Rapid computer prediction of total body water in fluid overload

Using computer analysis of the early plasma arterial disappearance curve of tritiated water (HTO), we sought the fewest points and earliest times needed to predict the final volume of dilution, total body water (TBW). In ten anesthetized adult female dogs weighing 19.1 +/- 0.5 kg, with bilateral ureteral ligation, 500 muC HTO were given IV. Arterial blood samples were taken until equilibrium (3 hours), when the approximate equivalent of extracellular fluid (ECF), 4,000 ml of lactated Ringer's solution, was given IV within 1 hour. The next day, in the second phase of the study, 1,000 muC of HTO were given IV and arterial blood samples were taken at intervals up to equilibrium (5 hours). TBW at 3 hours after the first HTO infusion was 63.3 +/- 1.2% body weight. Using a curve-fitting Fortran program (CFIT), the arterial plasma HTO concentrations were fitted to one or two exponentials. Although initial TBW could be predicted from arterial plasma concentrations of HTO during 20 minutes after injection in normally hydrated dogs, values during 60 minutes were required for accurate prediction of TBW after infusion of 4 L of fluid. TBW in normal and fluid-loaded animals was predicted within 2.3 +/- 0.6% of the final HTO equilibrium (r = 0.987).