Paediatric differences - Respiratory





Respiratory

Upper airway

  • Larger head
    • Tend to cause neck flexion
    • Body surface area of head
      • 19% at birth
      • 9% at 15 years of age
  • Shorter neck
  • Relatively larger tonge
  • Floor of mouth more easily compressible
  • Smaller face
  • Smaller nostril
  • Possible loose teeth

Larynx

  • Epiglottis ← !!
    • Longer, stiffer, and more U shaped
      • Horseshoe-shaped
    • Projects posteriorly at 45º
  • Larynx ← !!
    • Higher (C2/3)
      • c.f. C5/6 in adults
    • More anterior
  • Cricoid ring is the narrowest part ← !!
    • c.f. Larynx in adult
    • More funnel shaped (i.e. conical)
  • Mucuous membrane ← !!
    • More prone to oedema
    • Pseudo-stratified ciliated epithelium ... loosely bound to areolar tissue

EBM

Review article in Pediatric anaesthesia 2014 by Tobias found that:

  • Airway cross-section is more elliptical than circular
    • AP diameter greater than transverse
    • AP to transverse ratio does not change significantly with age
  • Narrowest portion of the airway was at the level of the vocal cords (or immediately below)

Clinical implication is that:

  • An circular uncuffed tube may have a leak...
    but still exert too much pressure on the lateral walls
  • Cuffed ETTs (esp Microcuff) may be preferable if:
    • Cuff pressure is appropriate
    • Cuff size is appropriate

NB:

Smallest size for Microcuff ETT is 3 mm
→ NOT appropriate for preterm or infants weighing less than 3 kgs

Trachea

  • Short and soft
  • Carina angle more symmetrical

Lungs

  • Relative immature at birth
    • Air-tissue interface surface area relatively small
  • Airway has smaller diameter
    • More likely to obstruct
    • Greater increase in resistance even with minor obstruction
  • Higher closing capacity
    • Increased gas trapping

Thorax

  • Ribs horizontal → Diaphragmatic breathing
  • Less type I fibres in diaphragm → More prone to fatigue
  • More compliant chest wall → Reduced FRC

Physiology

Limited reserves

  • Neonates and infants have limited reserves
  • Desaturate quickly

Apnoea

  • Common postop in premature infants
  • Consider caffeine 10 to 20 mg/kg PO/VI
  • Accompanied by
    • Desaturation
    • Bradycardia
  • Significant if >15 seconds

Work of breathing

  • Greater work of breathing
  • Up to 15% of oxygen consumption
  • c.f. 1% in adults

Mechanics

  • Expansion
    • Most of the impedance to expansion is due to lung
    • Critically dependent on surfactant
    • In adults, lung and chest wall contribute equally to impedance
  • Minute ventilation
    • Rate dependent
    • Tidal volume relatively fixed (6 to 8 mL/kg)
  • Closing volume
    • Closing volume > FRC until 6 to 8 years old
  • During spontaneous ventilation
    • Benefit from CPAP
  • During IPPV
    • Benefit from
      • Higher RR
      • PEEP
  • Dead space
    • High percentage of physiological dead space
      • 30%
    • Increased by anaesthetic equipment

Age-dependent

  • Infants ≤ 6 months old

    • Obligatory nasal breathers

  • 3 to 8 years olds

    • Adenotonsillar hypertrophy