PHASES OF FLUID AND ELECTROLYTE HOMEOSTASIS IN THE EXTREMELY LOW-BIRTH-WEIGHT INFANT

Objective. We had shown previously that preterm infants undergo three phases of fluid and electrolyte homeostasis; prediuretic, diuretic, an d postdiuretic. The objectives of the present study were: (1) to deter mine whether infants even more immature and infants cared for under th ermal environmental conditions different from those previously studied also undergo these three phases; and (2) to relate these phases to ch anges in renal function. Methods. Consecutive, timed urine collections were made during the first 5 days of life in 32 infants with birth we ights of 1000 g or less. Infants were cared for in radiant warmers for 24 hours and then transferred to nonhumidified incubators. Diuresis w as defined as urine flow rate (V) of 3 mL or more/kg per hour and weig ht loss of 0.8 g or more/kg per hour, The physiologic relationships am ong water and sodium balance, insensible water loss, arterial blood pr essure, and renal function were made during the three phases. Results. 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 diu resis 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 s odium excretion was 0.1 mEq/kg per hour; the glomerular filtration rat e (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 d iuretic phase, V and sodium excretion more than tripled; GFR and FENa doubled; and there was no change in urine osmolality. During postdiure sis, V and Na excretion decreased to values intermediate between the p rediuretic and diuretic phases, and FENa fell to prediuretic levels, b ut there was no change in GFR or urine osmolality. There was poor corr elation between blood pressure and GFR. Insensible water loss was high and variable during all phases, exceeding 190 mL/kg per day in the sm allest infants. Conclusions. Extremely low birth weight infants manife st three phases of fluid and electrolyte homeostasis, as do more matur e infants, independent of thermal environment. Diuresis and natriuresi s are the result of abrupt increases in GFR and FENa. We speculate tha t this may be the result of expansion of the neonatal extracellular sp ace as fetal lung fluid is reabsorbed.