Postnatal Weight Loss in Preterm Neonates <1500 g is Due to Isotonic Dehydration of the Extracellular Volume

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Abstract
Weight, extracellular volume (ECV; distribution volume of sucrose) and renal function were studied in 13 preterm infants at birth (age 6 h (2-12); median, range) and again when postnatal weight loss exceeded 5% of birth weight (age 84 (64-97) h). Gestational age was 28 (26-32) weeks, and birthweight was 1170 g (810-1 455). The infants were nursed in incubators and mechanically ventilated. Fluid therapy allowed a weight loss of up to 10% of birthweight. Body weight decreased significantly from 1101 .+-. 202g at birth to 1016 .+-. 198 g at day 3 and ECV from 499 .+-. 155 ml to 413 .+-. 118 ml. Mean weight loss of 85 .+-. 50 g was the same as mean ECV loss of 86 .+-. 48 ml, suggesting that postnatal weight loss is water loss from the ECV. Weight loss was preceeded by a marked increase in diuresis, exceeding fluid intake on day 2. Creatinine clearance did not change. The increased urine output led to a significant increase of sodium excretion without inducing hyponatremia but resulted in an isotonic reduction of ECV.