[Postnatal weight loss in ventilated premature infants below 1500 g: significance of renal and extra-renal fluid loss].

In 9 ventilated preterm infants (birth weight 810-1310 g, gestational age 26-30 weeks) we measured total body water (D2O dilution), extracellular volume (sucrose dilution) and renal function parameters at birth and when postnatal weight loss exceeded 5% of birth weight together with day to day changes of weight, fluid balance, and serum electrolytes. A significant postnatal weight loss (-89 +/- 47 g) occurred together with a reduction of total body water (-102 +/- 95 ml) and extracellular volume (-98 +/- 59 ml) in the first three days of life. The intracellular volume did not change. At the same time fluid balance was negative due to a high extrarenal water loss on day 1 and an increasing diuresis on the following days. The serum sodium concentration remained unchanged during postnatal water loss, because the negative fluid balance was accompanied by a negative sodium balance.