Chapter 9: Fluid Requirements in the Newborn Infant

INTRODUCTION

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, 3, and 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 ...