Sodium balance in very low-birth-weight infants

Fluid and electrolyte balance in extremely preterm infants <24 weeks of gestation in the first week of life

BACKGROUND

Extremely preterm infants, especially those <24 weeks of gestation, displayed extremes in changes of fluid/electrolyte balance. The purpose of the present study was to retrospectively investigate abnormalities in fluid/electrolyte balance during the first postnatal week.

METHODS

Of 32 subjects admitted during 20 years, 17 infants with no severe complications were examined, and 72 appropriate-for-dates infants at 24-28 weeks of gestation were taken as controls. The subjects were managed in closed incubators in a highly humidified environment and fluid/electrolyte balances were monitored in retrospect.

RESULTS

The subjects had higher urine contents, while the insensible water loss was higher than in controls. As for weight loss, no remarkable changes were noted between the two groups. On day 4 and thereafter, serum Na(+) levels were high, indicating a high complication rate of hypernatremia without differences in hyperkalemia. The decrease in blood base excess was rapid, requiring supplementation of sodium bicarbonate up to postnatal day 5 in all preterm infants.

CONCLUSION

Due to the relatively higher urine output and insensible water loss in extremely preterm infants during postnatal week 1, higher water intake was required. As such, metabolic acidosis was bound to have occurred.

Maturation of rat proximal tubule chloride permeability

We have previously shown that neonate rabbit tubules have a lower chloride permeability but comparable mannitol permeability compared with adult proximal tubules. The surprising finding of lower chloride permeability in neonate proximals compared with adults impacts net chloride transport in this segment, which reabsorbs 60% of the filtered chloride in adults. However, this maturational difference in chloride permeability may not be applicable to other species. The present in vitro microperfusion study directly examined the chloride and mannitol permeability using in vitro perfused rat proximal tubules during postnatal maturation. Whereas there was no maturational change in mannitol permeability, chloride permeability was 6.3 +/- 1.3 x 10(-5) cm/s in neonate rat proximal convoluted tubule and 16.1 +/- 2.3 x 10(-5) cm/s in adult rat proximal convoluted tubule (P < 0.01). There was also a maturational increase in chloride permeability in the rat proximal straight tubule (5.1 +/- 0.6 x 10(-5) cm/s vs. 9.3 +/- 0.6 x 10(-5) cm/s, P < 0.01). There was no maturational change in bicarbonate-to-chloride permeabilities (P(HCO3)/P(Cl)) in the rat proximal straight tubules (PST) and proximal convoluted tubules (PCT) or in the sodium-to-chloride permeability (P(Na)/P(Cl)) in the proximal straight tubule; however, there was a significant maturational decrease in proximal convoluted tubule P(Na)/P(Cl) with postnatal development (1.31 +/- 0.12 in neonates vs. 0.75 +/- 0.06 in adults, P < 0.001). There was no difference in the transepithelial resistance measured by current injection and cable analysis in the PCT, but there was a maturational decrease in the PST (7.2 +/- 0.8 vs. 4.6 +/- 0.1 ohms x cm2, P < 0.05). These studies demonstrate there are maturational changes in the rat paracellular pathway that impact net NaCl transport during development.

Sodium and potassium clearances by the maturing kidney: clinical-molecular correlates

A temporal dissociation exists between the early appearance of sodium absorptive and later detection of potassium secretory processes in the maturing rabbit collecting duct. To extend the latter findings to the human, we sought to correlate developmental changes in renal sodium and potassium clearances with the molecular expression of corresponding ion channels in kidneys of premature infants. In a longitudinal prospective study of 23- to 31-week gestational age (GA) infants, sodium, potassium, and creatinine clearances were measured weekly for 5 weeks and the absolute and fractional excretions of sodium (FE(Na)) and potassium (FE(K)) calculated. Gene-specific probes were used to assess steady-state abundance of mRNA encoding the sodium channel ENaC and potassium channel ROMK in homogenates of human kidneys (obtained from the Anatomic Gift Foundation). Although urinary losses of sodium in infants <approximately 28 weeks GA exceeded intake, leading to a state of negative sodium balance, most infants >/=28 weeks and all infants >approximately 32 weeks GA achieved a state of positive balance, a maturational process associated with a decrease in FE(Na )and increase in ENaC. Infants >approximately 30 weeks GA maintained a state of positive potassium balance. We noted a twofold reduction in FE(K )after approximately 26 weeks GA and no change in ROMK abundance during the developmental window studied. We speculate that the developmental regulation of renal ENaC expression contributes, at least in part, to the decrease in FE(Na )observed with advancing GA, and that in the human, as in the rabbit, there is a delay between the maturation of sodium absorptive and potassium secretory pathways.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

The moth and the aspen tree: sodium in early postnatal development

Over the past 25 years, our perception of the neonatal kidney has changed markedly from its being a "limited" organ compared with that of the adult to being extraordinarily well adapted in its role in maintaining homeostasis and making possible the rapid somatic growth necessary during this critical period of life. The present review focuses on the physiologic adaptations by the neonatal kidney in the maintenance of a positive sodium balance, which is necessary for normal growth not only in mammals but also in moths. There is a fine interplay between the developing brain, heart, thyroid, adrenals, and sympathetic nervous system, all converging on the kidney to conserve sodium, which is limited in the diet. The renin-angiotensin system plays a central role in this response and is balanced by developmental changes in the renal response to atrial natriuretic peptide, all of which contribute to sodium conservation. Over the next 25 years, advances in molecular genetics will doubtless elucidate many more facets of the mechanisms underlying neonatal sodium homeostasis. This will be particularly important as the survival of ever smaller preterm infants improves steadily.

Physiology of infants with very low birth weight

Advances in neonatal management have resulted in dramatic increases in survival in infants with birth weights less than 1,500 g. Extensive basic science and clinical research has led to a more comprehensive understanding of the physiological differences between the VLBW infant and larger neonates. Meticulous attention must be paid to appropriate fluid, electrolyte, nutrition, and temperature maintenance to achieve homeostasis and growth. Additionally, the clinician must be aware of the diagnostic and treatment modalities for the common complications seen in the premature infant to minimize mortality and long-term morbidity.

Sodium potassium adenosine triphosphatase activity in preterm and term infants and its possible role in sodium homeostasis during maturation

AIM

To investigate sodium (NA(+)) potassium (K(+)) adenosine triphosphatase (ATPase) activity in newborn infants at different gestational ages, to elucidate the mechanism underlying poor renal sodium conservation in preterm infants.

METHODS

Fifty three healthy newborn infants, gestational age 30-42 weeks, were studied. Umbilical cord red blood cell Na(+) K(+)ATPase activity, plasma renin activity, and plasma aldosterone activities were measured in all of them. Red blood cell Na(+) K(+)ATPase activity was re-examined in eight preterm infants, one and two weeks after birth. Total and ouabain sensitive ATPase activity was measured spectrophotometrically using a method that couples ATP hydrolysis with NADH oxidation.

RESULTS

Red blood cell Na(+) K(+)ATPase activity was significantly lower (p<0.01) in preterm babies with a gestational age below 35 weeks, compared with those with aged 35 weeks and above: 2.3 (0.8) and 6.7 (1.3) nmol NADH/minute/mg protein, respectively. There was no correlation between gestational age, Na(+) K(+)ATPase, plasma renin activity and aldosterone values either in the preterm or term babies. Two weeks after birth, irrespective of gestational age, the enzyme activity of the preterm babies increased to values similar to those observed in the term neonates at birth.

CONCLUSION

The differences in sodium homeostasis between term and preterm babies are modulated via changes in Na(+) K(+)ATPase activity.

Renal processing of glucose in well and sick neonates

AIMS

To determine the extent of renal processing of glucose in sick and well neonates.

METHODS

Glomerular filtration rate (GFR) and the renal processing of glucose, sodium, and water were measured using prolonged inulin infusion in 47 infants of 26-40 weeks of gestation, aged 1-13 days.

RESULTS

GFR rose by 15% after ventilatory support was withdrawn, and was unaffected by clinical instability. Fractional glucose excretion was low in the stable unventilated babies except at very high filtered loads, but rose in one unstable, unventilated baby. It was higher in ventilated babies, and remained high for at least six days after ventilation. For water and sodium, net differences between intake and urine excretion were not affected by ventilation, clinical stability, or glycosuria.

CONCLUSIONS

A combination of a low GFR and a high fluid intake, urine flow, and urine concentrating capacity, makes neonates very unlikely to develop an osmotic diuresis due to glycosuria while they have a blood glucose below 12 mmol/l, despite assertions to the contrary.

Acute renal failure in the newborn: incidence and outcome

OBJECTIVE

To describe an experience from a developing country of newborn renal disease particularly those without advanced neonatal care.

METHODOLOGY

Prospective evaluation from a referral hospital, North-East of Nigeria between 1 July 1990 and 30 June 1994. Babies admitted for any morbidities who were diagnosed as being in acute renal failure (ARF) during the study period (non-oliguric inclusive). Onset of ARF; day on which oliguria or anuria detected, or serum urea first exceeded 10 mmol/L. Urine output quantitated from carefully bagged urine and suprapubic bladder aspiration, and venous blood regularly obtained for serum electrolytes, urea and creatinine. Fractional excretion of sodium (FE(Na)) and renal failure index (RFI) were determined on some babies.

RESULTS

Forty-three neonates (M:F; 3.3:1) with ARF, the majority (27) of whom were out-born, and 14, 26 and three were preterm, full-term and post term, respectively. Encountered incidence was 3.9/1000 live births with a high prevalence rate; 34.5/1000 admissions. A significantly greater incidence was seen in the latter half of study; 10.7 vs 53.7/1000, P < 0.05. Early ARF occurrence (aged; 0-5 days) in 33 (77%) of babies. The aetiology was comprised of perinatal asphyxia, sepsis, obstructive uropathy and miscellaneous in 53.4%, 32.6%, 9.3% and 4.7%, respectively. Twenty-two (51.2%) deaths occurred; however, the exact causes were indeterminable. Fractional excretion of sodium (FE(Na)) and renal failure index (RFI) were of < 1.75% and < or = 2.0, respectively, significantly differentiated sepsis (intrinsic) from perinatal asphyxial (pre-renal) ARF; P < 0.01. No case of persistent renal failure occurred.

CONCLUSION

Our FE(Na) value (although less than reported in previous literature from affluent societies) remained sensitive (along with RFI) in differentiating aetiological group of ARF. Our data and medical management outcome, despite absence of level-III care, support the need for good resuscitation, careful monitoring and constant re-evaluation. The effect of salbutamol on hyperkalaemia is emphasized.

Developmental renal physiology of the low birth weight pre-term newborn

PURPOSE

The remarkable growth of the kidney and the rapid changes in renal function in the second half of gestation and early postnatal period are discussed.

MATERIALS AND METHODS

Adaptation to the extrauterine environment involves immediate postnatal natriuresis that is prolonged in the pre-term neonate, followed by the sodium retention necessary for growth. Glomerular filtration rate increases throughout the postnatal period, and it is modulated by the renin-angiotensin system and prostaglandins. Because of this, the fetus and neonate are particularly susceptible to renal injury following the administration of angiotensin converting enzyme inhibitors or nonsteroidal anti-inflammatory drugs. Renal tubular function in the neonate is characterized by reduced renal concentration and acidification ability, which can be further compromised by obstructive uropathy. Urine calcium excretion is high in the neonate, which can be aggravated by calciuric drugs, such as furosemide and glucocorticoids.

RESULTS

Reduced renal mass results in compensatory renal growth even in the fetus, a response that could prove maladaptive later in life through excessive glomerular hyperfiltration and progressive interstitial fibrosis.

CONCLUSIONS

These factors underscore the importance of attempting to maximize functional renal mass in the neonate or infant with renal impairment of any etiology.

Assessment of tubular reabsorption of sodium, glucose, phosphate and amino acids based on spot urine samples

Reference values for tubular transport of sodium, phosphate, glucose and amino acids are generally based on inulin or creatinine short-term clearances, which are difficult to obtain in children. Hence, quantitative assessment of tubular transport capacities is rarely performed. For a simplified procedure, reference values for fractional sodium excretion, phosphate reabsorption related to glomerular filtration rate, percent glucose and percent amino acid reabsorption were established in 62 children from spot urine and simultaneously obtained blood samples. Sodium excretion, and glucose and amino acid reabsorption were significantly lower in infants than children, whereas phosphate reabsorption decreased during the first year of life. Results using the proposed protocol and those obtained from timed urine specimens correlated well; the phenomenon of renal adaptation during childhood could equally well be demonstrated. Renal tubular dysfunction can be diagnosed without timed urine specimens.

Water balance, electrolytes and acid-base balance in extremely premature infants

There are few reported studies of water balance and electrolyte abnormalities in extremely low birthweight (ELBW) infants weighing < 1000 g nursed in high humidity. We retrospectively analyzed water balance, electrolyte and acid-base balance in 100 ELBW infants, of whom 72 were appropriate for gestational age (AGA) and 28 were small for gestational age (SGA). They were cared for in closed incubators at almost full ambient humidity. Fluid intake was restricted to 50-60 mL/kg on the first day of life and was adjusted to maintain normal serum Na concentration. Weight loss in AGA and SGA infants was 21.6 and 16.7%, respectively, and was associated with large urine volume rather than insensible water loss. The incidence of hypernatremia (> 150 mEq/L) and hyponatremia (< 130 mEq/L) was 8.0 and 33.3%, respectively. The incidence of hyperkalemia in AGA infants was 37.0%, and 14.8% in the SGA group. However, hyperkalemia requiring treatment was rare. The incidence of late metabolic acidosis in AGA and SGA infants was 84.6 and 37.5%, respectively (P < 0.01). The difference in water balance and electrolyte abnormalities in AGA and SGA infants needs to be taken into account in managing ELBW infants.

Body electrolytes in bronchopulmonary dysplasia and the effects of diuretic therapy

Body electrolytes and their regulatory hormones were studied in preterm infants who suffered from bronchopulmonary dysplasia under two groups: those who were not treated with diuretics (Group II), and those who were treated with diuretics (Group III). The values were compared with a group of matched healthy controls (Group I). Lower serum Na levels, a need of higher Na intake, and higher urinary Na concentrations and urinary specific gravity were found in Group II infants. FeNa was normal and the urinary flow rate was lower than the controls. These data suggest an inability of these infants to dilute urine. Group III infants who were treated with diuretics showed higher serum Na levels and lower urinary specific gravity than Group II infants. These values, as well as water and Na intake/output ratios, were all similar to the control values. Serum aldosterone level was highest in Group II but did not reach significance. Intracellular K concentration was not different between the groups indicating an optimum total body K balance. A significant negative correlation between serum Na and aldosterone levels was found in Group II infants, which was not noted in the controls. Significant correlations were also found between FeNa and plasma aldosterone level in the BPD groups, unlike the controls. The control group of infants showed significant positive correlation between Na balance and serum Na levels. Our results suggest that inability to dilute urine appropriately might be the reason for the BPD patients to retain body water. Water restriction and diuretic therapy therefore are reasonable therapeutic approaches in such cases.

Regulation of extracellular fluid volume in neonates

This review summarises mechanisms of control of extracellular fluid volume in the neonatal period. 'Normal' body fluid distribution and methods of its measurement are discussed as well as regulatory mechanisms with particular emphasis on hormonal and renal aspects.

Salt-deficient diet and early weaning inhibit DNA synthesis in immature rat proximal tubular cells

A maturational gradient exists between the inner and the outer cortical nephrons in infant rats. This study compares the putative growth-retarding effects of early weaning (EW) and a salt-deficient (SD) diet in proximal tubule (PT) cells in the inner and the outer cortex. The mitotic response was measured as tritiated-thymidine incorporation in PT cells from 18- to 22-day-old rats. Under basal conditions the mitotic index is the same in the inner and the outer cortex. EW retarded body growth, but had no significant effect on the kidney/body weight (KW/BW) ratio. EW caused a significant decrease in DNA synthesis in both the outer and the inner cortical PT cells, but the effect was significantly more pronounced in the outer cortex. The SD rats had significantly lower levels of serum sodium, lower urinary sodium excretion, slightly decreased BW, but no differences in KW/BW ratio or in dry/wet KW. SD caused a decrease in DNA synthesis in the PT cells in the outer cortex, but not in the inner cortex. In conclusion, two manipulations that can retard proliferation of PT cells, i.e. EW and a SD diet, have a more pronounced effect in immature than in mature PT cells.

Renal function in sick very low birthweight infants: 3. Sodium, potassium, and water excretion

Renal excretion of sodium, water, and potassium was measured on 434 occasions in a sample of 40 infants of 25.5-33 weeks' gestation, birth weight 720-2000 g, between the ages of 0.5 and 36 days. Water excretion varied between 1% and 30% of the glomerular filtration rate, or 15-350 ml/kg/day, and varied widely from day to day in individual infants. Nearly all infants became hyponatraemic before or after the first postnatal week. There were a few instances of hypernatraemia in the first week caused by high insensible water loss. There were high levels of sodium excretion up to 16% of filtered sodium, or 21 mmol/kg/day, in the first two postnatal weeks. Highest levels of sodium excretion were seen in the most immature infants in the first week. In most infants sodium excretion increased either in the first week or later before a subsequent decline. Potassium excretion was often high in the first week, as much as 96% of filtered potassium, or 5 mmol/kg/day, and is associated with early hyperkalaemia.

Salt and the newborn kidney

Renal function differs in term infants from that in adults, with lower glomerular filtration rate (GFR) and reduced proximal tubular reabsorption of sodium (Na) and water: nevertheless, it is adequate for their needs. This is not true of very preterm infants in whom hyponatraemia is common. Animal studies have shown that Na+, K(+)-ATPase and the Na+/K+ exchanger are poorly expressed at birth with rapid postnatal rises. Cell receptors for hormones that influence tubular Na transport are less numerous in the premature infant than later in life: intracellular second messenger systems may also be immature. The low GFR is due to vasoconstriction and may be necessary to prevent water and electrolyte wasting due to tubular overload. The hyponatraemia of prematurity could, in principle, be due either to Na loss or water excess and can be prevented either by giving additional Na or by restricting water intake. Na supplementation causes relative volume expansion (VE), water restriction volume contraction (VC); this is demonstrated by the effect of the two approaches on weight gain and on the levels of vasoactive hormones in the blood. We argue that moderate VE is more physiological than VC, both in attempting to simulate intrauterine conditions and in consideration of the infant's nutritional needs. The much less common complication of hypernatraemia is usually due to abnormal water loss and should be prevented by increasing water intake appropriately. The above applies to well, preterm babies: sick preterm infants are much more variable in their Na and water requirements than well infants of comparable gestation and weight and each needs an individually tailored regimen based on frequent clinical assessment and laboratory measurement.

Electrolyte balance

The kidney is structurally and biochemically immature at birth. As a consequence, renal function is low (3;10;ll;18). Glomerular filtration rate (related to body surface area or to body weight) is approximately 25% of that in adults. The capacity of several different tubular transport systems is lower in the infant than in the adult (2;5;13;21;28). A low transporting capacity of the neonatal kidney will sometimes result in undesired losses of electrolytes, amino acids, and peptides. The capacity to concentrate urine is low (7;29), and disturbances of serum tonicity, therefore, are common. The low concentrating capacity can be attributed to renal immaturity. The capacity of the newborn fullterm as well as preterm infant to release antidiuretic hormone is normal 7;31).

Special needs of the newborn infant in fluid therapy

The preterm baby, although less able to tolerate high fluid and solute loads, has a remarkable ability to adapt to a rather broad range of environmental challenges, and thus renal function in the preterm infant should no longer be considered impaired. Lower rather than higher fluid administration is the current trend in neonatal care because it is safer and more physiologic. Although changes in body weight over time provide a good indicator of water balance, determination of plasma and urine electrolytes and tonicity are also essential in the monitoring of fluid therapy in preterm infants.

The pharmacology and therapeutics of diuretics in the pediatric patient

Selection of appropriate diuretic therapy in children is hampered by a lack of age-specific pharmacokinetic and pharmacodynamic data, especially in premature neonates. Well-designed clinical trials in neonates, infants, and younger children are necessary prerequisites to safer and more efficacious diuretic therapy.

Mineral balance studies in sick preterm intravenously fed infants during the first week after birth. A guide to fluid therapy

Mineral balance studies were performed in 61 sick preterm infants given parenteral fluids only. Their gestational ages varied from 24 to 35 weeks, and 50 required mechanical ventilation. Two consecutive balance studies were performed; the first from admission to 48 hours in all babies given maintenance fluids of 10% Dextrose, and the second from 48 hours to 7 days in those babies given intravenous feeding (IVN). At the beginning and end of each balance period, the baby was weighed and an arterial blood sample taken for blood gases, electrolyte, urea, creatinine and protein determinations. During the balance period all urine was collected and analysed for electrolyte, urea, and creatinine composition, and all fluid intake was recorded. The balance of a mineral was calculated as the difference between parenteral intake and urine output. Infants requiring IVN were allocated alternatively to regimen X or regimen Y, which had the same calcium content of 9.5 mmol/L, but different phosphate contents, regimen X containing 7.3 mmol/L and regimen Y 11.6 mmol/L. In those infants requiring prolonged IVN, 12-24 hour balance studies were performed at weekly intervals after day 10. 1. Phosphate deficiency developed in infants given regimen X, who had higher urine calcium excretion, lower percentage calcium retention and lower plasma phosphate levels than those given regimen Y. These differences were apparent by day 7 and persisted after day 10. In infants given regimen Y, mean calcium retention from admission to day 7 was 3.9 mmol/kg, and after day 10 was 0.9 mmol/kg/day. 2. In the first 48 hours, urine output and creatinine clearance varied widely and were lower in infants with higher oxygen requirements at 48 hours. Ten babies had severe oliguria with outputs less than 10 mL/kg/day. Creatinine clearance was directly related to gestational age, mean arterial blood pressure, and plasma protein concentrations on admission. After 48 hours, urine output and creatinine clearance increased considerably. 3. In the first 48 hours, metabolic acidosis was produced by increased plasma non-protein metabolisable acid concentrations, which were associated with low creatinine clearances, and were thought to be due to lactic acid accumulation in response to decreased tissue perfusion. At 7 days, metabolic acidosis was of similar severity but was produced by decreased plasma non-metabolisable base concentrations, caused by increased urine loss of net base, and not directly by IVN.(ABSTRACT TRUNCATED AT 400 WORDS)

Endogenous digoxin-like factor in neonates: effect of age and relation to serum bilirubin levels

Endogenous digoxin-like immunoreactive factor(s) (DLIF) have been found in serum and urine of newborn infants, including those born prematurely. We assessed the effect of age on serum levels of DLIF in 73 samples obtained from 66 healthy full term newborn infants at birth and during the first two months of life. DLIF concentrations were highest at birth and fell progressively with age. In cord blood, DLIF levels were 0.73 +/- 0.35 ng/ml (mean +/- SD). DLIF concentrations were 0.45 +/- 0.11 ng/ml on day 1, 0.26 +/- 0.08 ng/ml on day 3, 0.19 +/- 0.07 ng/ml on day 5, 0.17 +/- 0.09 ng/ml on day 11, 0.11 +/- 0.02 ng/ml on days 15-30, and not detectable after 45 days of life. We also studied the relation between serum levels of DLIF and bilirubin in 23 jaundiced newborns between 3-5 days of life. We found a highly significant positive correlation between serum bilirubin concentrations and DLIF. These findings support the assumption that DLIF plays a role in impeding bilirubin excretion in the neonatal period, perhaps by inhibiting the activity of (Na-K)ATPase.

Hyponatremia in a neonate of a cocaine abusing mother

Neonatal hyponatremia can be caused by increased sodium losses, inadequate sodium intake, increased maternal or neonatal water load or by water retention secondary to excess of ADH release. Cocaine use by pregnant women has not as yet been reported to correlate with hyponatremia in the newborn infant. We present a case of an infant whose mother used cocaine regularly during the last stages of pregnancy and who developed hyponatremia in the first week of life. A mechanism is proposed and discussed.

Postnatal course of plasma levels of adrenocortical steroids in premature infants with and without NaCl supplementation

To assess the adrenocortical response of premature infants to alterations in sodium balance, the postnatal course of plasma progesterone, 11-deoxycorticosterone, corticosteronoe, aldosterone, 17-hydroxyprogesterone, 11-deoxycortisol, cortisol and cortisone was compared in healthy premature infants kept on low (1-2 mEq/kg per day) or high (3-5 mEq/kg per day) sodium diet. The mean birthweight (1470 g, range: 1210-1670 g vs 1410 g, range: 1130-1750 g) and mean gestational age (30.5 weeks, range: 29-32 weeks vs 30.2 weeks, range: 28-32 weeks) in the low and high sodium groups, respectively, were similar. Simultaneous steroid hormone measurements were made weekly up to the 5th week of life using mechanized Sephadex LH-20 multicolumn chromatography and standardized radioimmunoassays. It was demonstrated that in response to renal salt wasting and negative sodium balance there was a significant rise in plasma aldosterone concentration. The plasma levels of other individual corticosteroids generally declined with advancing age, the initial fall, however, was followed by a transient and insignificant but simultaneous increase in 11-deoxycortisol, cortisol, cortisone and corticosterone in prematures on low a sodium diet. This effect could be prevented by giving NaCl supplement. The NaCl-suppressible increase in adrenocortical activity may be the result of the combined effect of stress or angiotensin 11-induced adrenocorticotropic hormone (ACTH) release and/or prolactin-mediated enhanced adrenal response to ACTH.

Urinary sodium excretion in young infants: role of gestational and postnatal ages

Fractional urinary sodium excretion (FENa) and urinary sodium excretion (UNaE) were determined 88 times in 42 healthy, appropriate weight-for-gestational age infants. Gestational ages (GA) were 28-41 weeks; birthweights were 930-4135 g. Nineteen preterm infants were studied serially a total of 59 times between 1 and 55 days after birth. During the first 4 days after birth, multiple hierarchical regression analyses showed that FENa and UNaE were inversely related to GA (P less than 0.001). Postnatal age (PN) was not significantly related to FENa or UNaE. However the effect of GA on FENa and UNaE was not significantly greater than the effect due to postnatal age (PA) (P = 0.31 and P = 0.80, respectively). UNaE (1.6 +/- 0.2 mmol/kg per day) was significantly greater than sodium intake (1.1 +/- 0.2 mmol/kg per day) at 2.6 +/- 0.2 days (P less than 0.05). Longitudinal studies extending beyond 4 days indicated that GA and PA had interactive effects on FENa and UNaE; hence the contribution for a given GA was dependent on PA (and vice versa). Sodium intake remained stable (average 1.8 mmol/kg per day) beyond 7 days after birth and was consistently greater than UNAE (P less than 0.01). It is suggested that in healthy preterm infants beyond 2 weeks PA, a sodium intake of 2-3 mol/kg per day may be adequate to meet the renal sodium losses.

Postnatal development of renal function in very low birthweight infants

The postnatal development of renal function was compared in infants with a gestational age of 25-30 weeks, mean 27.8 weeks (GA 28), and in infants with a gestational age of 31-34 weeks, mean 32.5 weeks (GA 32). The infants were comparable with regard to postnatal course, fluid, caloric and salt intake. Observations were made during the 1st, 2nd and 4th-7th (mean 5th) postnatal weeks. From the 1st to the 5th postnatal week the creatinine clearance (CCr ml/min/1.73 m2), increased from 11 to 20 in GA 28 and from 15 to 30 in GA 32. At 2 weeks of age CCr was significantly lower in GA 28 than in GA 32. During the first week of life diuresis was lower in GA 28 than in GA 32 but thereafter was the same in both groups. We interpret this as a sign of dehydration in GA 28. Serum arginine vasopressin (S-AVP) concentrations were high in both groups at all ages. Mean urine osmolality was low (less than 300) regardless of postnatal age and S-AVP. Urinary sodium excretion was high at 1 week of age in both groups and decreased with increasing postnatal age. Na excretion was slightly higher in GA 28 than in GA 32 at 1 but not at 2 and 5 weeks. UK/UNa was below 1 in both groups during the first week of life and increased with postnatal age. Urinary aldosterone excretion was high in both GA 28 and GA 32 at all ages. Serum sodium levels were lower in GA 28 than in GA 32 at all ages.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular fluid volume changes in very low birth weight infants during first 2 postnatal months

Serial extracellular volume (ECV) changes were measured in 18 infants of less than 32 weeks gestation. Results were compared with changes in body weight, fluid and sodium intake, urine output, and serum sodium concentration. Mean +/- SD ECV decreased from 550 +/- 116 mL/kg on day 1 to 359 +/- 66 mL/kg on day 14. Thereafter, mean ECV/kg remained between 336 +/- 42 and 349 +/- 54 mL/kg. Clinical hydration and serum sodium concentration usually remained normal during this reduction of stabilization of ECV/kg. Six episodes of hyponatremia occurred at 11 to 31 days of age. Mean ECV/kg was significantly lower in infants with hyponatremia compared with infants of similar age with normal serum sodium concentration (303 +/- 36 mL/kg vs 368 +/- 56 mL/kg, P less than 0.01). Sodium intake in the two groups was similar. We conclude that ECV in the VLBW infant decreases postnatally and is regulated within a range similar to that in older infants, and that postnatal natriuresis in the first 2 weeks of life represents physiologic reduction of the expanded ECV of the fetus. Late hyponatremia may indicate excessive sodium loss and ECV depletion.

Water, sodium and acid-base balance in premature infants: therapeutical aspects

One of the main targets of fluid therapy in premature infants is to avoid variations in osmolality, which mainly means providing a stable sodium, glucose, and acid-base balance. Water, sodium, and acid-base balance were measured in 20 infants appropriate-for-gestational age with a gestational age less than or equal to 34 weeks. The infants were randomly assigned to one of two treatment groups. Fluid intake was restricted and air humidity in the incubator was high in order to minimize insensible water loss. Sodium intake in Group 1 was 2 mmol/kg/day and consisted of sodium chloride. Sodium intake in Group 2 was 4 mmol/kg/day and consisted of both sodium chloride and acetate. Weight loss was appropriate in both groups. In the high sodium intake group there was a tendency towards a more stable plasma sodium concentration than in the low sodium intake group. The use of sodium acetate was efficient and practical as normal acid-base balance was maintained. The protocol with restricted fluid intake (1st day 50 ml/kg, 2nd day 70 ml/kg, 3rd day 90 ml/kg, and 4th day 110 ml/kg), high air humidity, a sodium supply of 3 to 4 mmol/kg/day, and a slow correction of metabolic acidosis with sodium acetate, yields suitable guidelines in planning fluid and electrolyte therapy in premature infants less than or equal to 34 weeks' gestation.

Modern fluid and electrolyte management of the critically ill premature infant

In this article, the authors introduce the concept of a transitional physiology which governs fluid and electrolyte balance in the immediate postnatal period. The important impact of the extrauterine environment on fluid balance is also discussed. Finally, the pathophysiology of diuresis in RDS, and fluid shifts in the VLBW infant with therapeutic recommendations are presented.

Evaluation of renal function and acute renal failure in the neonate

The diagnosis of renal dysfunction in the neonate can be a challenging problem for the practicing pediatrician. Although there are real differences in renal function between term and preterm infants, overall function is quite adequate in both groups when fluid intake and environmental conditions are carefully controlled. When confronted with an infant with a pathologic decrease in urine output, the clinician must provide adequate fluid resuscitation for the infant with prerenal oliguria without inducing fluid overload in the infant with established, intrinsic renal failure. In addition, the infant with obstruction to urine flow must be distinguished. This requires careful assessment of physical findings and a few key laboratory determinations. Once the diagnosis of renal failure is made, frequent clinical monitoring with anticipation of potential complications is critical. Long-term management of renal failure in infancy and intervention for suspected urinary tract malformation in the fetus have emerged as difficult medical and ethical problems as our technology has advanced.

The postoperative response of the term and preterm newborn infant to sodium administration

Twenty surgical newborn infants aged less than 5 days were selected for study to determine the quantity of sodium administered during operation and the subsequent 48 hours, and to determine their response to this sodium load. There were 6 preterm infants with gestational age 35 weeks or less and 14 full-term infants aged more than 35 weeks. Measurements calculated at 12 hourly intervals for 48 hours after operation included sodium intake, sodium excretion, fractional excretion of sodium, and serum sodium. No special guidelines were utilized for fluid management. The mean sodium intake during the 48-hour study period for the term infants was 15.7 mEq/kg, of which 46% was given during the first 12 hours, and for the premature infants was 17.1 mEq/kg, of which 56% was given during the first 12 hours. In the two groups of patients, the amount of sodium given during the first 12 hours was 470% and 480% of their estimated maintenance requirements, respectively. The mean sodium output during the first 12 hours was low in the term group (1.2 mEq/L) and the premature group (1.3 mEq/L), and subsequently increased reaching maximum levels of 2.3 and 2.1 mEq/L, respectively, by 36 hours. The fractional excretion of sodium exceeded 1.0% in 53% of the term and 94% of the preterm infants. During the study period, the mean serum sodium levels exceeded 145 mEq/L (hypernatremia) in 64% of the term and 67% of the preterm infants.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium homeostasis in term and preterm neonates. I. Renal aspects

Eighty five 24 hour sodium balance studies and creatinine clearance measurements were performed in 70 infants of gestational age 27-40 weeks and postnatal age 3-68 days. The kidney's capacity to regulate sodium excretion was a function of conceptional age (the sum of gestational age and postnatal age) and an independent effect of postnatal age was also observed--extrauterine existence increased the maturation of this function. The sodium balance was negative in 100% of infants of less than 30 weeks' gestation, in 70% at 30-32 weeks, in 46% at 33-35 weeks, and in 0% of greater than 36 weeks, and the incidence of hyponatraemia closely paralleled that of negative sodium balance. Despite a low glomerular filtration rate (GFR) urinary sodium losses were highest in the most immature babies but fractional sodium excretion (FENa) was exponentially related to gestational age. An independent effect of postnatal age could be identified on FENa but not in GFR. These findings indicate that in infants of greater than 33 weeks' gestation sodium conservation is possible because of a favourable balance between the GFR and tubular sodium reabsorption, but that below this age GFR exceeds the limited tubular sodium reabsorption capacity. The rapid increase in sodium reabsorption in the first few postnatal days seems to be due to maturation of distal tubular function, probably mediated by aldosterone. We suggest that the glomerulotubular imbalance for sodium is a consequence of the immaturity of the tubuloglomerular feedback mechanism, and we estimate that the minimum sodium requirement during the first 2 weeks of extrauterine life is 5 mmol (mEq)/kg/day for infants of less than 30 weeks' gestation and 4 mmol (mEq)/kg/day for those born between 30 and 35 weeks.

Sodium homeostasis in term and preterm neonates. II. Gastrointestinal aspects

Eighty five 24 hour balance studies were performed on 70 healthy newborn infants of gestational age 27-40 weeks; dietary intake and stool losses of sodium were measured. There was a relation between gastrointestinal sodium absorption and conceptional age (the sum of gestational and postnatal age), whether expressed as absolute stool sodium losses or as the ratio of stool sodium to dietary sodium intake. The stool K:Na ratio rose appreciably with maturation, although stool content of potassium was not greatly increased. These findings suggest that intestinal sodium absorption is inefficient in immature babies and that the degree of malabsorption is inversely related to conceptional age.

Role for glucagon as a mineralocorticoid antagonist during the fetal-perinatal period

The progressive decrease in total body water (TBW) and sodium during the fetal-perinatal period; the normally noted 5-10 per cent largely water weight loss noted in term infants during the first 3-4 days postnatally; and, the even greater TBW and sodium losses incurred by premature infants during the initial post-natal week may, in part, be secondary to a glucagon-mediated, renal distal tubular hyporesponsiveness to aldosterone.

Water balance in very low-birth-weight infants: relationship to water and sodium intake and effect on outcome

The clinical effects of fluid therapy designed to maintain different degrees of negative water balance during the first five days of life were determined prospectively in 88 very low-birth-weight infants. Infants with birth weights of 750 to 1,500 gm were matched for birth weight in 250 gm increments. RDS or no RDS, asphyxiated or not, and inborn or outborn. Each infant was randomized to either Group 1--fluids managed to allow 1 to 2% loss of BW per day to a maximum loss of 8 to 10%, or Group 2--fluids managed to allow 3 to 5% loss of BW per day to a maximum loss of 13 to 15%. The mean five-day cumulative fluid input in Group 2 was 220 ml/kg less than in Group 1, yet Group 2 lost only 41 gm/kg more than did Group 1 (8.8% of BW lost in Group 1 vs 12.9% of BW lost in Group 2, P less than 0.001). There were no statistically significant differences between the groups in incidence of clinically significant patent ductus arteriosus, intracranial hemorrhage, bronchopulmonary dysplasia, necrotizing enterocolitis, dehydration, acute renal failure, or metabolic disturbances. There was no difference in duration of respiratory support required, in time to regain BW, or in time to discharge. There was no difference in the neonatal mortality rate. Fluid input in VLBW infants can be flexible to allow the gradual loss of 5 to 15% of birth weight during the first week of life without adversely affecting outcome.

Implications of limitation of renal function for the nutrition of low birthweight infants

Renal function is immature in low-birth-weight infants. The glomerular filtration rate is low during the entire first month of life. It is 20-50% of that observed in older children and adults. This limits the excretory capacity of the kidney and might set an upper limit for the protein intake. The capacity to reabsorb bicarbonate is not fully developed. This predisposes the low-birth-weight infant to metabolic acidosis. The capacities to excrete sodium when in positive sodium balance and to retain sodium when in negative sodium balance are limited. If the daily sodium balance is not well monitored, conditions of negative sodium balance with hyponatremia as well as of positive sodium balance with hypernatremia might occur.

Postnatal development of renal sodium handling in premature infants

To estimate the contribution of the specific defect in proximal and distal tubular reabsorption of sodium to renal salt wasting, fractional sodium excretion, distal tubular sodium delivery, and distal tubular sodium reabsorption were determined in 11 healthy premature infants. The study was performed on the seventh day and at weekly intervals thereafter up to the sixth week of life. Sodium clearance and fractional sodium excretion decreased significantly with increasing postnatal age (P less than 0.001). There was no significant alteration in either osmolar or free-water clearances. Distal tubular sodium delivery steadily decreased from 4.96 +/- 0.66 (mean +/- SE) in the first week to 3.3 +/- 0.41 ml/minute/dl GFR in the sixth week of life (P less than 0.05). Distal tubular sodium reabsorption was 69.5 +/- 2.36% in the first week, then rose significantly to reach a value of 83.7 +/- 1.85% in the second week (P less than 0.001) and remained practically unchanged thereafter. It is suggested that the rapid improvement of distal tubular sodium reabsorption in premature infants might result from forced stimulation by the excessively activated renin-angiotensin-aldosterone system.