Urinary acidification in extremely low birth weight infants.

Premature infants often present metabolic acidosis without protein load in the early neonatal period, around days 4-6. In order to elucidate the cause of acidosis, we investigated urinary acidification of infants in the early neonatal period. Urine pH, fractional excretion of HCO(3)(-) (FEHCO(3)), excretion of HCO(3)(-) and NH(4)(+) of the appropriate-for-date infants were measured on days 0-2 and on days 4-6 of life. Extremely low birth weight (ELBW) infants showed higher urine pH than more than 1500 g birth weight infants. FEHCO(3) and HCO(3)(-) excretion were of high values in ELBW infants on days 0-2, but decreased on days 4-6. Urine NH(4)(+) excretion rate was lower in ELBW infants than in birth weight more than 1000 g on days 0-2 of life and still remained at a low rate on days 4-6. These data indicated that insufficiency of NH(4)(+) excretion is the main cause for metabolic acidosis of ELBW infants in the early neonatal period.