Validation of a revised slow-stop flow recirculation method

Slow flow/stop flow methods have replaced the three needle technique as methods of choice for measuring recirculation. However, the time delay after reducing blood flow may affect the BUN in the systemic (slow flow/stop flow arterial line) sample and therefore limit the accuracy of this methodology. It has been observed that recirculation does not occur in a properly cannulated access unless the access blood flow rate is less than the dialyzer blood flow rate (BFR). This suggests that the systemic sample could be obtained at a higher than usual blood pump rate. We studied 50 patients and compared a revised slow-stop flow (S/SF) recirculation technique in which the systemic sample was drawn after the blood pump rate was reduced to 120 ml/min for 10 seconds and then stopped, to a non-urea based method that utilized indicator velocity dilution (IVDM). Seven patients were found to have recirculation by IVDM; all had recirculation by S/SF of more than 10% (minimum 16.7%) and an access BFR that was less than the dialyzer BFR. In the 43 patients without recirculation by IVDM, the mean recirculation by S/SF was 1.9 +/- 3.2% (mean +/- SD). Five patients without recirculation by IVDM had more than 5% recirculation by S/SF (range, 5.9 to 8.3%). Although there was a small systematic tendency to overestimate recirculation, this modified urea based method was still able to detect recirculation with good reliability. Single values above 10% are highly likely to indicate the presence of true recirculation. Repeated values over 5%, are also likely to be significant, indicating the presence of true recirculation and its clinical correlate, marginal access blood flow.