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Despite having incompletely developed kidneys very-low-birth-weight infants appear to eliminate balanced amounts of bicarbonate in their urine. Researchers chemically analyzed blood and urine samples taken during the first four days of life from 22 very-low-birth-weight infants. Urinary bicarbonate levels increased and then decreased following the three stages of the fluid balance cycle as documented for pre-term infants. The infants retained more bicarbonate than they eliminated during the study period. Most (88%) of the sodium eliminated during the middle stage was accompanied by chloride and not bicarbonate., Objective. To test the hypothesis that due to the immaturity of their kidneys extremely low birth weight infants lose large amounts of bicarbonate in their urine. Methods. Urine and blood samples collected every 8 to 12 hours for the first 4 days of life from 22 preterm infants 23 to 29 weeks' gestation weighing 540 to 982 g at birth were prospectively studied. Results. As described previously, three phases of fluid homeostasis were identified. The first phase (prediuresis) was a period of low urine output followed by a period of spontaneous diuresis/natriuresis (diuretic phase) and then by a phase when urine output varied according to fluid intake (postdiuresis). Sodium, potassium, chloride, and bicarbonate excretion rates and bicarbonate balance (bicarbonate or acetate infused minus bicarbonate excreted) were calculated for each of the three phases. Urinary excretion of sodium, potassium chloride, and bicarbonate increased from the prediuretic to the diuretic phase and decreased from the diuretic to the postdiuretic phase. During the diuretic phase 88% of renal sodium excretion was accompanied by excretion of chloride. Bicarbonate balance was positive in all three fluid phases. Cumulative renal bicarbonate loss over the first 4 days of life was 1.9 [+ or -] 0.5 meq/kg (SD) and the cumulative bicarbonate balance was [+ or -]4.4 [+ or -] 4.1 meq/kg (SD). The glomerula' filtration rate, filtered load of bicarbonate, and absolute tubular reabsorption of bicarbonate significantly increased from the prediuretic to the diuretic phase, while fractional reabsorption of sodium and chlor de decreased between these two phases. The fractional reabsorption of bicarbonate did not change from prediuresis to diuresis, but increased from diuresis to postdiuresis and consequently from prediuresis to postdiuresis. Conclusions. Contrary to our original hypothesis, the total renal bicarbonate excretion of extremely low birth weight infants in the first 4 days of life is low and the net bicarbonate balance is positive. The anion predominantly accompanying the excretion of sodium in all three phases is chloride and not bicarbonate. Bicarbonate excretion appears to be independent of sodium excretion during these phases. The increase in renal tubular bicarbonate reabsorption during the first week of life may be associated with extracellular volume contraction. Pediatrics 1996;98:256-261; renal bicarbonate excretion, renal sodium excretion, renal chloride excretion, extremely low birth weight infants, renal function., ABBREVIATIONS. [Mathematical Expression Omitted], bicarbonate; ECV, extracellular volume; [Mathematical Expression Omitted], plasma bicarbonate; GFR, glomerular filtration rate; FENa, fractional excretion of sodium; ELBW, extremely low birth weight; NICU, neonatal intensive [...]

Kidney function appears to go through three phases in very low birth weight babies. Researchers evaluated urine output and kidney function in 32 infants weighing 1,000 gm or less at birth. The infants followed a similar pattern: for about the first day after birth, urine output was low. This was followed by a marked increase in urine output and sodium loss. The first two phases were not correlated with intravenous fluid intake or with transfer from an overhead heating bed to an incubator, which could have affected water loss through the skin. Finally, the infant established a urinary excretion pattern responsive to fluid intake. This same pattern was seen in a study where infants were kept in a humid environment. These data suggest that the pattern may be a normal adaptation to life outside the womb. Intravenous fluids and ion balance must be monitored carefully and tailored individually., Objective. 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. Methods. 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 (V7) 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. 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 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. Conclusions. 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. Pediatrics 1995,96:484-489; neonatal renal function, postnatal diuresis, postnatal adaptation, fetal-to-neonatal transition, insensible water loss., ABBREVIATIONS. RDS, respiratory distress syndrome; GFR, glomerular filtration rate; FENa, fractional excretion of sodium; ELBW, extremely low birth weight; IWL, insensible water loss; O/I, urine output to fluid intake ratio. [...]