Question 6

Outline how the respiratory system of a neonate differs from that of an adult.

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College Answer

This question required an outline of the anatomical, mechanical and functional differences. It was expected that factors leading to an increased work of breathing and oxygen cost would be mentioned. The mechanics of expiration were not often included in candidates’ answers. Immaturity of the alveoli and peripheral chemoreceptors were common omissions. Inaccuracies regarding upper airway anatomy and compliance of the chest wall cost some candidates marks. The question did not call for an explanation of the relative difficulty of intubation. Discussion of pathophysiology due to airway obstruction, causes of central apnoea or sensitivity to drugs was not required. Many answers included inaccurate information. Points which were often missed were difference in bronchial angles, number of alveoli, number of type 1 fibres in diaphragm, ciliary function and peripheral chemoreceptors.

Discussion

Differences in Respiratory physiology of the Adult and the Neonate

Domain Differences in the neonate In the adult
Airway
  • Small mandible
  • Large tongue
  • Larger tonsils and adenoids
  • Superior laryngeal position
  • Soft, narrow, short trachea
  • Thus, intubation is more difficult
  • Intubation is easier
Airway resistance
  • Respiratory resistance is increased at birth: bronchi arre smaller and lung volumes are smaller
  •  low airway resistance
Lung volumes and spirometry variables
  • FRC is similar to adult
  • VT is similar to adult
  • Minute volume is increased
  • Resp rate is increased
  • ERV is reduced
  • Closing capacity is increased
  • Anatomical dead space is increased (3.0 ml/kg)
  • Normal anatomical dead space is 2.2 ml/kg
Compliance
  • Lung compliance is decreased (less surfactant)
  • Chest wall compliance is increased (cartilaginous ribs)
  • Good lung compliance
  • Low chest wall compliance

Gas exchange

  • Increased shunt (10-25%, due to patent ductus arteriosus)
  • Foetal haemogloin = left shift of the OHDC (poor affinity for 2,3-DPG) 
  • Postatal increase in 2,3-DPG = right shift of the OHDC
  • Oxygen toxicity includes retinopathy
  • High oxygen-carrying capacity of blood because of higher Hb and haematocrit
  • Normally shunt should be minimal
  • Oxygen is relatively nontoxic
Control of respiration
  • Immature respiratory centre, rhythmogenesis and reflex responses:
  • Decreased response to hypercapnia
  • Parradoxuic resposne to hypoxia
  • Periodic apenoeas and cyclical oscillating respiratory rate 
  • Mature reflxes and rhythmogenesis
Respiratory enegetics
  • The total oxygen consumption of the neonate is increased (6-10ml/kg/min)
  • Work of breathing is increased
  • Ideal efficiency is at a resp rate between 30 and 50
  • Diaphragm is more susceptibel to fatigue (fewer Type 1 fibres)
  • Oxygen consumption is 3ml/kg/min
  • Max efficiency at resp rate 12-14

References

Davis, Ryan P., and George B. Mychaliska. "Neonatal pulmonary physiology." Seminars in pediatric surgery. Vol. 22. No. 4. WB Saunders, 2013.

Neumann, Roland P., and Britta S. von Ungern‐Sternberg. "The neonatal lung–physiology and ventilation." Pediatric Anesthesia 24.1 (2014): 10-21.

LoMauro, Antonella, and Andrea Aliverti. "Physiology masterclass: extremes of age: newborn and infancy." Breathe 12.1 (2016): 65-68.