Fluid Requirements in the Newborn Infant

               R. S. Cohen

One of the very first issues that must be dealt with when a newborn infant is admitted to intensive care is that of fluid management. The vast majority of newborn intensive care unit (NICU) patients will need intravenous (IV) fluids initially, and certainly almost all very low birth weight (VLBW) ones will. Thus, the question of how much fluid a baby needs must be answered on the admission orders for most NICU patients.

With rare exceptions, neonates are born well hydrated. Indeed, total body water for neonates is higher than older children and adults. Total body water changes with maturation from up to 85%–90% in the very preterm infant, decreasing to about 75% at term, and eventually becoming about 65% in older children and adults.¹ Total body water includes different compartments—intracellular and extracellular. The extracellular water can also be subdivided into interstitial and plasma fluid volume. The drop in weight over the first few days postpartum is almost entirely caused by loss of excess extracellular water. For well term infants, this increased extracellular water acts as a reserve to prevent dehydration during the first days of life until maternal milk production increases and provides an adequate fluid intake.

Particularly for preterm infants, this loss of water is both physiologic and necessary. Many studies have shown that, for premature infants, delayed water loss results in a worse outcome, specifically an increased incidence of chronic lung disease and persistent patency of the ductus arteriosus.^2–4 Therefore, initial fluid intake for premature infants should be restricted to the least fluid necessary. The degree of fluid restriction is limited by the need to use intravenous fluids to deliver other necessities to the patient. For example, because preterm infants are at risk for hypoglycemia and cannot take adequate nutrition by mouth initially, intravenous dextrose is used as a glucose source. Extrapolating from studies of hepatic glucose production in lambs, a rate of about 4 to 6 mg/kg per minute was estimated for term infants.⁵ To provide this rate of dextrose delivery, most NICUs restrict premature infants initially to about 60 to 80 mL/kg IV daily of D10W (10% dextrose by volume in water); 60 mL/kg of D10W provides 4.167 mg/kg per minute of dextrose, and 80 mL/kg of D10W provides 5.56 mg/kg per minute of dextrose. Remember that hydrous dextrose is not equivalent to anhydrous glucose; the conversion factor is simplified as 0.91. Thus, more accurately, intravenous D10W at 60 or 80 mL/kg daily is the equivalent of a glucose infusion rate of about 3.8 or 5.05 mg/kg per minute of glucose, respectively.

Because hepatic glucose production may continue in extremely low gestational age neonates (ELGANs) despite the infusion of intravenous dextrose, some of these patients will become hyperglycemic on this regimen. This may require slowing the infusion rate further or decreasing the dextrose concentration. Most units will not go below a dextrose concentration of about 3% to avoid using overly hypotonic solutions. Occasionally, an insulin infusion may be needed to control this hyperglycemia. This is generally a last resort because premature infants are relatively insulin resistant.

Assuming the initial intravenous rate results in euglycemia, further adjustments of intravenous fluid intake over the first few days of life are then determined by changes in weight, urine output, and serum chemistries. Excessive intravenous fluid administration during the immediate postpartum period will result in either weight gain or inadequate weight loss. Furthermore, a drop in serum sodium levels will often accompany this. Overly restricting fluid intake at this time will result in too much weight loss and a rise in serum sodium. Kidneys of very premature infants have a limited ability to compensate and will neither fully excrete a fluid overload nor be able to decrease obligate free water losses adequately if overly restricted. Thus, it is imperative to monitor weight and serum electrolytes closely over the first few days of life in premature infants and increase or decrease the intravenous fluid rate accordingly.

There are known predictable causes of increased fluid requirements for NICU patients. One is the marked increase in transepidermal water loss (TEWL) seen in ELGANs, generally those born at about 26 weeks gestation or earlier, with very immature skin. Usually, this can be recognized on physical exam because of the shiny red translucent appearance of the skin. These infants can have TEWL as much as 15 times that of term infants,^6,7 which if untreated can rapidly result in hypernatremic dehydration. Much higher intravenous rates are needed for these ELGANs, commonly 2–3 times as high or more, which places them at great risk for hyperglycemia or hyponatremia and requires great vigilance.

Attempts to limit the TEWL have focused on 2 different approaches. One is to use humidified incubators to decrease evaporative water losses. Acceptance of this approach has been hindered by the risk of bacterial or fungal contamination of the water supply, resulting in increased infections, and the lack of convincing evidence of improved long-term outcome.^8,9 However, newer technologies for incubator humidification may have eliminated the risk of waterborne infection and resulted in improved outcomes.^10,11 It appears that neonatology is arriving at a point at which knowledge and technology have advanced to allow us to know how to safely start and modulate humidification for ELGANs. Starting at a high relative humidity of about 85% and then weaning over the first few weeks of life allows for the advantages of decreased TEWL initially while avoiding the increased risk of infection seen with earlier technology and supporting appropriate epidermal maturation.⁷

The other approach to decreasing TEWL has been to use various skin coverings. Many barriers, including semipermeable membranes and ointments, have been tried. Although decreased TEWL has been reported with ointments,^12,13 concerns remain about possible increased skin infection.¹⁴ Wraps have been shown to decrease TEWL.^15–17 They also decrease the incidence of hypothermia, but their impact on long-term outcome is unclear.¹⁸

Other causes of increased TEWL include phototherapy, which can increase TEWL by over 20%,^19,20 and for ELGAN infants, total intravenous fluid rates should be increased accordingly. TEWL may be increased in disorders of epithelial development, such as “collodion baby” ichthyosis, epidermolysis bullosa, harlequin ichthyosis, and more. Abdominal wall defects (gastroschisis, omphalocele) and spina bifida can also result in an increased insensible water loss that must be compensated.

After the diuresis of extracellular fluid during the first few days of life, fluid administration is advanced slowly to support adequate renal function and provide nutrition. How much to advance is critical given the concerns about fluid overload and the possible adverse consequences thereof.⁴ Minimal maintenance fluids are in the 100- to 120-mL/kg daily range, and growth usually requires somewhat more intake. Intake in the 130- to 140-mL/kg daily range seems to allow ductal closure,^21,22 and higher intravenous rates in the range of 170 mL/kg daily are associated with an increased risk for prolonged patency.³

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