The dilution of the blood and the retention of infused Ringer's solution in the body increases in the presence of hypovolemia, which can be attributed chiefly to a reduction of the elimination rate constant.

The relative efficiency of crystalloid infusion fluids differs depending on whether the entire dilution-time profile or only the maximum dilution is compared. Kinetic analysis and simulation is a useful tool for the study of such differences.

The kinetics of crystalloid solutions in humans have not been adequately described previously. Therefore, we measured blood haemoglobin concentration during and for 120 min after i.v. infusion of 25 ml kg-1 of Ringer's acetate solution over 15, 30, 45 and 80 min, and 12.5 ml kg-1 over 30 min in six adult female volunteers. The dilution-time profiles were analysed according to a new kinetic model adapted for fluid spaces. Volume expansion produced by Ringer's solution approached steady state in an exponentially decaying manner when plasma volume had increased by approximately 550 ml. The size of the fluid space expanded by Ringer's solution was only 4.8 litre (95% confidence interval 3.8-5.8 litre) except for the fastest infusion, where it averaged 9.0 litre. The rate of fluid elimination could be predicted as the product of plasma dilution and a constant averaging 95 (95% confidence interval 68-122) ml min-1.

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.

The disposition of fluid given by i.v. infusion can be studied by fitting one-volume and two-volume kinetic models to the fractioned dilution of blood haemoglobin and serum albumin concentrations over time. However, the two-volume model is sometimes associated with a high standard error in estimating the size of the secondary (peripheral) body fluid space, V2. To examine if a fixed elimination rate constant (kr) determined by urinary excretion can be used to make the model more stable, we infused Ringer's acetate 25 ml kg-1 over 30 min in 15 male volunteers (mean age 35 yr). A fixed kr increased the total residual error when curve-fitting was applied according to the one-volume model. The two-volume model was improved when there was a strong within-patient covariance between kr and V2 (r2< or = -0.98). The size of V2 was 10 litre when the fixed and model-generated values of kr agreed fully.