1.4 The transition from intra-uterine to extra-uterine life

Question 1

What is the key aspect of transition from intrauterine to extrauterine life?

Answer 1

The newborn must take over oxygenation for survival

Question 2

List pathophysiological features of intrauterine life

Answer 2

• O2 diffuses across placental membrane from mother’s blood to foetus
• Foetal alveoli are expanded but liquid filled
• Minimal blood flow to the foetal lungs (~8%) as lungs are not the source of receiving O2 or removing CO2
• Blood vessels perfusing foetal lungs are constricted
• -> Due to subsequent ^ resistance to flow in constricted vessels of lungs, blood from Rt) side of heart (~92%) takes path of lower resistance across ductus arteriosus into aorta and systemic circulation

Question 3

Why is it normal for a healthy newborn infant to appear cyanotic in first few minutes post birth?

Answer 3

• Foetal Spo2: 50-60%

- Intrapartum Spo2: 40-50%

Question 4

List pathophysiological features of extrauterine life

Answer 4

• Umbilical arteries and umbilical vein constrict and are clamped
• -> ceases placental circulation (thereby ceasing O2) and SVR increases
• The newborn must take first breath which initiates complex series of events which switch GE from placenta to the lungs
• Cardiovascular shunts close over several days resulting in fully formed CV system
• The transition from fetal to extra-uterine life is then complete.

Question 5

What causes the CV shunts to close and how long does it take for this to occur?

Answer 5

• Changes during first breath + biochemical factors controlling ductus smooth muscle constriction cause closure of these shunts
• Complete within ~96 hours

Question 6

What events occur during the newborn taking their first breath to switch GE from placenta to the lungs?

Answer 6

• Hydrostatic pressure created during inspiration causes lung liquid to move out of alveoli and into surrounding lung tissue (takes approx. 5-10 breaths)
• Liquid cleared from tissue via blood vessels and lymphatics (approx. 1-4 hours)
• –> Osmotic gradient induced by Na reabsorption aids liquid movement into the lung tissue
• 8-10x increase in blood flow to lungs with onset of effective ventilation due to large decrease in pulmonary vascular resistance (caused by pulmonary capillary recruitment and relaxation of blood vessels due to lung aeration and ^ blood oxygen content)
• ^ in pulmonary blood flow due to decrease in pulmonary vascular resistance + increase in SVR reverses pressure gradient across ductus arteriosus resulting in shunting of blood from aorta into pulmonary circulation