Most normal infants require little pressure to spontaneously open the lungs after birth (5 to 10 cm H2O). With the onset of breathing, pulmonary vascular resistance decreases, partly a result of the mechanics of breathing and partly a result of the elevated arterial oxygen tensions. The increased blood flow to the lungs increases the volume of pulmonary venous blood returning to the left atrium; left atrial pressure now exceeds right atrial pressure, and the foramen ovale closes. As the flow through the pulmonary circulation increases and arterial oxygen tensions rise, the ductus arteriosus begins to constrict. In a term infant, this constriction functionally closes the ductus arteriosus within 1 day after birth. A permanent closure requires thrombosis and fibrosis, a process that may take several weeks. In a premature infant, the ductus arteriosus is less sensitive to the effects of oxygen; if circulating levels of vasodilating prostaglandins are elevated, the ductus arteriosus may remain patent. This patency is a common problem in a premature infant with respiratory distress syndrome.
Ventilation, oxygenation, and normal pH and Pco2 levels immediately reduce pulmonary artery vasoconstriction bycausing smooth muscle relaxation. Remodeling of the medialmuscle hypertrophy begins at birth and continues for thenext 3 months, resulting in a further reduction of pulmonary vascular resistance and a further increase of pulmonaryblood flow. Persistence or aggravation of pulmonary vasoconstriction caused by acidosis, hypoxia, hypercapnia, hypothermia, polycythemia, asphyxia, shunting of blood from thelungs, or pulmonary parenchymal hypoplasia results in persistent pulmonary hypertension of the newborn (PPHN). Failure to replace pulmonary alveolar fluid completely withair can lead to respiratory distress (transient tachypnea of the newborn).
Silver nitrate (1%) instilled into both eyes without being washed out is an indicated effective therapy for the prevention of neonatal gonococcal ophthalmia, which can result in severe panophthalmitis and subsequent blindness. Silver nitrate may produce a chemical conjunctivitis with a mucopurulent discharge and is not effective against C. trachomatis. Many hospitals use erythromycin drops to prevent neonatal gonococcal and chlamydial eye disease.
Bacterial colonization of a newborn may begin in utero if the fetal membranes have been ruptured. Most infants undergo colonization after birth and acquire the bacteria present in the mother’s genitourinary system, such as group B streptococci, staphylococci, E. coli, and clostridia. Antiseptic skin or cord care is routine in most nurseries to prevent the spread of pathologic bacteria from one infant to another and to prevent disease in the individual infant. Staphylococcal bullous impetigo, omphalitis, diarrhea, and systemic disease may result from colonization with virulent S. aureus. Triple antibiotic ointment (polymyxin B, neomycin, and bacitracin) or bacitracin may be applied to the umbilical cord to reduce its colonization with gram-positive bacteria. Epidemics of
S. aureus nursery infections are managed with strict infectious disease control measures (cohorting, hand washing, and monitoring for colonization).
Vitamin K prophylaxis (intramuscular) should be given to all infants to prevent hemorrhagic disease of the newborn. Before discharge, infants should receive the hepatitis B vaccine and be screened for various diseases (Tables 58-4 and 58-5).