Response to Rapid Volume Expansion During the Postnatal Period

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Abstract
The hypothesis that the postnatal diuresis observed in healthy neonates during the 2nd h of life is due to an expansion of their extracellular volume as a result of a purposeful or spontaneous placental blood transfusion was tested. Healthy infants (7) were given infusions of a 5% glucose solution, 0.3-2.5 ml/kg/min over a period of 14-70 min during the 3rd or 4th h 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 2- to 4-fold 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 < 30 min following the end of infusion. The fraction of exogenous fluid load excreted was < 5% in 5 of 7 infants. Apparently, during the first few h of postnatal life, the kidney of the new-born infant is capable of responding to a water load in a fashion similar to the kidney of the neonate aged a few days. The transient postnatal diuresis observed in these infants could be due partly to the expansion of extracellular volume by placental blood transfusion.