Volume kinetics is a method used for analyzing and simulating the distribution and elimination of infusion fluids. Approximately 50 studies describe the disposition of 0.9% saline, acetated and lactated Ringer's solution, based on repeated measurements of the hemoglobin concentration and (sometimes) the urinary excretion. The slow distribution to the peripheral compartment results in a 50-75% larger plasma dilution during an infusion of crystalloid fluid than would be expected if distribution had been immediate. A drop in the arterial pressure during induction of anesthesia reduces the rate of distribution even further. The renal clearance of the infused fluid during surgery is only 10-20% when compared with that in conscious volunteers. Some of this temporary decrease can be attributed to the anesthesia and probably also to preoperative psychologic stress or dehydration. Crystalloid fluid might be allocated to "nonfunctional" fluid spaces in which it is unavailable for excretion. This amounts to approximately 20-25% during minor (thyroid) surgery.
