The role of Doppler ultrasonography in vascular access surveillance-controversies continue

Abstract

Chronic hemodialysis therapy required regular entry into the patient's blood stream with adequate flow. The use of arteriovenous fistulas and grafts is linked with lower morbidity and mortality than the use of catheters. However, these types of accesses are frequently affected by stenoses, which decrease the flow and lead to both inadequate dialysis and access thrombosis. The idea of duplex Doppler ultrasound surveillance is based on the presumption that in-time diagnosis of an asymptomatic significant stenosis and its treatment prolongs access patency. Details of performed trials are conflicting, and current guidelines do not support ultrasound surveillance. This review article summarizes the trials performed and focuses on the reasons of conflicting results. We stress the need of precise standardized criteria of significant access stenosis and the weakness of the metaanalyses performed.

Keywords: Hemodialysis; arteriovenous fistula; arteriovenous graft; hemodialysis vascular access; ultrasonography.

Figure 1.

Correct orientation of the Doppler angle in the vascular lumen. The cosinus of the Doppler angle is a part of the equation transforming the Doppler frequency shift into the velocity. Therefore, it should be always the same for the follow-up. Manufacturers set it usually to 60°.

Figure 2.

Flow volume measurement in the graft. The flow volume calculation is based on the following formula: Qa = πr × TAMEAN, where r is the radius of the examined graft and TAMEAN is the time-averaged mean velocity inside. The latter is an integral value during the heart cycle of the average velocity layer (flow velocity is the fastest in the middle and the slowest along the vascular wall). TAMEAN must not be replaced by TAMAX, which is the time velocity integral of the fastest velocity only—using TAMAX would lead to significant overestimation of Qa. Pulse wave Doppler sample size should be wide enough to cover most of the vessel lumen because of the different speeds of blood inside the vessel from the center to the wall.

Figure 3.

RD measurement. There is excessive intimal hyperplasia in the outflow vein causing the stenosis. By diameter reduction, this stenosis would be significant if compared with the left or right part of the vein. Nevertheless, the RD is 2.35 mm, so this stenosis was considered borderline, and soon, reevaluation (within 6–8 weeks) was indicated.