THE PRETERM INFANT
Saturday, August 16, 2008
Premature infants account for the majority of high-risk newborns. The preterm infant faces a variety of physiologic handicaps:
Delivery Room Care
See Perinatal Resuscitation section, above.
Care in the Nursery
A. Thermoregulation: Maintaining a stable body temperature is a function of heat production and conservation balanced against heat loss. Heat production in response to cold stress can occur through voluntary muscle activity, involuntary muscle activity (shivering), and thermogenesis not caused by shivering. Newborns produce heat mainly through the last of these three mechanisms. This metabolic heat production depends on the quantity of brown fat present, which is very limited in the preterm infant. Heat loss to the environment can occur through the following mechanisms: (1) radiation—transfer of heat from a warmer to a cooler object not in contact; (2) convection—transfer of heat to the surrounding gaseous environment, influenced by air movement and temperature; (3) conduction—transfer of heat to a cooler object in contact; and (4) evaporation—cooling secondary to water loss through the skin. Heat loss in the preterm newborn is accelerated because of a high ratio of surface area to body mass, reduced insulation of subcutaneous tissue, and water loss through the immature skin.
The thermal environment of the preterm neonate must be regulated carefully. The infant can be kept warm in an isolette, in which the air is heated and convective heat loss is minimized. Alternatively, the infant can be kept warm on an open bed with a radiant heat source. Although evaporative and convective heat losses are greater when the radiant warmer is used, this system allows easy access to a critically ill neonate. Ideally, the infant should be kept in a neutral thermal environment (Figure 1–5). The neutral thermal environment allows the infant to maintain a stable core body temperature with a minimum of metabolic heat production through oxygen consumption. The neutral thermal environment for a given infant depends on size, gestational age, and postnatal age. The neutral thermal environment (for either isolette or radiant warmer care) can be obtained by maintaining an abdominal skin temperature of 36.5°C. Generally, when infants reach 1700–1800 g, they can maintain temperature while bundled in a bassinet.
B. Monitoring the High-Risk Infant: Care of the high-risk preterm infant requires sophisticated monitoring techniques. At a minimum, equipment to monitor heart rate, respirations, and blood pressure should be available. Oxygen saturation can be assessed continuously using pulse oximetry. This determination can be correlated with arterial oxygen tension (PaO2) as needed. Transcutaneous PO2 and PCO2 can also be used to assess oxygenation and ventilation. Finally, arterial blood gases, electrolytes, glucose, calcium, bilirubin, and other chemistries must be measured on small volumes of blood. Early in the care of a sick preterm infant, the most efficient way to sample blood for tests as well as to provide fluids and monitor blood pressure is through an umbilical arterial line. Once the infant is stable and the need for frequent blood samples is reduced (usually 4–7 days), the umbilical line should be removed. All indwelling lines are associated with morbidity from thrombosis or embolism, infection, and bleeding.
C. Fluid and Electrolyte Therapy: Fluid requirements in preterm infants are a function of (1) insensible losses (skin and respiratory tract), (2) urine output, (3) stool output (< 5% of total), and (4) others, such as nasogastric losses. In most circumstances, the fluid requirement is determined largely by insensible losses plus urine losses. The major contribution to insensible water loss is evaporative skin loss. The rate of water loss is a function of gestational age (body weight), environment (losses are greater under a radiant warmer than in an isolette), and the use of phototherapy. Respiratory losses are minimal when infants are breathing humidified oxygen. The renal contribution to water requirement is influenced by the decreased ability of the preterm neonate to concentrate the urine and conserve water.
Electrolyte requirements are minimal for the first 24–48 hours until there is excretion in the urine. Basal requirements are as follows: sodium, 3 mEq/ kg/d; potassium, 2 mEq/kg/d; chloride, 2–3 mEq/ kg/d; and bicarbonate, 2–3 mEq/kg/d.
In the infant born before 30 weeks’ gestation , sodium and bicarbonate losses in the urine are frequently elevated, increasing the infant’s requirement for these electrolytes.
Initial fluid management after birth is determined by the infant’s size. Infants weighing more than 1500 g should start at 80–100 mL/kg/d of 10% dextrose in water (D10W), whereas those weighing less should start at 100–120 mL/kg/d of either D10W or 5% dextrose in water (D5W) (infants < 800 g and < 26 weeks’ gestation often become hyperglycemic on D10W). The most critical issue in fluid management is monitoring. Measurements of body weight, urine output, fluid and electrolyte intake, serum and urine electrolytes, and glucose allow fairly precise determinations of the infant’s water, glucose, and electrolyte needs. Parenteral nutrition should be started early and continued until an adequate enteral intake is achieved.
D. Nutritional Support: The average caloric requirement for the growing premature infant is 120 kcal/kg/d. Expected weight gain for the adequately nourished preterm infant is 10–30 g/d.
Infants initially require intravenous glucose infusions to maintain blood glucose concentration in the range of 60–100 mg/dL. Infusions of 5–7 mg/kg/min (approximately 80–100 mL/kg/d of a 10% dextrose solution) are usually needed. Nutritional support in the very low-birth-weight infant generally is started at 24–48 hours of age with parenteral alimentation solutions given either peripherally or centrally via an umbilical vein line or Silastic catheter (Table 1–19). Small-volume trophic feeds with breast milk or 20 kcal/oz premature formula should be started by gavage at 10–25% of the infant’s nutritional needs as soon as possible and slowly advanced to full caloric needs over 3–7 days once the infant is stable. Intermittent bolus feedings are preferred because these appear to stimulate the release of gut-related hormones and may accelerate maturation of the gastrointestinal tract. The more rapid advancement schedule is used for infants weighing over 1500 g and the slowest schedule in the infant weighing less than 1000 g. Note:{ebd} In the extremely low-birth-weight infant (< 1000 g) or the postsurgical neonate, continuous-drip feeds are sometimes better-tolerated.
In general, long-term nutritional support for infants of very low birth weight consists either of breast milk supplemented to increase protein, caloric density, and mineral content or of infant formulas modified for preterm infants. In all of these formulas, protein concentrations (approximately 2 g/dL) and caloric concentrations (approximately 24 kcal/oz) are relatively high. In addition, premature formulas contain some of the fat as medium-chain triglycerides—which do not require bile for emulsification—as an energy source. Increased amounts of calcium and phosphorus are provided to enhance bone mineralization. The infant should be advanced gradually to feedings of higher caloric density after the full volume of either breast milk or formula (20 kcal/oz) is tolerated. Success of feedings is assessed by passage of feeds out of the stomach, abdominal examination free of distention, and normal stool pattern.
When the preterm infant approaches term, the nutritional source for the bottle-fed infant can be changed to a transitional formula (22 kcal/oz) until age 6–9 months. Iron supplementation (2–4 mg/kg/d) is recommended for premature infants, beginning at about age 2 months. This can be provided by iron-supplemented formulas. In some infants, iron supplementation may be indicated earlier. In particular, infants treated with erythropoietin (epoetin alfa) for prevention of anemia of prematurity require supplemental iron at a dosage of 4–8 mg/kg/d.
- The ability to suck, swallow, and breathe in a coordinated fashion is not achieved until 34–36 weeks’ gestation. Therefore, enteral feedings must be provided by gavage. Furthermore, preterm infants frequently have gastroesophageal reflux and an immature gag reflex, which increases the risk of aspiration of feedings.
- Decreased ability to maintain body temperature.
- Pulmonary immaturity–surfactant deficiency, often with structural immaturity in infants of less than 26 weeks’ gestation. Their condition is complicated by the combination of noncompliant lungs and a compliant chest wall.
- Immature control of respiration, leading to apnea and bradycardia.
- Persistent patency of the ductus arteriosus, leading to further compromise of pulmonary gas exchange because of overperfusion of the lungs.
- Immature cerebral vasculature, predisposing the infant to subependymal or intraventricular hemorrhage and periventricular leukomalacia.
- Impaired substrate absorption by the gastrointestinal tract, compromising nutritional management.
- Immature renal function (including both filtration and tubular functions), complicating fluid and electrolyte management.
- Increased susceptibility to infection.
- Immaturity of metabolic processes, predisposing to hypoglycemia and hypocalcemia.
Delivery Room Care
See Perinatal Resuscitation section, above.
Care in the Nursery
A. Thermoregulation: Maintaining a stable body temperature is a function of heat production and conservation balanced against heat loss. Heat production in response to cold stress can occur through voluntary muscle activity, involuntary muscle activity (shivering), and thermogenesis not caused by shivering. Newborns produce heat mainly through the last of these three mechanisms. This metabolic heat production depends on the quantity of brown fat present, which is very limited in the preterm infant. Heat loss to the environment can occur through the following mechanisms: (1) radiation—transfer of heat from a warmer to a cooler object not in contact; (2) convection—transfer of heat to the surrounding gaseous environment, influenced by air movement and temperature; (3) conduction—transfer of heat to a cooler object in contact; and (4) evaporation—cooling secondary to water loss through the skin. Heat loss in the preterm newborn is accelerated because of a high ratio of surface area to body mass, reduced insulation of subcutaneous tissue, and water loss through the immature skin.
The thermal environment of the preterm neonate must be regulated carefully. The infant can be kept warm in an isolette, in which the air is heated and convective heat loss is minimized. Alternatively, the infant can be kept warm on an open bed with a radiant heat source. Although evaporative and convective heat losses are greater when the radiant warmer is used, this system allows easy access to a critically ill neonate. Ideally, the infant should be kept in a neutral thermal environment (Figure 1–5). The neutral thermal environment allows the infant to maintain a stable core body temperature with a minimum of metabolic heat production through oxygen consumption. The neutral thermal environment for a given infant depends on size, gestational age, and postnatal age. The neutral thermal environment (for either isolette or radiant warmer care) can be obtained by maintaining an abdominal skin temperature of 36.5°C. Generally, when infants reach 1700–1800 g, they can maintain temperature while bundled in a bassinet.
B. Monitoring the High-Risk Infant: Care of the high-risk preterm infant requires sophisticated monitoring techniques. At a minimum, equipment to monitor heart rate, respirations, and blood pressure should be available. Oxygen saturation can be assessed continuously using pulse oximetry. This determination can be correlated with arterial oxygen tension (PaO2) as needed. Transcutaneous PO2 and PCO2 can also be used to assess oxygenation and ventilation. Finally, arterial blood gases, electrolytes, glucose, calcium, bilirubin, and other chemistries must be measured on small volumes of blood. Early in the care of a sick preterm infant, the most efficient way to sample blood for tests as well as to provide fluids and monitor blood pressure is through an umbilical arterial line. Once the infant is stable and the need for frequent blood samples is reduced (usually 4–7 days), the umbilical line should be removed. All indwelling lines are associated with morbidity from thrombosis or embolism, infection, and bleeding.
C. Fluid and Electrolyte Therapy: Fluid requirements in preterm infants are a function of (1) insensible losses (skin and respiratory tract), (2) urine output, (3) stool output (< 5% of total), and (4) others, such as nasogastric losses. In most circumstances, the fluid requirement is determined largely by insensible losses plus urine losses. The major contribution to insensible water loss is evaporative skin loss. The rate of water loss is a function of gestational age (body weight), environment (losses are greater under a radiant warmer than in an isolette), and the use of phototherapy. Respiratory losses are minimal when infants are breathing humidified oxygen. The renal contribution to water requirement is influenced by the decreased ability of the preterm neonate to concentrate the urine and conserve water.
Electrolyte requirements are minimal for the first 24–48 hours until there is excretion in the urine. Basal requirements are as follows: sodium, 3 mEq/ kg/d; potassium, 2 mEq/kg/d; chloride, 2–3 mEq/ kg/d; and bicarbonate, 2–3 mEq/kg/d.
In the infant born before 30 weeks’ gestation , sodium and bicarbonate losses in the urine are frequently elevated, increasing the infant’s requirement for these electrolytes.
Initial fluid management after birth is determined by the infant’s size. Infants weighing more than 1500 g should start at 80–100 mL/kg/d of 10% dextrose in water (D10W), whereas those weighing less should start at 100–120 mL/kg/d of either D10W or 5% dextrose in water (D5W) (infants < 800 g and < 26 weeks’ gestation often become hyperglycemic on D10W). The most critical issue in fluid management is monitoring. Measurements of body weight, urine output, fluid and electrolyte intake, serum and urine electrolytes, and glucose allow fairly precise determinations of the infant’s water, glucose, and electrolyte needs. Parenteral nutrition should be started early and continued until an adequate enteral intake is achieved.
D. Nutritional Support: The average caloric requirement for the growing premature infant is 120 kcal/kg/d. Expected weight gain for the adequately nourished preterm infant is 10–30 g/d.
Infants initially require intravenous glucose infusions to maintain blood glucose concentration in the range of 60–100 mg/dL. Infusions of 5–7 mg/kg/min (approximately 80–100 mL/kg/d of a 10% dextrose solution) are usually needed. Nutritional support in the very low-birth-weight infant generally is started at 24–48 hours of age with parenteral alimentation solutions given either peripherally or centrally via an umbilical vein line or Silastic catheter (Table 1–19). Small-volume trophic feeds with breast milk or 20 kcal/oz premature formula should be started by gavage at 10–25% of the infant’s nutritional needs as soon as possible and slowly advanced to full caloric needs over 3–7 days once the infant is stable. Intermittent bolus feedings are preferred because these appear to stimulate the release of gut-related hormones and may accelerate maturation of the gastrointestinal tract. The more rapid advancement schedule is used for infants weighing over 1500 g and the slowest schedule in the infant weighing less than 1000 g. Note:{ebd} In the extremely low-birth-weight infant (< 1000 g) or the postsurgical neonate, continuous-drip feeds are sometimes better-tolerated.
In general, long-term nutritional support for infants of very low birth weight consists either of breast milk supplemented to increase protein, caloric density, and mineral content or of infant formulas modified for preterm infants. In all of these formulas, protein concentrations (approximately 2 g/dL) and caloric concentrations (approximately 24 kcal/oz) are relatively high. In addition, premature formulas contain some of the fat as medium-chain triglycerides—which do not require bile for emulsification—as an energy source. Increased amounts of calcium and phosphorus are provided to enhance bone mineralization. The infant should be advanced gradually to feedings of higher caloric density after the full volume of either breast milk or formula (20 kcal/oz) is tolerated. Success of feedings is assessed by passage of feeds out of the stomach, abdominal examination free of distention, and normal stool pattern.
When the preterm infant approaches term, the nutritional source for the bottle-fed infant can be changed to a transitional formula (22 kcal/oz) until age 6–9 months. Iron supplementation (2–4 mg/kg/d) is recommended for premature infants, beginning at about age 2 months. This can be provided by iron-supplemented formulas. In some infants, iron supplementation may be indicated earlier. In particular, infants treated with erythropoietin (epoetin alfa) for prevention of anemia of prematurity require supplemental iron at a dosage of 4–8 mg/kg/d.
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