Adaptation to extrauterine life
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Background
At the end of pregnancy, the fetus must take the journey of childbirth to leave the reproductive female mother. Upon its entry to the air-breathing world, the newborn must begin to adjust to life outside the uterus.
Breathing and circulation
Perfusing its body by breathing independently instead of utilizing placental oxygen is the first challenge of a newborn. With the first breaths, there is a fall in pulmonary vascular resistance, and an increase in the surface area available for gas exchange due to the loss of low-resistance placental circulation. Overall, there is an increase in total peripheral resistance (TPR). There is a rapid subsequent increase in oxygenation due to the cardiovascular system changes. The decrease in venous return results in left atrial pressure being slightly higher than right atrial pressure, which closes the ductus arteriosus and foramen ovale fetal circulation shunts, and another increase in pulmonary blood flow from approximately 4% to 100% of cardiac output.
All of these changes result in the adaptation of the cardiovascular system from fetal circulation patterns to an adult circulation pattern. At this point, most congenital heart disease –which was not symptomatic in utero – starts to cause cyanosis or other symptoms. Usually, these problems are caused by cardiac or respiratory problems, though other causes may rarely contribute.
At birth, there is a maturation of the alveoli and capillary networks of the lungs, as well as the deployment of surfactants to keep the alveoli open. At this point, a baby's rhythmic breathing also commences. If there are any problems with breathing, management can include stimulation, bag and mask ventilation, intubation and ventilation. Cardiorespiratory monitoring is essential to keeping track of potential problems. Pharmacological therapy such as caffeine can also be given to increase heart rate. A positive airway pressure should be maintained, and neonatal sepsis must be ruled out.
Potential neonatal respiratory problems include apnea, transient tachypnea of the newborn (TTNB), respiratory distress syndrome (RDS), meconium aspiration syndrome (MAS), airway obstruction, and pneumonia.
Energy metabolism
Energy metabolism in the fetus must be converted from a continuous placental supply of glucose to intermittent feeding. While the fetus is dependent on maternal glucose as the main source of energy, it can use lactate, free-fatty acids, and ketone bodies under some conditions. Plasma glucose is maintained by glycogenolysis.
Glycogen synthesis in the liver and muscle begins in the late second trimester of pregnancy, and storage is completed in the third trimester. Glycogen stores are maximal at term, but even then, the fetus only has enough glycogen available to meet energy needs for 8-10 hours, which can be depleted even more quickly if demand is high. Newborns will then rely on gluconeogenesis for energy, which requires integration, and is normal at 2-4 days of life.
Fat stores are the largest storage source of energy. At 27 weeks gestation, only 1% of a fetus' body weight is fat. At 40 weeks, that number increases to 16%.
Inadequate available glucose substrate can lead to hypoglycemia, fetal growth restriction, preterm delivery, or other problems. Similarly, excess substrate can lead to problems, such as infant of a diabetic mother (IDM), hypothermia or neonatal sepsis.
Anticipating potential problems is the key to managing most neonatal problems of energy metabolism. For example, early feeding in the delivery room or as soon as possible may prevent hypoglycemia. If the blood glucose is still low, then an intravenous (IV) bolus of glucose may be delivered, with continuous infusion if necessary. Rarely, steroids or glucagon may have to be employed.
Temperature regulation
Newborns come from a warm environment to the cold and fluctuating temperatures of this world. They are usually naked, wet, have a larger surface area than in the uterus (on account of not being curled up), with variable amounts of insulation, limited metabolic reserves, and an inability to shiver. As such, it is not surprising that some newborns may have problems regulating their temperature. As early as the 1880s, infant incubators were used to help newborns maintain warmth, with humidified incubators being used as early as the 1930s.
Basic techniques for keeping newborns warm include keeping them dry, wrapping them in blankets, giving them hats and clothing, or increasing the ambient temperature. More advanced techniques include incubators (at 36.5°C), humidity, heat shields, thermal blankets, double-walled incubators, and radiant warmers.