Dilution kinetics of chemicals used for estimation of water content of body compartments in perinatal medicine.

Dilution kinetics of markers commonly used for estimation of body water content and distribution in perinatal medicine (p-aminohippurate, inulin, antipyrine, H218O, bromide, and T1824) were studied in pregnant and neonatal baboons. Amniotic fluid concentrations of p-aminohippurate and inulin decreased exponentially after intraamniotic injection of these markers; from 2-24 h after injection, concentrations decreased linearly on semilogarithmic plot (r = 0.96-1.00). Plasma concentrations of antipyrine decreased exponentially during the first 60 min after intravenous injection, then linearly from 1-5 h (r = 0.92-0.90). Plasma concentrations of 18O decreased linearly from 1-6 h after injection in three or four cases (r = 0.94-0.99). Plasma concentrations of bromide decreased during the first 2 h after injection, then stabilized for at least 3 h. Plasma concentrations of T1824 decreased linearly from 10-60 min after intravenous injection (r = 0.97-1.00). Then the decline became exponential until 5 h. These data allow us to make specific recommendations regarding the optimal time and method of amniotic fluid and blood sampling during body water studies.