ESTIMATION OF HEMODIALYSIS ACCESS BLOOD-FLOW RATES BY A UREA METHOD IS A POOR PREDICTOR OF ACCESS OUTCOME

Blood flow in peripheral arteriovenous fistulae and grafts as used for hemodialysis access can be derived from simultaneous measurements of 1) the amount of access recirculation (AR) induced by reversing the di alysis blood lines, and 2) the dialyzer blood flow rate (Qb). The hemo dynamic monitor (HDM) uses magnetic principles to measure AR. The meas urement is based on differential conductivity between arterial (A) and venous (V) blood flows in the dialysis blood tubing sets after the in jection of hypertonic saline into the V line as a conductivity tracer. Access blood flow rates (Qa) derived from AR measurements by the HDM are predictive of access outcome. The measurement of AR is traditional ly done from the comparison of urea levels simultaneously taken from t he A and V blood lines and from the systemic circulation. Thus, the ur ea method can also be used to estimate access blood flow rates. The pu rpose of this study was to determine whether urea based Qa values are also predictive of outcome. Forty-one patients with arteriovenous fist ulae (n = 25) or Core-Tex grafts (n = 16) were studied by a standard p rotocol. The protocol involved temporarily reversing the A and V lines , taking three blood samples for urea estimation, performing an HDM re circulation test, and recording Qb as per the machine blood pump setti ng. The data allowed calculation of Qa by the HDM (Qa [HDM]) and urea (Qa [urea]) methods. Qa (HDM) was 1,177 +/- 887 ml/min (mean +/- stand ard deviation) and Qa (urea) 964 +/- 793 ml/min, a statistically signi ficant difference (paired t-test p < 0.001). There was a significant l inear correlation between the results (r = 0.94, p < 0.0001), but the regression equation also showed that Qa (urea) values were less than Q a (HDM). The influence of the Qa value on access outcome was determine d after an 8 month follow-up. Nine of the 41 accesses were lost to clo tting. Chi-square and discriminate analyses showed that Qa (HDM) signi ficantly (p = 0.005) predicted access outcome, whereas Qa (urea) did n ot (p = 0.164). The specificity of a low Qa (HDM) in predicting access clotting was 0.78, compared with 0.62 for Qa (urea). The data show th at although Qa can be estimated by the urea method, the finding of a l ow Qa (urea) is a poor predictor of access outcome and may lead to cos t ineffective investigations.