[Method of determining glomerular filtration rate and extracellular fluid volume in the perioperative phase]

In the course of an investigation concerning the renal response in abdominal surgery, we paid special attention to the precision of determination of the glomerular filtration rate (GFR) and extracellular fluid volume (ECF). The influence of the applied mathematical model on calculation using slope techniques was examined. ECF and GFR were measured by single injection of the radioactive tracer 51Cr-EDTA; plasma samples were taken during the first 4 h postinjection. The results were computed according to the compartmental and noncompartmental model. The correlation between two-compartmental and noncompartmental GFR and ECF values was highly significant. However, values calculated from two-compartmental model were significant lower (p less than 0.01) and differences up to 20% were observed. Calculating ECF by the most widely used approximative method (assuming an early equilibrium between the extra- and intravascular part of ECF) yields incredible high values and correlation to other methods was poor. Based on our data, we conclude that GFR and ECF values obtained by compartmental analysis depend on flow rates between the theoretical compartments. Also the use of two or three exponential components for the plasma curve in the course of noncompartmental analysis (as done by most authors) is not any better than compared conventional compartmental analysis. Noncompartmental analysis should be preferred especially when small changes in ECF and GFR are observed, as it is the more accurate.