Flow measurements in dialysis shunts: lack of agreement between conventional Doppler, CVI-Q, and ultrasound dilution

BACKGROUND: Measuring flow in dialysis shunts is recommended to predict imminent thrombosis. Multiple methods for measuring blood flow are in use. Numerous ultrasound protocols exist which determine volume flow using a conventional Doppler (CD) frequency shift analysis technique. All of these are subject to potentially large errors. Quantitative colour velocity index (CVI-Q) does not make use of the Doppler equation and is more precise in vitro. Ultrasound dilution (UD) measures access flow during dialysis in a non-operator-dependent way. The aim of the present study was to compare these three methods of measuring access flow in vivo for agreement with each other. METHODS: In 38 accesses flow was measured by CD, CVI-Q, and UD. All measurements were done during dialysis. Agreement was determined by intraclass correlation coefficient (ICC=R(i)) and Bland-Altman analysis. RESULTS: ICC between UD and CVI-Q was R(i)=0.56. ICC between UD and CD was R(i)=0.10, and ICC between CD and CVI-Q was R(i)=0.16. Bland-Altman analysis revealed a bias (mean difference) of -38 ml/min between UD and CVI-Q, a bias of 1129 ml/min between UD and CD, and a bias of 1167 ml/min between CVI-Q and CD. CONCLUSIONS: CD measurements did not agree with UD or CVI-Q much higher values were recorded with the former than with the latter two techniques. The agreement between UD and CVI-Q measurements is low but reasonable. Caution must be applied in comparing and interpreting values of access flow measured by different techniques.