BACKGROUND: Changes in the volume of the fluid space expanded by i.v. infusion of crystalloid solution have been analysed recently using mathematical models. The models are based on the assumption that the body strives to maintain volume homeostasis of fluid spaces and that the rate of restoration is a function of deviation from resting volume.

METHODS: In this work mathematical models were developed to represent the kinetics of volume changes of fluid spaces associated with infusion of Ringer's solution. Based on the characteristics of fluid transportation, two expansion models were tested. The single-fluid space model has a single volume into which fluid is fed and from which fluid is left, and the two-fluid space model has another space in addition to the first volume so that fluid exchanges between these two spaces are possible. Volunteers (six men) were given Hartman's solution for 30 min and the changes in blood hemoglobin were detected.

RESULTS: From the comparison with experimental data, the single-fluid space model was found to represent adequately in all volunteers, however, two-fluid space model was found to represent 75% of volunteers.

CONCLUSIONS: These models may be useful in the description and analysis of the effects of i.v. fluid therapy.