ACCURACY OF DILUTION TECHNIQUES FOR ACCESS FLOW MEASUREMENT DURING HEMODIALYSIS

Access flow is now widely measured by creating artificial recirculatio n with the dialysis lines reversed and using dilution methods that sen se either ultrasound velocity, electrical impedance, optical, or therm al changes, This study identifies and quantifies factors that influenc e the accuracy of access flow measurements and recommends ways to redu ce these errors. Two major sources of access flow measurement error ar e identified, arising firstly from the second pass of the indicator by recirculation through the cardiopulmonary system (cardiopulmonary rec irculation, CPR), and secondly from changes in venous line blood flow (Qb) and vascular access flow induced by the pressure of venous bolus injections. These errors are considered from theory, by direct measure ment of access flow in a sheep model, and by analysis of clinical data . Two extremes for the venous introduction of indicator can be conside red in access flow measurements, a slow infusion, which perturbs neith er the venous line flow nor access flow but increases the error attrib utable to the second pass of the indicator by recirculation through ca rdiopulmonary system, or rapid injection, which eases separation of th e second pass of the indicator signal but generates changes in the ven ous flow and access flow. If CPR is not eliminated, the area added to that of the first pass of indicator ranges up to 40%. Good time resolu tion could permit the separation of the areas generated by the first a nd second passage of the indicator. In sheep experiments, injections o f 5 or 10 mt into a venous port close to the vascular access caused Qb to change by 20% to 40%. Both the animal experiments and analysis of raw data collected during routine clinical dialysis showed that moving the injection site sufficiently far from the patient, before or into the venous bubble trap, reduced the increase in Qb to only approximate ly 5% during the critical time when the concentration curve is changin g for most tubing brands (Baxter, Belco, Gambro, Hospal, Medlsystem, a nd National Medical Care). Because of the smaller volume of the venous bubble chamber in Cobe tubing (Cobe, Centrysystem 3), this brand show ed approximately a 20% increase in ab. Moving the site of bolus infect ions to before the bubble trap in the sheep experiments also eliminate d the influence of changes in access flow. An additional error in acce ss flow measurement of 20% or more arises from the use of flow reading taken from pump setting rather than a measured flow. The discrepancy between the real flow and pump setting is attributable to needle size, vascular access conditions, or pump calibration. The results show tha t problems can be minimized by using a dual sensor system that retains the precise timing necessary for separation of access recirculation f rom CPR; by accurate measurement of dialyzer blood flow; by moving the site of injection to before the venous bubble trap, sufficiently far from the patient, and correcting for any remaining deviations in flow in the venous line concurrent with the dilution curve. (C) 1998 by the National Kidney Foundation, Inc.