Urinary excretion as an input variable in volume kinetic analysis of Ringer's solution

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, V-2. To examine if a fixed elimination rate constant (k(r)) 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 k(r) 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 k(r) and V-2 (r(2) less than or equal to -0.98). The size of V-2 was 10 litre when the fixed and model-generated values of k(r) agreed fully.