1 L or so per minute, in order to remain patent. In contrast, AVF are functional even at lower blood flow rates of 500 ml/min or lower (even down to 200 ml/min). The connection of a high-pressure system to a low-pressure system causes turbulent flow and eddy currents, felt as a thrill and auscultated as bruit. In well-functioning access, the thrill/bruit is present through the entire AVF and AVG and felt both during systole and diastole. The location, intensity, and timing of the thrill and bruit are affected by the flow rates and development of stenosis and/or thrombosis, and this information is used in the diagnosis of any developing problems.