SALT AND WATER HOMEOSTASIS IN THE NEWBORN

Abstract

Fluid and electrolyte administration to infants has been empiric and is critical to the survival of low birth weight infants.Inulin clearance/CIN/, sodium reabsorption and clearance/CNA+/ and free water clearance/CH2O/ by constant infusion with free voiding technique were compared at I.V. infusion rates of 3.6/L.R./or 10.3/H.R./ ml/kg/hr in 32 healthy appropriate for gestational age infants 545–3900 gm. Fractional Na+ reabsorption was similar in both groups, 98.8±.79%/L.R./and 98.5±1.5/H.R./.CH2O in 9 infants 2000 grams studied at L.R. and, in 5 infants at H.R. were 2.1±O.98 and 5.O±2.7 ml/min/100 ml GFR respectively /p= .025/. Urine volumes were 5.96 and 9,5ml/min/100 ml GFR respectively. Na+ reabsorption in the distal nephron /CH2O/CH2O+cNa/ was 64.7± 17% in the L.R. group. These data demonstrate that infants can excrete water and readsorb Na+ efficiently at L.R. Our results suggest that H.R. of fluid can be given to low weight infants if the osmotic load is not excessive. They respond to increased I.V. infusion rates by increasing delivery from the proximal nephron /V. x100/GFR/ but can reabaorb the increased sodium load presented to their distal nephrons and increase CH2O.