Total body water in newborns

Abstract

In this chapter, the modifications that occur in water distribution from the fetus to the newborn, focusing on the neonatal period, are reviewed. This is a critical period in life as the newborn infant has to adapt itself to a new environment. The main water compartments, intra- and extracellular, undergo changes throughout infancy, with an increase in intracellular space and a decrease in extracellular space as the child grows. The mechanisms of water dynamics behind these modifications are discussed. The flow of water through the cell membranes occurs through the cells (i.e., transcellular) or between the cells (i.e., paracellular). Water crosses the cell membranes through the lipid bilayer or through membrane pores (aquaporins). In paracellular flow, the flux occurs through wide spaces between the cells. Water molecules can also cross the membrane through diffusion in response to osmotic or hydrostatic pressure differences. Motionally distinct water fractions have been described: free bulky water and bound water, where bound water can be tightly bound or loosely bound water. Water can be liberated according to the need of the organism from the bound water fraction irrespective of its location, in the intra- or extracellular compartments. Healthy newborn infants lose approximately 5-10% of their body weight during the first week of life. This loss in birth weight is attributed to fluctuation of total body water. After birth, the extracellular water compartment undergoes a contraction, followed by natriuresis, diuresis, and weight loss; the infant is oliguric during the first 24-48 h of life. Gestational age is a strong determinant of water loss in newborn, mainly preterm, infants. Insensible water loss through the skin plays a major role in water depletion, as newborns possess a greater surface area, and it has been shown that preterm infants have greater transepidermal water loss than term infants, and this is more pronounced in appropriate-for-gestational-age infants compared to small-for-gestational-age infants. Small-forgestational- age newborn infants have been shown to possess greater total body water than is appropriate for gestational age infants. The knowledge of water distribution in neonates and the differences between the two major groups of newborns (appropriate and small-for-gestational-age infants) is fundamental for administering fluids for those infants in the intensive care units without challenging their homeostasis.