Modelling the volume of expandable body fluid spaces during i.v. fluid therapy.

We have developed mathematical models to represent the changes in volume of fluid spaces associated with i.v. administration of a crystalloid solution. Input data for parameter estimations were dilution of blood, measured as reduction of blood haemoglobin concentration. The models were 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. Two models were derived; the first had a single fluid space into which fluid was administered and from which fluid left, the other model had a second fluid space communicating with the first. These models may be useful in the description and analysis of the effects of i.v. fluid therapy.