Ultrasonography in Vascular Diagnosis by Wilhelm Schäberle

Author:Wilhelm Schäberle
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

4.4 Shunt Maturation and Flow Volume Measurement

A decreased flow through the access fistula, due to complications such as stenosis, impairs hemodialysis function. Only a high-grade shunt stenosis becomes functionally relevant, which, according to Kathrein (1988, 1991), is defined as a decrease in the volume flow rate below 250 mL/min. Although this would seem to be the most obvious thing to do, blood flow is not measured directly in the affected access vein. This is because abrupt changes in diameter, especially in older shunts, and changes in the lumen shape (elliptical) give rise to errors. Determination of mean velocity within the fistula is also impaired by turbulent flow (spectral broadening). For these reasons, the flow volume in an AV hemodialysis access can be determined most reliably by measuring the mean flow velocity in the main feeding artery (typically the brachial artery). The flow volume is the mean velocity (calculated from the Doppler waveform of the feeding artery by means of the implemented software) multiplied by the cross-sectional area (see Sect. 1.1.2.4). Flow volume measurements performed on different ultrasound machines may vary by up to 30%. One reason is the use of different methods for determining the cross-sectional area (direct planimetric measurement or calculation from diameter, leading-edge method). Another is the way in which flow velocity is determined: it may be calculated as the mean velocity in the vessel cross section or as the median velocity. An inadequate receive gain can thus produce measurement errors. Calibration measurements are rarely done before flow volumes are measured. The discrepancies are less relevant as long as serial measurements are performed with the same equipment.

Grosser et al. (1991) investigated volume flow measurements performed in the brachial artery, radial artery, and shunt vein and found the best reproducibility for measurements in the brachial artery. Pitfalls are the calculation of the cross-­sectional areas of small arteries (blooming effect) and the determination of mean flow velocity when flow is very turbulent.

The most accurate results are obtained by calculating the difference in arterial blood volumes before and after the shunt anastomosis. Alternatively, the volume in the inflow artery can be determined relative to that of the contralateral artery. It should be noted, however, that the blood flow to the arm is negligible in most cases compared with the high shunt flow.

Measurement of flow volumes in the draining vein is discouraged, especially in older shunts, as it is subject to the above-mentioned errors. However, when there is complex branching of veins, an attempt can be made to determine the functionally relevant shunt flow of the access vein. To do so, the Doppler sample volume must be placed in a straight segment of vein with little turbulence and without major caliber variation.

The flow volume can serve to evaluate the maturity of a newly created fistula. A wide range of flow rates from 500 to 1,200 mL/min make an AV fistula suitable for hemodialysis.

A retrospective study investigated the accuracy of ultrasound in the evaluation of AV fistula maturity in 69 patients using different criteria such as minimum venous diameter (Robbin et al.

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