Response to rapid volume expansion during the postnatal period.

This study was designed to test the hypothesis that the postnatal diuresis observed in healthy neonates during the second hour of life is due to an expansion of their extracellular volume as a result of a purposeful or spontaneous placental blood transfusion. Seven healthy infants were given infusions of a 5% glucose solution, 0.3 to 2.5 ml/kg/min, over a period of 14 to 70 minutes during the third or fourth hour of life. Plasma and urine were analyzed for osmolality, inulin, and p-aminohippuric acid in order to estimate osmolal and free water clearances as well as glomerular filtration rate and renal plasma flow. Despite individual variation in initial values and in response, glucose infusion caused a prompt two- to fourfold increase in urine output with a similar increase in glomerular filtration rate and renal plasma flow but no consistent change in urine osmolality. The response was of short duration, subsiding within less than 30 minutes following the end of infusion. The fraction of exogenous fluid load excreted was less than 5% in five of seven infants. This study indicates that during the first few hours of postnatal life, the kidney of the newborn infant is capable of responding to a water load in a fashion similar to the kidney of the neonate aged a few days. It can also be concluded that the transient postnatal diuresis observed in these infants could, in part, be due to the expansion of extracellular volume by placental blood transfusion.