Phases of fluid and electrolyte homeostasis in the extremely low birth weight infant

We had shown previously that preterm infants undergo three phases of fluid and electrolyte homeostasis; prediuretic, diuretic, and postdiuretic. The objectives of the present study were: (1) to determine whether infants even more immature and infants cared for under thermal environmental conditions different from those previously studied also undergo these three phases; and (2) to relate these phases to changes in renal function.

Consecutive, timed urine collections were made during the first 5 days of life in 32 infants with birth weights of 1000 g or less. Infants were cared for in radiant warmers for 24 hours and then transferred to nonhumidified incubators. Diuresis was defined as urine flow rate (V) of 3 mL or more/kg per hour and weight loss of 0.8 g or more/kg per hour. The physiologic relationships among water and sodium balance, insensible water loss, arterial blood pressure, and renal function were made during the three phases.

Twenty-eight (87%) of the 32 infants underwent the three homeostatic phases. The median ages of onset and cessation of diuresis were 25 and 96 hours, respectively. There was no correlation between onset of diuresis and change of thermal environment. During the prediuretic phase, V averaged 1.6 mL/kg per hour, and 17 of 28 infants had at least one collection period in which V was less than 1 mL/kg per hour; urinary sodium excretion was 0.1 mEq/kg per hour; the glomerular filtration rate (GFR) was 0.22 mL/kg per hour; fractional excretion of sodium (FENa) was 6.2%; and urine osmolality was dilute (221 mOsm/kg). During the diuretic phase, V and sodium excretion more than tripled; GFR and FENa doubled; and there was no change in urine osmolality. During postdiuresis, V and Na excretion decreased to values intermediate between the prediuretic and diuretic phases, and FENa fell to prediuretic levels, but there was no change in GFR or urine osmolality. There was poor correlation between blood pressure and GFR. Insensible water loss was high and variable during all phases, exceeding 190 mL/kg per day in the smallest infants.

Extremely low birth weight infants manifest three phases of fluid and electrolyte homeostasis, as do more mature infants, independent of thermal environment. Diuresis and natriuresis are the result of abrupt increases in GFR and FENa. We speculate that this may be the result of expansion of the neonatal extracellular space as fetal lung fluid is reabsorbed.