FLUID ELECTROLYTE STATUS OF THE NEWBORN

ASSESSING FLUID AND ELECTROLYTE STATUS OF THE NEWBORN
A number of conditions can impact neonatal renal function adversely. The presence of several of these can be suspected on the basis of information found in the prenatal and neonatal history.
Maternal history
• A newborn's fluid and electrolyte (FE) status partially reflects the mother's FE status. For example, excessive administration of oxytocin or hypotonic intravenous fluid (IVF) to the mother can cause hyponatremia in the neonate at birth.

• Placental dysfunction (eg, from hypertension in pregnancy) can affect intrauterine growth adversely. Infants who are growth retarded at birth (<10th percentile for gestational age) tend to grow poorly despite adequate nutrition after birth.

• Poorly controlled maternal diabetes may be associated with renal vein thrombosis. This can affect an infant's renal function adversely.

• Maternal use of angiotensin-converting enzyme (ACE) inhibitors, such as captopril, during pregnancy can lead to acute renal failure in infants. Other medications administered to the mother, including indomethacin, furosemide, and aminoglycoside, also may affect renal function in the neonate.

• Antenatal steroids may increase skin maturation, thereby decreasing IWL and the risk of hyperkalemia.

Newborn history
• The presence of polyhydramnios or oligohydramnios can be associated with either congenital nephrotic syndrome or congenital renal dysfunction.

• Severe in utero hypoxemia or birth asphyxia may lead to acute tubular necrosis.

• Posterior urethral valves can be suspected when weak urinary stream and dribbling are present.

• The environment in which an infant is cared for also affects fluid loss. An environment with high ambient humidity decreases IWL, while the use of a radiant warmer or phototherapy may significantly increase an infant's IWL.

Clinical evaluation
• Weight factors
o Sudden changes in an infant's weight do not necessarily correlate with changes in intravascular volume. An infant's weight rises significantly for a number of reasons while intravascular volume has decreased. Examples include the long-term use of paralytic agents and peritonitis, both of which can lead to increased interstitial fluid volume and increased body weight but decreased intravascular volume.
o While growth charts are valuable in following growth parameters and nutritional status over time, they play little role in the daily management of fluid and electrolyte balances.

• Skin and mucosa manifestations: Altered skin turgor, a sunken anterior fontanelle (AF), and dry mucous membranes are not sensitive indicators of dehydration in babies. Remembering that premature infants have poorly keratinized skin that leads to a marked elevation in IWL is important.

• Cardiovascular signs
o Tachycardia can result either from too much ECF (as can be seen in congestive heart failure [CHF]) or from too little ECF (as can be seen in hypovolemia).
o Although delayed capillary refill occurs in low cardiac output states, it also can be seen in infants with peripheral vasoconstriction resulting from cold stress.
o Hepatomegaly can occur in neonates with ECF excess, especially in CHF.
o As a result of an infant's compensatory mechanisms, blood pressure (BP) readings usually are normal, with mild or moderate hypovolemia. With severe hypovolemia, hypotension is present almost invariably.

Laboratory evaluation
Depending on the clinical situation and the suspected etiology of fluid and electrolyte derangements, some or all of the following tests may be warranted:
• Serum electrolyte, urea nitrogen, creatinine, and plasma osmolarity levels: Keep in mind that over the first 12-24 hours, results of these tests may still reflect maternal values.

• Accurate total urine output and total fluid intake

• Urine electrolytes and specific gravity: If the infant is being treated with diuretics, such as furosemide, results of these tests are difficult to interpret.

• Calculation of the fractional urinary excretion of sodium in relation to creatinine (FENa)

• Blood gas analysis: Metabolic acidosis may be a marker of inadequate tissue perfusion.