Distribution of crystalloid fluid changes with the rate of infusion: a population-based study

Background: Crystalloid fluid requires 30 min for complete distribution throughout the extracellular fluid space and tends to cause long-standing peripheral edema. A kinetic analysis of the distribution of Ringer's acetate with increasing infusion rates was performed to obtain a better understanding of these characteristics of crystalloids. Methods: Data were retrieved from six studies in which 76 volunteers and preoperative patients had received between 300 ml and 2375 ml of Ringer's acetate solution at a rate of 20-80 ml/min (0.33-0.83 ml/min/kg). Serial measurements of the blood hemoglobin concentration were used as inputs in a kinetic analysis based on a two-volume model with micro-constants, using software for nonlinear mixed effects. Results: The micro-constants describing distribution (k(12)) and elimination (k(10)) were unchanged when the rate of infusion increased, with half-times of 16 and 26 min, respectively. In contrast, the micro-constant describing how rapidly the already distributed fluid left the peripheral space (k(21)) decreased by 90% when the fluid was infused more rapidly, corresponding to an increase in the half-time from 3 to 30 min. The central volume of distribution (V-c) doubled. Conclusion: The return of Ringer's acetate from the peripheral fluid compartment to the plasma was slower with high than with low infusion rates. Edema is a normal consequence of plasma volume expansion with this fluid, even in healthy volunteers. The results are consistent with the view that the viscoelastic properties of the interstitial matrix are responsible for the distribution and redistribution characteristics of crystalloid fluid.