Perfusion flow rate, analyte recovery determination

At typical flow rates, the concentration in the dialysate, Cout, is less than the actual concentration in the extracellular fluid, Cext (23). The ratio of Cout/Cext is defined as relative recovery, R, and must be considered for probe calibration and sampling optimization. In vitro, R is easily calculated because the dialysate and the extracellular fluid are homogenous therefore, probe calibration is easily obtained. However, in in vivo studies, calculation of R is difficult because of the active removal of neurotransmitters by uptake and tortuosity. Movement of analytes is impeded by tissue that surrounds the probe, and this movement cannot be easily accounted for with in vitro calibrations. Therefore, the most common method to determine concentrations in vivo is the zero-net flux method, in which known analyte concentrations are added to the perfusate (Cin), and then the analyte concentration is measured at the probe outlet (Cout)- The difference between analyte concentration at the inlet and outlet is used to establish the actual analyte concentration in the tissue, and the relative recovery rate can be calculated. This calibration method can be used to estimate basal levels of neurotransmitters. For example, the zero-net flux method has been used to determine that basal concentrations of dopamine are approximately 1-3.5 nM (24, 25). Although basal level concentrations... 

In ultrafiltration, analyte molecules are basically carried along with the flow of water and electrolytes. The factors determining recovery in ultrafiltration are membrane characteristics, temperatme, and chemistry of the analyte. Unlike microdialysis, recovery is not dependent on flow rate, membrane surface area, or probe size. Recovery tends to be higher than for dialysis, since there is no perfusion medium to dilute the collected analyte. Ultrafiltration recovery rates are typically in the 90-100% range. This high recovery rate simplifies determination of in vivo analyte concentrations. Table I illustrates some in vitro recoveries obtainable with ultrafiltration probes. 

---