12
Adaptation to extrauterine life

The transition from intrauterine to extrauterine life involves a complex sequence of physiologic changes that begin before birth. Remarkably, although infants experience some degree of intermittent hypoxemia during labor, most undergo this transition smoothly and uneventfully. If not, cardiorespiratory depression requires prompt and appropriate resuscitation.

Physiologic changes in fetal–neonatal transition

• Before birth, the lungs are filled with fluid. Oxygen is supplied by the placenta. On reaching the right atrium, some of the oxygenated blood from the placenta flows directly to the left atrium via the patent foramen ovale, bypassing the lungs. This ensures that the most oxygenated blood goes to the heart and brain. In addition, the blood vessels that supply and drain the lungs are constricted (providing high pulmonary vascular resistance), so most blood from the right side of the heart bypasses the lungs and flows through the ductus arteriosus into the lower aorta (Fig. 12.1a).
• Shortly before and during labor, lung liquid production is reduced.
• During descent through the birth canal, the infant’s chest is squeezed and some lung liquid exudes from the trachea.
• Multiple stimuli (thermal, chemical, tactile) initiate breathing. Serum cortisol, ADH (antidiuretic hormone), TSH (thyroid-stimulating hormone) and catecholamines dramatically increase.
• The first gasp is usually within a few seconds of birth. A negative intrathoracic pressure is generated to achieve this. Most lung liquid is absorbed into the bloodstream or lymphatics within the first few minutes of birth.
• Aeration of the lungs is accompanied by increased arterial oxygen tension; the pulmonary artery blood flow increases and the pulmonary vascular resistance falls.
• Contraction of the umbilical arteries restricts access to the low resistance placental circulation. This results in increased peripheral vascular resistance and an increase in systemic blood pressure.
• The fall in pulmonary vascular resistance and the rise in systemic vascular resistance result in near equalization of pressures across the duct and virtual cessation of ductal flow and also closure of the foramen ovale (Fig. 12.1b).

Fig. 12.1 Changes in the circulation at birth. (a) Fetal circulation. (b) Newborn circulation.

Abnormal transition from fetal to extrauterine life

The transition may be altered by a variety of antepartum or intrapartum events, resulting in cardiorespiratory depression, asphyxia or both (Table 12.1).

Table 12.1 Conditions assciated with abnormal neonatal adaptation to extrauterine life.

Fetal	Maternal	Placental
Preterm/post-dates	General anesthetic	Chorioamnionitis
Multiple birth	Maternal drug therapy, e.g. narcotics, magnesium sulfate	Placenta previa
Forceps or vacuum-assisted delivery	Pregnancy-induced hypertension	Placental abruption
Breech or abnormal presentation	Chronic hypertension	Cord prolapse
Shoulder dystocia	Maternal infection
Emergency cesarean section	Maternal diabetes mellitus
Intrauterine growth restriction (IUGR)	Polyhydramnios
Meconium-stained amniotic fluid	Oligohydramnios
Abnormal fetal heart rate trace
Congenital malformations
Anemia, infection

The Apgar score

The Apgar score, named after Virginia Apgar, an anesthesiologist, is used to describe an infant’s condition during the first few minutes of life (Table 12.2). It is assigned at 1 and 5 minutes of life. If the score is still below 7 or the infant is requiring resuscitation, it is continued every 5 minutes until normal or 20 minutes of age. Although often assigned, few babies truly attain a score of 10, because it is uncommon for the baby to be pink all over. The Apgar score is useful as a shorthand record of the newborn infant’s condition after birth.

Table 12.2 Apgar score.

	Apgar score
	0	1	2
Heart rate	Absent	<100 beats/min	>100 beats/min
Respiration	Absent	Slow, irregular	Good, crying
Muscle tone	Limp	Some flexion of extremities	Active motion
Reflex irritability (response to stimulation)	No response	Grimace	Cough, sneeze, cry
Color	Blue or pale	Body pink, blue extremities	Pink

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Questions

Does resuscitation alter how the Apgar score is assigned?

No. The Apgar score is assigned according to the infant’s condition irrespective of whether or not resuscitation is being performed.

Can one determine Apgar scores in preterm infants?

Yes. However, the extremely preterm infant’s maximum score is reduced by poor muscle tone and weaker response to stimulation than term infants.

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Question

What is the long-term significance of a low Apgar score (3 or less)?

An infant with a low Apgar score at 1 minute but responding rapidly to resuscitation has an excellent prognosis.

An infant with a low Apgar score beyond 10 minutes of age in spite of adequate resuscitation is at markedly increased risk of neurologic damage resulting in cerebral palsy; the longer the score remains low, the greater the risk.

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Key point

The Apgar score is not used to determine the need for resuscitation.

Evaluation for resuscitation is made second by second and is based on breathing, heart rate and tone.

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Asphyxia

Sustained, severe asphyxia (Fig. 12.2) in utero or during labor or postnatally results in the infant making increased respiratory effort, followed by a period of apnea (primary apnea). During primary apnea the heart rate falls to about half its normal rate but the blood pressure is initially maintained.

Fig. 12.2 Schematic representation of physiologic responses to intrapartum asphyxia and neonatal resuscitation.

(Adapted from Resuscitation Council UK Newborn Life Support.)

With continuing asphyxia, the infant starts to gasp, the heart rate slowly falls, as does the blood pressure. After several minutes, after a last gasp, there is secondary apnea. Anaerobic metabolism produces lactic acidosis and cardiac function deteriorates. To recover, positive pressure ventilation, if necessary accompanied by cardiac compressions, is required.