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

Risk factors for late-onset hyponatremia and its influence on neonatal outcomes in preterm infants

Late-onset hyponatremia (LOH), hyponatremia occurring after two weeks of age with the achievement of full feeding, is the result of a negative sodium balance caused by inadequate salt intake or excessive salt loss due to immature renal or intestinal function in preterm infants. The aims of our study were to identify the risk factors for LOH and its influence on neonatal outcomes. This was a retrospective cohort analysis of 161 preterm infants born before 34 weeks of gestation between June 2009 and December 2010 at Seoul National University Hospital. LOH was defined as a sodium level ≤ 132 mEq/L or 133-135 mEq/L with oral sodium supplementation. LOH occurred in 49 (30.4%) of the studied infants. A lower gestational age, a shorter duration of parenteral nutrition, the presence of respiratory distress syndrome, the use of furosemide, and feeding with breast milk were significant risk factors for LOH. In terms of neonatal outcomes, the infants with LOH had longer hospital stays and higher risks of bronchopulmonary dysplasia and retinopathy of prematurity requiring surgery. LOH lasting at least 7 days significantly increased moderate to severe bronchopulmonary dysplasia, periventricular leukomalacia, and extra-uterine growth retardation. LOH is commonly observed in preterm infants; it may be a risk factor for bronchopulmonary dysplasia and retinopathy of prematurity or a marker of illness severity.

Predictors of delayed first voiding in newborn

AIM

Delay>24 h of age in neonates' first voiding attracts attention, although the phenomenon is usually benign. Earlier studies indicate that stress increases the infant's arginine vasopressin (AVP) and aldosterone secretion during birth. Our aim was to seek predictors of delayed first voiding and indirect evidence of AVP effect behind this phenomenon.

METHODS

The study population comprised 20 normal-term newborns whose first voiding was delayed>24 h of age (cases), and 19 age-matched control infants who voided for the first time at <24 h of age (controls). The first urine was collected and osmolality (U-Osm) and sodium content (U-Na) measured.

RESULTS

The median of U-osm in cases was 432.50 (284-519) and in controls 337.50 (169-497) mOsm/L (p=0.005), and U-Na 21.50 (9-241) and 40.00 (13-226) mmol/L (p=0.001), respectively. Cases were more frequently born to primiparous mothers than controls (70% vs. 21%, p=0.004). Duration of labour was longer in cases than controls, first stage 10.5 h (3.92-20.50 h) versus 5.7 h (1.17-16.00 h) (p=0.045) and second stage 0.42 h (0.08-1.25 h) versus 0.17 h (0.08-0.92 h) (p=0.015). All seven (35%) abnormal cardiotocographies were recorded with cases (p=0.008).

CONCLUSIONS

Delayed voiding appears to be related to a prolonged and stressful birth. Laboratory findings in the first urine suggest increased AVP and aldosterone secretion in such cases.

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.

Water and electrolyte metabolism of the micropremie

There are five problem schemas presented in this article that indicate potential contradictions in therapeutic goals: (1) shock and edema presenting upon premature birth; (2) the hyperosmolar state, problematic in patients less than 750 g birth weight; (3) the respiratory distress syndrome and respiratory failure, often complicated by patent ductus arteriosus; (4) bronchopulmonary dysplasia, resulting from prematurity and mechanical ventilation; and (5) late onset of hyponatremia, sometimes accompanied by growth failure in the recovering premature. These five problems considered together comprise a developmental continuum of illness and recovery, where appropriate fluid management has recently been demonstrated to benefit outcomes greatly. Clinicians over the past decade have encountered all of the fluid and electrolyte nightmares. Although there are many different formulations for treating each of these scenarios, recommending one approach for all patients is likely to be incorrect much of the time.

The effect of hypoxia from birth on the regulation of aldosterone in the 7-day-old rat: plasma hormones, steroidogenesis in vitro, and steroidogenic enzyme messenger ribonucleic acid

Adaptation to hypoxia in the neonate requires an appropriate adrenocortical response. The purpose of this study was to examine the adaptation of the aldosterone pathway in rat pups exposed to hypoxia in vivo from birth to 7 days of age. Neonatal rats (with their lactating dams) were exposed to normoxia (21% O2) or hypoxia (12% O2) continuously for 7 days from birth. Trunk blood was collected, and entire adrenal glands were processed from 7-day-old rats to study the activity of the steroidogenic pathway in dispersed cells and isolated mitochondria, for measurement of expression of the steroidogenic enzyme messenger RNAs (mRNAs) by RT-competitive PCR and in situ hybridization histochemistry, for measurement of zona glomerulosa width by immunohistofluorescent staining for P450c11AS protein, and for measurement of mitochondrial number and distribution by transmission electron microscopy. Exposure to hypoxia for 7 days from birth resulted in a marked increase in plasma ACTH, corticosterone, and aldosterone with no change in PRA. Aldosteronogenesis and P450c11AS activity were both augmented in dispersed cells; this effect was lost in isolated mitochondria (from entire adrenal glands) using a permeable substrate for P450c11AS. There was no significant effect of hypoxia on expression of the steroidogenic enzyme mRNAs measured by RT-competitive PCR or in situ hybridization histochemistry. Finally, hypoxia had no effect on mitochondrial number or stereology as assessed by transmission electron microscopy or on zona glomerulosa width as assessed by staining for P450c11AS protein. We conclude that, as opposed to that in adults, hypoxia in the neonate results in an augmentation of aldosteronogenesis. This effect is not accounted for by a change in steroidogenic enzyme mRNA expression, zona glomerulosa width (i.e. hyperplasia), or mitochondrial number or distribution. This functional augmentation of aldosteronogenesis may be due to a change in mitochondrial permeability to steroid substrates and/or the effect of cytosolic factors that control mitochondrial steroidogenesis.

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.

Late hyponatremia in very-low-birth-weight infants: incidence and associated risk factors

OBJECTIVE

To determine the incidence of late hyponatremia in very-low-birth-weight infants and to identify associated risk factors. Low serum sodium concentration in otherwise healthy premature infants beyond 2 weeks of life is referred to as the hyponatremia.

DESIGN

Retrospective cohort review.

SETTING/SUBJECTS

The intensive care nursery at St Luke's Hospital Perinatal Center, Kansas City, Mo. Criteria for subject selection were birth weight of 1,500 g or less; survival for more than 21 days; development of late hyponatremia or hospitalization for 42 days or more; and measurement of serum sodium concentration at least once between the 14th and 56th day of life. Of 515 infants admitted to the nursery for the 1992 calendar year, 124 had a birth weight of 1,500 g or less and survived for more than 21 days; however, 16 of these infants were discharged at 42 days of life or less, 11 did not have a serum sodium concentration measurement after the second week of life, and 1 did not have complete medical records. Thus, the final sample was 96 subjects.

STATISTICAL ANALYSES

Percentages to determine incidence of late hyponatremia; t test of chi 2 test to determine differences between infants with and without late hyponatremia; multiple logistic regression to determine the strongest indicators of late hyponatremia.

RESULTS

Incidence of late hyponatremia was 62.5%. Significant risk factors for late hyponatremia were birth weight of 1,000 g or less (P < .001), feedings of fortified human milk (P < .013), and occurrence of an intraventricular hemorrhage (P < .036). Fortified human milk feeding was a significant risk factor for late hyponatremia in both weight groups (ie, birth weight greater than or less than 1,000 g).

CONCLUSION

Despite standard fortification, human milk may contain an insufficient quantity of sodium to met the needs of very-low-birth-weight infants.

Urinary excretion of aldosterone, arginine vasopressin and cortisol in premature infants with maximum renal acid stimulation

Of 452 low-birth-weight infants who were routinely screened for maximum renal acid stimulation (MRAS) (urine pH < 5.4), 149 episodes of incipient late metabolic acidosis (urine pH < 5.4 on 2 consecutive days) were randomly allocated to either a control group or treatment with NaHCO3 or NaCl (2 mmol/kg/day each) for 7 days. Urinary excretion of aldosterone-18-glucuronide (Aldo), arginine vasopressin (AVP) and cortisol was determined in timed urine samples. On day 1, patients with MRAS showed a tendency towards increased urinary excretion of Aldo compared with infants without MRAS. In patients who received alkali therapy, urinary excretion of Aldo, AVP and cortisol decreased or showed a trend to lower values from day 1 to day 7, whereas in patients with MRAS but no specific therapy, Aldo and AVP showed a tendency to increase. We concluded that persistent MRAS is not only characterized by a reduced rate of weight gain and a tendency to decreased nitrogen assimilation, but also increased secretion of Aldo and AVP.

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.

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.

Sodium balance and extracellular volume regulation in very low birth weight infants

We assessed sodium balance and extracellular volume regulation in very low birth weight infants by examining the effect of differences in sodium intake on postnatal sodium homeostasis and body water composition. Twenty infants (mean birth weight 1103 +/- 216 gm, mean gestation 28.5 +/- 1.7 weeks) were randomly assigned to receive sodium in doses of either 1 or 3 mmol.kg-1.day-1 for the first 10 postnatal days. Extracellular volume (estimated by the bromide dilution method), sodium excretion, creatinine clearance, fractional sodium excretion, plasma atrial natriuretic factor level, urine aldosterone concentration, and vasopressin excretion were measured on postnatal days 1, 5, 10, 20, and 30. The corrected bromide space was large at birth and decreased in both groups during the first 5 days of observation, concomitant with a negative sodium balance. After 5 days of age, sodium excretion decreased in both groups so that sodium balance became positive and the corrected bromide space increased in proportion to increasing body weight. Differences in sodium intake were associated with differences in tubular sodium reabsorption; corrected bromide space and net sodium balance were similar in the two groups. Serum sodium concentration was significantly lower in the low-sodium intake group. Creatinine clearance, plasma atrial natriuretic factor level, and excretion of aldosterone and vasopressin were not significantly different between the two groups. We conclude that very low birth weight infants are able to regulate sodium balance by altering renal sodium excretion. However, the renal response to sodium intake may be insufficient to prevent changes in serum sodium concentration. The roles of specific renal and hormonal mechanisms regulating sodium excretion in very low birth weight infants remain incompletely defined.

Autosomal recessive polycystic kidney disease

The clinical features of 55 cases of autosomal recessive polycystic kidney disease (ARPCKD) have been reviewed. Each had evidence of ARPCKD. The outcomes of 87% were known; 24 had died. Twenty-four of 31 were seen between 1980 and 1986; 7 could not be traced. Forty-five percent presented under 1 month; 38% between 1 month and 1 year; and 9 cases over 1 year. Hyponatraemia occurred in 15 out of 19 aged less than 3 months; hypertension occurred in 65%; splenomegaly in 47% of those surviving more than 3 months. Portocaval shunts were done in 5 aged 2-12 years. Thirteen died of renal failure, 6 under 1 year, and 7 between 1 year and 13 years. Life-table survival rates calculated from birth revealed that 86% were alive at 3 months, 79% at 1 year, 51% at 10 years, and 46% at 15 years. Calculations based on patients who survived to 1 year of age showed that 82% were alive at 10 years and 79% at 15 years. These results reveal an improved prognosis for a condition once assumed to be fatal.

Inappropriate fluid response in congenital diaphragmatic hernia: first report of a frequent occurrence

Safe management of the newborn infant with congenital diaphragmatic hernia (CDH) requires precise fluid administration to avoid hypovolemia or fluid overload. Twenty-two CDH patients and 12 infants who underwent abdominal operations were studied for three postoperative days to determine whether the postoperative neonatal renal response to fluid administration was appropriate or inappropriate. Each response was categorized, on the basis of urine and blood measurements, as: (1) appropriate urine output and concentration, (2) inappropriate urine output and concentration with fluid retention or (3) renal failure. Fluid intake was similar in all groups. The CDH group had a significantly lower urine output, higher urine osmolarity, and lower serum osmolarity. All of the control group (100%) responded appropriately to intake. Sixty-four percent of the CDH group inappropriately retained water during the first 16 hours (appropriate, 27%; renal failure, 9%). By 24 hours, 34% still had inappropriate urine output and fluid retention. The majority of patients with CDH initially responded inappropriately to postoperative fluid intake. If this response is not recognized and fluid intake is not adjusted, serious fluid overload will result.

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.

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.

Renal response to arginine vasopressin in premature infants with late hyponatraemia

To assess the influence of late hyponatraemia on the renal responsiveness to endogenous arginine vasopressin (AVP), urinary excretion and plasma concentration of sodium, plasma and urine osmolality, free water clearance, and urinary AVP concentration and excretion were measured in 11 healthy premature infants with a mean birth weight of 1360 g and mean gestational age of 31 weeks. Studies were performed on days 1, 5, and 19. The development of late hyponatraemia was associated with a pronounced decline in urine osmolality, whereas urine flow rate and free water clearance increased significantly. Mean (SEM) urine AVP concentration and excretion also rose significantly from 2.15 (0.31) pg/ml and 0.36 (0.55) pg/min/m2 on the first day to 6.5 (0.96) pg/ml and 3.85 (0.63) pg/min/m2 on the 19th day, respectively. When renal response to AVP was compared at different ages the highest urine osmolality and steepest response curve was observed on the first day. With development of hyponatraemia the renal response became blunted. It is concluded that the limited tubular sodium transport and hyponatraemia hinders the establishment of intrarenal osmotic gradient, impairs renal response to AVP, and prevents excessive water retention and further fall of plasma sodium.