Respiratory Changes in Neonates and Children

Transition at Birth

Transition from placental gas exchange to pulmonary gas exchange occurs within 20s after birth:

• Compression of the thorax through the vaginal canal expels foetal lung water

• Elastic recoil, combined with cooling of the skin and mechanical stimulation (which stimulate the respiratory centre), facilitate first breath

• The rapid drop in pulmonary vascular resistance with spontaneous breathing drives the changes in the cardiac circulation

• The first three breaths establish functional residual capacity
  Large changes in intrathoracic pressure in the first three breaths pressure drive alveolar amniotic fluid into the circulation, and establish FRC.

Neonates and Children

• Compliance
  Neonatal chest walls are highly compliant relative to their lungs (due to both a reduced lung compliance and increased chest wall compliance), as compared to adults where lung and chest compliance is equal. Therefore elastic work of breathing is largely determined by the lungs.

• Oxygenation
  • O₂ consumption is ~10ml.kg^-1.min^-1 in neonates, and 6ml.kg^-1.min^-1 in children
  • There is a ~10% shunt after birth which contributes to a greater A-a gradient

• Ventilation
  • Obligate nose breathers
  • Increased CO₂ production due to higher metabolic rate
  • Increased minute ventilation, which is due to increased respiratory rate (25-40 breaths per minute)

• Neurological control of breathing
  Respiratory patterns change following birth, and complete change to adult respiratory patterns may take some weeks. Patterns include:
  • Periodic breathing is a slowly oscillating respiratory rate and V_T
  • Periodic apnoea is intermittent apnoea interspersed with normal breathing.

• Volumes and capacities
  • Closing capacity is increased relative to adults, causing shunt
  • Functional residual capacity is unchanged
  • Tidal volume and dead space are unchanged

• Laryngeal anatomy
  • Large head
  • Large tongue
  • Large, stiff, U-shaped epiglottis
  • Elevated larynx
    Glottis is at C-3C4 (C6 in adults).
  • Upper airway is narrowest at the cricoid ring (rather than the glottis).
  • Trachea is shorter and narrower
    4-5cm long, 6mm diameter in the neonate.

• Small airways
  • Reduced bronchial smooth muscle so bronchospasm is uncommon
  • Bronchioles contribute 50% of airways resistance
    Bronchiolitis much more distressing in neonates and children.

References

1. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.
2. CICM March/May 2013