Fluids, Electrolytes, and Nutrition

Electrolyte disturbances occur frequently in premature neonates who are at high risk for sodium and glucose disturbances and have difficulty with adequate calcium and phosphorus stores. Neonatal nutrition is one of the most important aspects of caring for an infant, particularly one who is preterm. Because of the delicate balance of electrolytes in preterm infants, it is typical to check serum electrolytes regularly during the first days and weeks of hospitalization in a neonatal intensive care unit (NICU).

This section covers the following topics:

• Neonatal Electrolyte Abnormalities

• Hypoglycemia

• Fluid Requirements

• Nutrition (Parenteral and Enteral Neonatal Nutrition)

• Growth

Neonatal Electrolyte Abnormalities

Preterm infants have the highest percentage of total body water relative to body weight. The percentage of total body water decreases with increasing gestational age and into adulthood. Infants also have a high ratio of surface area to body weight, which results in high evaporative losses. Total body water is defined as intracellular fluid (ICF) plus extracellular fluid (ECF). ECF consists of interstitial fluid plus intravascular fluid. Most body fluid in the earliest preterm infants is ECF. ECF decreases as other components (e.g., fat) increase over time.

Preterm infants have increased insensible water loss (IWL). IWL is measured as fluid intake minus urine output plus change in weight. IWL increases with decreasing gestational age, radiant warming, and exposed abdominal contents (e.g., gastroschisis). Maintaining temperature and hydration is important in the resuscitation of infants at any gestation age and even more so at earlier gestational ages. In infants ≤28 weeks’ gestation at the time of delivery, humidification and other methods of reducing hypothermia, such as placing the baby in a plastic bag in the delivery room, are employed to decrease IWL further.

For more information on electrolyte abnormalities, see Acid-Base/Electrolyte Disturbances in the Pediatric Critical Care guide.

Sodium

Due to increased IWL, preterm infants are at risk of developing hypernatremia during the first few days of life and require up to 200 mL/kg/day of maintenance fluids to maintain fluid losses and normal sodium levels. Hyponatremia can also occur in neonates from iatrogenic causes (including excessive fluid resuscitation), donor human-milk intake, and pathologic processes (e.g., syndrome of inappropriate antidiuretic hormone secretion).

Potassium

Hyperkalemia is a common finding in preterm and term infants. Causes for hyperkalemia include acidemia and congenital adrenal hyperplasia. Iatrogenic hypokalemia is also common, mostly due to inadequate maintenance of potassium in intravenous fluids or use of diuretics for conditions such as chronic lung disease. A hemolyzed blood sample will lead to apparent hyperkalemia that is not reflective of the infant’s potassium level.

Calcium and Phosphorous

Perturbations in calcium and phosphorous often occur in preterm infants. Most preterm infants did not obtain adequate calcium and phosphorous stores during the third trimester and have greater need for calcium and phosphorous for bone growth after birth, as compared with term infants. Preterm formulas include higher concentrations of calcium and phosphorous and are supplied in a 2:1 ratio in parenteral nutrition.

Hypoglycemia

Hypoglycemia is one of the most common metabolic disturbances in both preterm and term infants. The American Academy of Pediatrics (AAP) and Pediatric Endocrine Society (PES) have not reached a consensus on the definition of hypoglycemia, although both recognize the importance of assessing for, and treating, the condition.

• The AAP defines hypoglycemia as a serum glucose <40 mg/dL during the first 24 hours of life.

• The PES defines hypoglycemia as <60 mg/dL by 48 hours of life.

Risk factors for hypoglycemia include prematurity, large or small for gestational age (LGA or SGA), infant of a diabetic mother, stress, sepsis, congenital disorders (e.g., inborn errors of metabolism and Beckwith-Wiedemann syndrome), polycythemia, and maternal medication exposures (e.g., beta-blockers, insulin).

Infants are screened for hypoglycemia based on their risk-factor profile. The AAP recommends screening within the first hour of life after receiving an initial feed (formula or breast milk). Management of hypoglycemia focuses on increasing the level of the serum glucose with feeding, dextrose gel, or intravenous (IV) glucose administration. Many NICUs have developed local algorithms for treating hypoglycemia in preterm and other at-risk infants.

Infants of diabetic mothers are often LGA and at risk for additional conditions, including shoulder dystocia; hypocalcemia and hypomagnesemia (see Nutrition and Growth below); respiratory distress (respiratory distress syndrome vs. transient tachypnea of the newborn; see Common Respiratory Disorders); polycythemia and hyperbilirubinemia; and congenital malformations including cardiac defects (e.g., interventricular septal hypertrophy, tetralogy of Fallot, and transposition of the great arteries), small left colon syndrome, caudal regression syndrome, and spinal dysraphism (neural tube defects).

Nutrition

Depending on gestational age at birth, preterm infants miss at least part or all of the third trimester, which is typically rich in the transfer of nutrients including proteins and electrolytes. Therefore, preterm infants are vulnerable to delayed growth and development. Appropriate nutrition can improve neurodevelopmental outcomes. In this section, we describe the basics of neonatal nutrition.

Fluid Administration

Neonates have much larger total-body-water compartments than older children and adults (see Electrolyte Disturbances). Because the percentage of total body water decreases with increasing gestational age, the amount of fluid initiated in a neonate depends on gestational age. The amount of fluid an infant requires per day is based on weight loss or gain, urine output, and electrolyte measurements.

Enteral Nutrition

Neonatologists strive to begin enteral feeding in stable infants as soon as possible to promote intestinal growth and achieve adequate nutrition. Although most infants ≤34 weeks’ gestation are unable to take nutrition by mouth, a nasogastric tube inserted into the stomach can deliver enteral nutrition until an infant begins to develop feeding cues and oral-motor abilities needed to eat by mouth. Both breast milk and formula are used for enteral feeds in preterm infants.

Breast milk: Repeated studies have shown that breast milk, particularly maternal breast milk, provides optimal nutrition and immunoglobulins to infants and reduces the risk of necrotizing enterocolitis in the most premature infants. Pasteurized donor breast milk, provided by milk banks, is another source of breast milk for premature infants when maternal breast milk is unavailable or volumes are inadequate. Although breast milk is recommended as the primary source of enteral nutrition for infants, vitamin D is variably transferred via breast milk. Therefore, supplementation is required in all exclusively breastfed infants.

Formula: Infant formula can be used for enteral feeding if maternal or donor milk is not available or if an infant has a specific medical condition that requires specialized formula. Formulas are categorized as follows:

• Preterm formulas supply additional protein, vitamins, minerals, and calories specific to the needs of a premature infant.

• Post-discharge or transitional formulas are ideal at the time of discharge to supply additional calcium, phosphorus, and calories (compared with term formulas) required for additional growth.

• Hypoallergenic and elemental formulas are used in situations in which the proteins need to be broken down for digestion, including short gut syndrome, milk protein allergy, and metabolic conditions for which the formula is specialized for the metabolic abnormality.

• Term formulas

Fortification: Both breast milk and formula sometimes need to be fortified with additional calories for preterm infants to supply them with adequate proteins, minerals, and calories for appropriate growth. Fortification is usually achieved with a human-milk fortifier for breast milk or increased concentration of formula. Institutional guidelines vary on when to fortify during enteral feed advancement. Preterm infants are particularly at risk of anemia due to the absence of iron transfer during the third trimester and from iatrogenic blood loss. All preterm infants should receive vitamin D and iron supplementation once they reach full-volume feeds, and all term breastfed infants should receive vitamin D supplementation.

Contraindications: Although enteral feeding is the preferred route to promote adequate growth, it is not always possible. Contraindications include necrotizing enterocolitis, intestinal obstruction, or any gastrointestinal pathology that reduces blood flow to the mesentery and compromises the gut (e.g., hemodynamic instability, particularly in patients receiving vasopressors). The presence of an umbilical arterial catheter represents a relative contraindication to enteral feeding, although trophic feedings are still often feasible. Enteral feeding should be advanced cautiously if an infant's mother was receiving magnesium therapy prior to delivery or if the infant has other signs of poor gut motility.

Parenteral Nutrition

Parenteral nutrition (PN) is used when preterm infants are unable to meet their full nutritional needs with enteral feeding. Parenteral nutrition is also indicated when gut rest is necessary, as in necrotizing enterocolitis. Calories are provided from amino acids, glucose, and lipids. Electrolytes, minerals, and vitamins are added to parenteral nutrition as necessary and monitored frequently with serum testing.

Serum glucose and triglyceride levels should be monitored when increasing or decreasing PN volume. Electrolytes should be monitored every few days for calcium, magnesium, and phosphorus values as amino acids are increased. Daily or every-other-day measurement of electrolytes helps guide basic sodium and potassium concentrations in the individual infant’s PN preparation.

Growth

Term and preterm infants can lose up to 10% or 15% of birth weight, respectively, in the first week of life and then are expected to regain this weight by day 10 to 14. Once infants begin gaining weight, the expected rate of gain is 15-20 g/kg/day for infants weighing <2 kg and 20-30 g/kg/day for infants weighing >2 kg. Infants typically grow 1 cm/week in length, and 1 cm/week in head circumference.

Growth charts have been developed for term and preterm infants. For term infants, the World Health Organization (WHO) growth chart is most often used and follows the growth of a typical breastfed infant. Several versions of the preterm growth chart are available. Preterm infant growth is based on intrauterine growth curves. The Fenton growth curve combines information from the WHO and intrauterine growth charts to estimate growth at varying gestational ages. The Olsen growth chart for preterm infants combines multicenter data to reflect actual growth in the NICU, compared with expected intrauterine growth.

(Source: The WHO Growth Charts. Centers for Disease Control and Prevention, National Center for Health Statistics 2010.)