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An assessment of body water metabolism and electrolyte balance plays an important role in the early medical management of preterm infants and sick term infants coming to neonatal intensive care. Intravenous or intra-arterial fluids given during the first several days of life are a major factor in the development, or prevention, of morbidities such as intraventricular hemorrhage, necrotizing enterocolitis, patent ductus arteriosus, and bronchopulmonary dysplasia. Therefore it is crucial that an infant's clinician pay close attention to the details of maintaining and monitoring body water and serum electrolytes, and the management of fluid infusion therapies.

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Bodily fluid balance is a function of the distribution of water in the body, water intake, and water losses. Body water distribution gradually changes with increasing gestational age of the fetus, from the very immature fetus with 90% of body weight as water, to the late preterm fetus with 75–80% body weight as water. At birth these gestational changes in body water are reflected in the developing maturity of renal function, transepidermal insensible water losses, and neuroendocrine adaptations. A clinician must account for these variables when deciding the amount of infusion fluids to administer to an infant.

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  1. Body water

      1. Total body water (TBW). Water accounts for nearly 75% of the body weight in term infants and as much as 85–90% of body weight of preterm infants. TBW is divided into two basic body water compartments: intracellular water (ICW) and extracellular (ECW). ECW is composed of intravascular and interstitial water. For the fetus there is a gradual decrease in ECW to 53% at 32 weeks' gestation and a gradual increase in ICW. Thereafter the proportions remain fairly constant until 38 weeks' of gestation when increasing body mass of protein and fat stores reduce ECW further by approximately 5%.

        At birth there begins a further contraction of the ECW as a function of the normal transition from intrauterine to extrauterine life. A diuresis occurs that reduces body weight proportionally to gestational age. For the very low birthweight preterm infant body, weight losses of 10–15% can be expected, whereas full-term infants usually lose 5% of body weight. These losses are largely accounted for as water and, to a lesser extent, body fat stores.

      1. TBW balance in the newborn

        • Renal. Fetal urine flow steadily increases from 2–5 mL/h to 10–20 mL/h at 30 weeks' gestation. At term, fetal urine flow reaches 25–50 mL/h and then drops to 8–16 mL/h (1–3 mL/kg/h). These volume changes illustrate the large exchange of body water during fetal life and the abrupt changes forcing physiologic adaptation at birth. Despite marked fetal urine flow in utero, glomerular filtration rates (GFR) are low. At birth, GFR remains low but steadily increases in the newborn period under the influence of increasing systolic blood pressure, increasing renal blood flow, and increasing glomerular permeability. Infant kidneys are able to produce dilute urine within limits dependent on GFR. The low GFR of preterm infants is the result of low renal blood ...

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