Modelling of peripheral fluid accumulation after a crystalloid bolus in female volunteers - a mathematical study

To simultaneously model plasma dilution and urinary output in female volunteers.

Ten healthy female non-pregnant volunteers, aged 21-39 years (mean 29), with a bodyweight of 58-67 kg (mean 62.5 kg) participated. No oral fluid or food was allowed between midnight and completion of the experiment. The protocol included an infusion of acetated Ringer's solution, 25 ml/kg over 30 min. Blood samples (4 ml) were taken every 5 min during the first 120 min, and thereafter the sampling rate was every 10 min until the end of the experiment at 240 min. A standard bladder catheter connected to a drip counter to monitor urine excretion continuously was used. The data were analysed by empirical calculations as well as by a mathematical model.

Maximum urinary output rate was found to be 19 (13-31) ml/min. The subjects were likely to accumulate three times as much of the infused fluid peripherally as centrally; 1/μ = 2.7 (2.0-5.7). Elimination efficacy, E(eff), was 24 (5-35), and the basal elimination k(b) was 1.11 (0.28-2.90). The total time delay T(tot) of urinary output was estimated as 17 (11-31) min.

The experimental results showed a large variability in spite of a homogenous volunteer group. It was possible to compute the infusion amount, plasma dilution and simultaneous urinary output for each consecutive time point and thereby the empirical peripheral fluid accumulation. The variability between individuals may be explained by differences in tissue and hormonal responses to fluid boluses, which needs to be further explored.