A Comparison of Chloride, Bromide, and Sucrose Dilution Volumes in Neonatal Pigs 1

Abstract Use of 36Cl, 82Br, and [3H]sucrose to estimate extracellular water volume was evaluated in 14 piglets (7–14 days old). 36Cl and 82Br were distributed in approximately the same volume, but a period of 5–6 hr after injection was required to reach equilibrium in the neonatal pig. Dilution volumes calculated before equilibration (2–5 hr) for 36Cl (326 ± 11 ml/kg) and 82Br (328 ± 13 ml/kg) were different from equilibration (6–8 hr) phase volumes (356 ± 13 ml/kg and 355 ± 13 ml/kg, respectively; P < 0.001). A 3-hr sample estimated the same volume distribution calculated by extrapolation of the 6- to 8-hr period because of the relationship between the two slopes of the plasma clearance curves. After the 82Br and 36Cl had achieved equilibration, each was distributed in a volume equivalent to total body chloride space (362 ± 29 ml/kg) measured by neutron activation; no statistical differences were found (P = 0.6). The early equilibration phase measured a 10% smaller, faster exchangeable fraction of total body Cl. Sucrose dilution volume (332 ± 19 ml/kg) required multiple plasma samples for extrapolation and measured a dilution volume 7% smaller (P < 0.05) than total body chloride space.