Adaptation to extrauterine life
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.
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 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.
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
womb (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.
