Determination of access blood flow from ionic dialysance: theory and validation

Several noninvasive techniques have been recently developed for calculating blood flow rate of vascular access in hemodialyzed patients from the on-line measurement of recirculation ratio by injecting a saline bolus when the blood lines are reversed. Here we describe a new noninvasive method based on ionic dialysance measurements without the need of a saline bolus.

Mathematical modeling allows to calculate blood flow in vascular access (QA) from the recirculation ratio (Rrev) measured when the blood lines are reversed, without the need to stop ultrafiltration, by using the formula: QA = (QB - QF) 1 - Rrev/Rrev where QB is the blood flow at the dialyzer inlet and QF the ultrafiltration rate. Because the ionic dialysance takes recirculation into account, we tested a new method to assess QA from the measurement of ionic dialysance at normal (D) and reverse (Drev) positions of the blood lines for the same QB. Assuming the absence of access recirculation at normal position of the blood lines, mathematical modeling provides the following relationship: QA = (D - QF)Drev/D - Drev. The estimation of QA from measurement of ionic dialysance (QA-ID) was compared in 28 patients to the estimation of QA by ultrasound dilution technique (QA-US).

The two methods were strongly correlated (QA-ID = 1.24 * QA-US, r2 = 0.86, P < 0.0001). The difference between QA-ID and QA-US was 107 +/- 387 ml/min (mean +/- SD).

Our method provides a valuable estimation of the vascular access flow and is fully noninvasive, easy to perform (no need of bolus injection and of accurate measurement of QB) and totally inexpensive. Consequently this method is suitable for monitoring access blood flow in hemodialyzed patients in order to predict access thrombosis and to treat significant stenotic lesions before thrombosis.