Renal Handling of Sodium in Premature and Ful-Term Neonates: a Study Using Clearance Methods during Water Diuresis

Abstract

Summary: A study using fractional clearances during orally induced water diuresis was designed to delineate the mechanism underlying defective tubular reabsorption of sodium in very low-birth-weight neonates. The use of clearance methodology during maximal water diuresis may give an indirect estimate of distal sodium delivery [urine volume (V), C_H2O + C_Na+K], sodium reabsorption at the diluting segments (C_H2O), and proportion of the distal load reabsorbed distally (C_H2O/C_H2O + C_Na+K), when all values are corrected to 100 ml glomerular filtration rate.

The study was carried out in 28 healthy newborn infants who were grouped according to conceptual age (CA): 13 infants with mean birth weight of 1370 ± 330 g and mean CA of 31.8 wk (range, 28-34 wk), and 15 infants with mean birth weight of 2330 2 550 g and mean CA of 37.9 wk (range, 35-41 wk). All studies were performed at 6-7 days of age. It was demonstrated that higher urinary osmolality (67.5 ± 23.2 versus 52.9 ± 9.4 mOsm/kg, P < 0.0025) and higher fractional sodium excretion (2.3 ± 1.8 versus 0.9 2 0.5 ml/dl glomerular filtration, P < 0.01) observed in the group of very preterm infants resulted from significantly decreased proximal (V: 18.7 2 6.0 versus 13.3 ± 3.6 ml/dl glomerular filtration, P < 0.005; C_H2O + C_Na+K: 17.1 ± 5.2 versus 11.9 ± 3.3 ml/dl glomerular filtration, P < 0.005) and distal (C_H2O/C_H2O + C_Na+K × 100: 81.9 ± 8.2 versus 88.2 ± 4.5%, P < 0.01) tubular sodium reabsorption. Distal sodium delivery correlated significantly and inversely with CA when all subjects were considered, but when correlations were assessed separately in each group, only in the group of very preterm infants were significant correlations present between CA and urinary osmolality, sodium and chloride clearances, and percentage of distal sodium reabsorption.

These results suggest that the urinary sodium loss observed in very preterm infants depends on the incapacity of the distal tubule to cope with the arrival of an increased fractional load of sodium. A modest defect in urinary dilution is observed as an obligatory consequence.

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