Hypernatremia in the very low birthweight infant.

The incidence and cause of hypernatremia in twenty-five very low birthweight infants were determined by a retrospective analysis of their clinical course during the first 72 hours of life. Ten (40%) developed a serum sodium greater than or equal to 150 mEq/l. This was associated with a greater weight loss than found in normal infants (5.3 +/- 2.6%/day vs 1.7 +/- 2.3%/day, p less than .05) and with more time spent under radiant energy, but not with the amount of water, or sodium received. The severity of hypernatremia correlated with birthweight (r = -.47, p less than .05) and with postnatal weight loss but not with sodium intake. The excess weight loss in the hypernatremic group, when added to their average measured fluid intake suggests an estimated optimal fluid requirement of 150 ml/kg/day in the environment described. The hypothesized mechanism for hypernatremia is dehydration, possibly through insensible water loss.