Edward F. Bell, MD and Michael J. Acarregui, MD
Peer Review Status: Internally Peer Reviewed

Fluid therapy for the neonate can be rationally planned if several physiologic concepts are kept in mind. Firstly, the neonate has an excess of total body water at birth, particularly extracellular water, which must be redistributed and excreted. The renin-angiotensin system is in high gear during the first week after birth; thus not only is plasma angiotensin II likely to be elevated, but also aldosterone, which is a mineralocorticoid and has the potential to modulate sodium excretion/reabsorption. The surface of the newborn is large and increases with decreasing size; therefore there is a greater likelihood of excess insensible water loss (IWL), which may be exaggerated as birth weight and gestational age decrease and open radiant warmers rather than incubators are used. Finally, in most instances the neonatal kidney has the capacity to not only dilute urine, but also to concentrate it, reaching values of 600-700 m0sm/L (specific gravity ≤ 1.015). It should be noted, however, that this is less than that seen in adults or term infants. These observations are contrary to previous "beliefs," and each of these aspects of the neonate are reviewed elsewhere (J Pediatr 101:387, 1982).

Our goal in the low-birth-weight (LBW) infant ≤1599 g is to allow a gradual weight loss over the first week, i.e., 5-6% over the first 24 h and 12-15% by the end of the first week. We also attempt to maintain urine output ≥ 0.5 ml/kg hr. If IWL plus urine output significantly exceeds intake, weight loss may be greater than desired and occur more rapidly in the preterm LBW infant. This in turn may result in development of hypernatremia since fluid losses through the skin are essentially free water. To take each of these into account, the first approach to fluid therapy is adequate monitoring and appropriate supportive care. Thus, all infants ≤1000 g birth weigh should be maintained on a bed scale, kept on "strict" input and output measurements, and covered with a "saran blanket" to minimize IWL, especially if cared for in an open radiant warmer.

It is estimated that the nongrowing neonate requires 60-75 kcal/kg/day and that fluid losses are closely related to caloric expenditure. Thus, in the first 1-3 days after birth fluid requirements are likely to be in the range of 65-75 ml/kg/day in a neutral thermal environment. To accomplish this we use 10% dextrose in water (D10W). Therefore, at 24 h the fluid should be changed to D10 with 1/4 isotonic saline. The addition of KCl to the infusate should be considered by day 3 if there are no contraindications, e.g., poor renal function or hemolytic disease, at 2-3 mEq/kg/day. Although negative potassium balance occurs with this approach it is quickly corrected.

Our approach to fluid therapy has been to gradually increase the volume to approximately 75-80 ml/kg/day on day 2, 90-95 ml/kg/day on day 3, and -125 ml/kg/day by day 7. At 14 days most infants are receiving about 135 ml/kg/day.

References:

  1. J Pediatr 101:387, 1982.
  2. J Pediatr 101:423, 1982.
  3. The Body Fluids in Pediatrics, Winters RW (ed), Boston, Little Brown & Co., 1973.
  4. Clinics in Perinatology 9:483, 1982.
  5. The Micropremie: The Next Frontier, 99th Ross Conference on Pediatric Research, Columbus, OH, 1990.