Renal handling of sodium in premature and full-term neonates: a study using clearance methods during water diuresis

Hyponatraemia despite isotonic maintenance fluid therapy: a time series intervention study

OBJECTIVE

To examine the prevalence of dysnatraemias among children admitted for paediatric surgery before and after a change from hypotonic to isotonic intravenous maintenance fluid therapy.

DESIGN

Retrospective consecutive time series intervention study.

SETTING

Paediatric surgery ward at the Children's Hospital in Lund, during a 7-year period, 2010-2017.

PATIENTS

All children with a blood sodium concentration measurement during the study period were included. Hypotonic maintenance fluid (40 mmol/L NaCl and 20 mmol/L KCl) was used during the first 3 years of the study (646 patients), and isotonic solution (140 mmol/L NaCl and 20 mmol/L KCl) was used during the following period (807 patients).

MAIN OUTCOME MEASURES

Primary outcomes were sodium concentration and occurrence of hyponatraemia (<135 mmol/L) or hypernatraemia (>145 mmol/L).

RESULTS

Overall, the change from hypotonic to isotonic intravenous maintenance fluid therapy was associated with a decreased prevalence of hyponatraemia from 29% to 22% (adjusted OR 0.65 (0.51-0.82)) without a significantly increased odds for hypernatraemia (from 3.4% to 4.3%, adjusted OR 1.2 (0.71-2.1)). Hyponatraemia <130 mmol/L decreased from 6.2% to 2.6%, and hyponatraemia <125 mmol/L decreased from 2.0% to 0.5%.

CONCLUSIONS

Routine use of intravenous isotonic maintenance fluids was associated with lower prevalence of hyponatraemia, although hyponatraemia still occurred in over 20% of patients. We propose that the composition and the volume of administered fluid need to be addressed.

Fluid and electrolyte balance during the first week of life and risk of bronchopulmonary dysplasia in the preterm neonate

BACKGROUND

Early fluid and electrolyte imbalances may be associated with an increased risk of bronchopulmonary dysplasia.

OBJECTIVE

We sought to establish an association between fluid and electrolyte balance in the first week of life and the risk of bronchopulmonary dysplasia.

METHODS

Clinical charts of 205 neonates <32 weeks gestational age and/or <1,250 g birth weight (admitted to our NICU between 1997 and 2008) were analyzed. Clinical features, fluid and electrolyte balance were analyzed for the first 7 days of life using multivariate models of generalized estimation equations. A p value <0.05 was considered significant in all of the hypothesis tests.

RESULTS

The prevalence of bronchopulmonary dysplasia was 22%. Lower gestational age and birth weight, male gender, less frequent use of antenatal steroids, respiratory distress syndrome, use of surfactant, patent ductus arteriosus, duration of invasive ventilation and NICU stay were significantly associated with bronchopulmonary dysplasia. The variation in serum values of potassium, phosphorus and creatinine during the first week of life also revealed an association with bronchopulmonary dysplasia. Higher mean plasma calcium values were associated with spontaneous closure of the patent ductus arteriosus. The use of indomethacin to induce patent ductus arteriosus closure was significantly higher in bronchopulmonary dysplasia patients.

CONCLUSIONS

Differences in renal function and tubular handling of potassium and phosphorus are present during the first week of life among preterm neonates who will develop bronchopulmonary dysplasia. The higher rate of patent ductus arteriosus and indomethacin use may influence these differences. Serum levels of calcium also appear to play a role in spontaneous ductus arteriosus closure.

The urinary activity of angiotensin-converting enzyme in preterm, full-term newborns, and children

The urinary activity of the angiotensin-converting enzyme (U(ACE)) is not yet completely documented in human neonates. We measured the U(ACE) in 36 premature neonates on the 1st day and in the 1st, 2nd, 3rd, and 4th weeks of life, in 22 full-term neonates between the 1st and 2nd days, and in 30 nursing and preschool children between 1 month and 6 years of age. The urinary excretion of sodium (U(Na)/U(Cr)) and the potassium/sodium index (U(K)/U(Na)) were analyzed in the neonates. U(ACE) was greater in premature than in full-term neonates and greater in both than in older children (p<0.001). In the premature neonates, U(ACE) peaked at the 2nd week, the U(Na)/U(Cr) index decreased, and the U(K)/U(Na) index increased between the 1st day and the 2nd week (p<0.001). The U(Na)/U(Cr) index on the 1st day and in the 1st and 2nd weeks was greater in premature than in full-term neonates (p<0.001). There was no significant correlation between the U(ACE) and the U(Na)/U(Cr) index. In conclusion, the U(ACE) profile was shown to be age dependent and related to the postnatal renal development. The increase in U(ACE) activity may reflect the high activity of the neonatal intrarenal renin-angiotensin system (RAS).

Endogenous and exogenous glucocorticoid regulation of ENaC mRNA expression in developing kidney and lung

Lung liquid absorption at birth is crucial for the successful onset of respiration. Na absorption by the renal collecting duct plays an important role in renal fluid and electrolyte homeostasis during the early postnatal period. The epithelial Na channel (ENaC) plays a central role in mediating these functions, and its subunit expression is developmentally regulated in a temporal and tissue specific pattern. Several lines of evidence suggest that the prenatal increase in circulating glucocorticoids may play an important role in increasing ENaC expression during maturation. We tested the role of the prenatal surge using corticotropin-releasing hormone (CRH) knockout (KO) mice. Relative ENaC expression in lungs of KO mice increased at the same rate as in wild-type (WT) mice, but absolute expression was only 20-30% of WT. In contrast, relative and absolute expression of all three subunits in kidneys was not different between KO and WT mice. Dexamethasone (Dex) increased alpha-ENaC mRNA in fetal lung and kidney explants within 24 h but had different effects on beta- or gamma-ENaC. Dex increased beta- and gamma-ENaC in lung, but only after >48 h of exposure, and had no effect on kidney. The results suggest that the kidney metabolizes endogenous glucocorticoids, but the lung does not. Furthermore, the marked difference between lung and kidney responsiveness to glucocorticoids in beta- and gamma-ENaC expression suggests that factors other than steroids may be important in regulating functional ENaC expression during development.

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.

Developmental regulation of ENaC subunit mRNA levels in rat kidney

To assess the role of distal nephron apical Na channel (ENaC) gene expression in Na wasting by the immature kidney, ENaC alpha-, beta-, and gamma-subunit mRNA levels were examined in the rat by RT-PCR. In microdissected nephron segments, all three ENaC subunit mRNAs were detected in the distal convoluted tubule, connecting tubule, cortical collecting duct, and outer medullary collecting duct. The inner medullary collecting duct and all other nephron segments were consistently negative. The mRNA levels were quantified in kidneys at different developmental stages by multiplex RT-PCR with "primer dropping," with endoplasmic reticulum-specific cyclophilin mRNA as an internal standard. All three ENaC mRNA levels were low or undetectable on gestational day 16 and only slightly higher 3 days before birth. A sharp rise was observed between 3 days before and 1-3 days after birth; the levels at postnatal days 1-3 were already similar to those of adult kidneys. The results suggest that ENaC subunit gene expression is not a limiting factor in the full-term newborn rat kidney, but low levels of expression may limit distal Na absorption in more immature kidneys, such as those of very premature human infants.

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.

Renal function in very low birth weight infants: normal maturity reached during early childhood

Development of glomerular and tubular renal function is delayed in preterm infants. To study the pattern of maturation during infancy and childhood, we re-evaluated renal function in 22 very low birth weight infants--in 14 of the infants at 18 months postconceptional age (9 months corrected age) and in the remaining 8 infants at 8 years of age. The glomerular filtration rate remained lower at 9 months corrected age than in term infants of the same postconceptional age: 82 +/- 23 versus 125 +/- 18 ml/min per 1.73 m2 (p < 0.001). At 8 years of age the glomerular filtration rate did not differ from that of healthy control subjects. Effective renal plasma flow, filtration fraction, albumin excretion, maximal concentrating ability, and kidney size determined by ultrasonography were all normal at 8 years of age. We conclude that renal function, which is markedly reduced during the neonatal period in very low birth weight infants, reaches normal maturity by 8 years of age but not by 9 months corrected age.

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.

Mechanisms regulating renal sodium excretion during development

The present review focuses on the ontogeny of mechanisms involved in renal sodium excretion during renal maturation. The effect of birth on renal excretion of sodium and the role played by the different tubular segments in the regulation of sodium excretion during maturation are discussed. The influence of circulating catecholamines and renal sympathetic innervation in regulating sodium excretion during renal development is reviewed. The effects of aldosterone, atrial natriuretic factor, and prostaglandins on sodium regulation during renal maturation are discussed. Special emphasis is given to the potential role of glucocorticoids in modulating sodium excretion early in life.

Factors influencing plasma renin and renin substrate in premature infants

Concentrations of plasma renin (PRC) and plasma renin substrate (PRS) were measured during the first week of life in 52 infants born at less than 37 weeks' gestation (mean (SEM) gestation 30 (0.4) weeks, mean (SEM) birth weight 1.35 (0.08) kg). Both PRC (median 35, interquartiles 16.3, 94.5 ng/ml/hour) and PRS (median 2.3, interquartiles 1.3, 5.0 micrograms/ml) were raised compared with adults. The proportional rise in PRC was much greater than that in PRS, suggesting that PRS may be rate limiting in the generation of angiotensin I. Log10 PRC was inversely proportional to gestational age and a high urinary sodium loss was associated with a significantly raised log10 PRC. In hypoxaemic infants, there was a strong correlation between log10 PRS and haemoglobin concentration; this is a new observation in human infants but consistent with data available from other species.

Interrelationship between cortisol and atrial natriuretic factor in the immature ovine fetus

1. In chronically cannulated ovine fetuses (100-130 days of gestation) the infusion of cortisol (86.7 +/- 15 micrograms/h for 4 h) or human atrial natriuretic factor (ANF; 4.4 micrograms for 2 h) resulted in highly significant increases in the excretion of sodium, chloride, potassium and water. 2. Cortisol had no significant effect on fetal plasma ANF concentrations. All values are mean and s.e.m. Plasma immunoreactive ANF was 53 +/- 5 and 67.3 +/- 13 pmol/L in the 4 h saline infused fetuses, and 51.3 +/- 14.3 and 74 +/- 13.3 pmol/L in cortisol-infused fetuses (n = 7). A separate group of fetuses received 2 h infusions of saline or hANF (4.4 micrograms/h), and plasma IR-ANF values were measured (n = 3). The values, at 0, 60, 90 and 120 min were, respectively, 19.7 +/- 3, 17.3 +/- 0.7, 18.7 +/- 3.7 and 20.7 +/- 3.7 pmol/L in the saline infused group, and 25.3 +/- 5.3, 80.7 +/- 32.3, 123.3 +/- 4.3 and 100 +/- 15 pmol/L in the ANF-infused fetuses. 3. Blood cortisol concentrations, in fetuses infused for 4 h with 0.9% NaCl, were 3.1 +/- 0.8 nmol/L (n = 7); in fetuses infused with 0.9% NaCl for 2 h were 3.6 +/- 1 nmol/L (n = 3); in fetuses infused for 4 h with cortisol were 19.9 +/- 1.9 nmol/L (n = 7); and in fetuses infused with hANF for 2 h were 6.0 +/- 3.0 nmol/L (n = 5). 4. There was no effect of fetal hANF infusion on maternal or fetal blood aldosterone concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Mortality, morbidity, growth and development of babies weighing 501-1,000 grams and 1,001-1,500 grams at birth

During the five-year period, January 1980 to December 1984, 149 babies of extremely low birthweight (ELBW; 501-1,000 g) and 296 of very low birthweight (VLBW; 1,001-1,500 g) were admitted to King's College Hospital, Neonatal Intensive Care Unit. The survival rates were 51.7% and 82.8% respectively. There were more peri- and postnatal problems in the ELBW babies than the VLBW babies and these differences were highly significant. All surviving babies born between January 1980 and December 1982 were followed up until aged two. The ELBW children had a higher incidence of neuro-developmental sequelae than those of VLBW especially when both major and minor problems were considered. Their developmental quotients were lower at one and two years. Significantly more ELBW children were of short stature and low weight (less than 3rd centile). By two years of age the differences between ELBW and VLBW children in stature and mean DQ had lessened.

Postnatal development of renal function during the first year of life

Several aspects of renal function vary considerably during the 1st year of life and differ markedly from the equivalent values in the adult. Glomerular filtration rate (GFR) increases little, prior to the time an infant reaches a conceptional age of 34 weeks, the point in renal development from which the absolute GFR (ml/min) increases gradually to mature values when linear growth is completed during adolescence. GFR corrected for body size is not comparable with adult normal values until after 12 months of age; therefore, whether GFR is estimated from Scr or measured by timed urine collection, there is no easily recalled range of normal values for infants. One must know the changes in the renal function of normal infants that take place following birth during the 1st year of life. Despite several attempts to do so, renal function during the 1st year of life cannot be assessed from urine flow rate. A urine flow rate of less than 1 ml/kg per hour may be normal and appropriate and may not be harmful either to preterm or full-term infants with normal GFR. Impaired concentrating ability of the neonatal kidney is probably of no clinical significance in all but the most extreme circumstances and is not a major factor in an infant becoming dehydrated, developing hypernatremia or being at greater risk of acute renal injury. Acid-base status in infants must be interpreted appropriately to know when alkali therapy should be introduced to avoid growth failure secondary to true metabolic acidosis. When plasma renin activity is measured in the infant with renal failure of hypertension, one must compare the result with the normal range of values related to postnatal age of normal infants.

Role of atrial natriuretic peptide in sodium homeostasis in premature infants

To examine the possible involvement of atrial natriuretic peptide (ANP) in sodium homeostasis in premature infants, two groups of low birth weight infants with different dietary sodium regimens were studied. Sodium balance and plasma concentration of ANP were measured at weekly intervals for 5 weeks. At 1 week of age the study was started by dividing infants into two groups, group 1 with low and group 2 with increased sodium intake. Mean plasma concentrations of ANP were 47.7 +/- 7.6 and 51.4 +/- 9.5 fmol/ml, respectively. A steady decrease in plasma ANP concentration to 18.8 +/- 2.9 fmol/ml was observed in infants with sodium intake 1.5 mmol/kg/d (group 1), which was related to the decrease in serum sodium concentration in this group. In contrast, supplementation with NaCl 4.6 mmol/kg/d (group 2) was associated with a 30% increase in plasma ANP concentration, significantly different (P less than 0.025) from that in infants not given supplement, and was also higher than the values in full-term neonates. Our data suggest that altered sodium homeostasis induces regulatory changes in plasma ANP levels. ANP may provide a sensitive and important hormonal system for the control of sodium balance, even in premature neonates.

Late hyponatremia in premature infants: role of aldosterone and arginine vasopressin

To assess the possible involvement of arginine vasopressin in the pathogenesis of late hyponatremia in preterm infants, serial measurements of sodium balance, fractional sodium excretion, plasma and urine osmolality and sodium concentration, and urinary aldosterone and arginine vasopressin excretion were performed at weekly intervals in nine healthy preterm infants. During the course of late hyponatremia, there was a significant increase in urinary aldosterone and arginine vasopressin excretion, from 0.94 +/- 0.16 to 4.30 +/- 0.76 micrograms/day and from 0.38 +/- 0.08 to 1.19 +/- 0.26 ng/day, respectively, from the first to the fourth to fifth weeks. A significant negative correlation was found between fractional sodium excretion and urinary aldosterone excretion. Aldosterone excretion, however, correlated positively with urinary arginine vasopressin excretion in seven of the nine infants. The parallel increase in urinary aldosterone and arginine vasopressin excretion in salt-losing premature infants may occur in response to the protracted contraction of the extracellular fluid compartment, and may contribute to the restoration of volume in the body fluid compartments and to the development of late hyponatremia.