Humidification

Inspired gas is warmed and humidified by the structures of the upper airway. When these structures are bypassed (i.e. by endotracheal intubation), artificial humidification systems must be used to achieve the target absolute humidity of >30g/m3.

Long-term tracheostomy patients undergo metaplasia of tracheal epithelium, reducing (or removing) the need for humdification systems - although this may be required in an acute respiratory deterioration (e.g. infection).

Physiological Importance

Inadequate humidification of inspired gas leads to:

  • Impaired ciliary function
  • Tracheobronchial epithelial damage
  • ↑ Mucous viscosity
    • Airway obstruction
  • Atelectasis
    Obstruction of small airways.

Humidification Systems

Include:

  • Cold-water humidifiers
  • Hot-water humidifiers
    Thermostatically-controlled heated water resevoir, with gas containing high water content produced.
    • Small risk of infection from aerosolised particles in the water resevoir
    • Effective over a wide range of minute ventilation
      Particularly at high levels.
    • Can be either:
      • Blow-by
        Gas passed over the water pool.
      • Bubble/cascade
        Gas passed into the water pool.
  • Heat-Moisture Exchanger
    Water vapour (and it’s heat content) is captured in a hygroscopic membrane during expiration, and is returned during the subsequent breath during inspiration.
    • May include viral/bacterial filters
    • Can become obstructed with blood/oedema/sputum
    • Add to circuit dead space
      Usually 30-90mL.
    • Less effective than hot-water humidification systems

Complications

  • Inadequate humidification
    Practically only of concern with an HME.
  • Over-humidification
    Risk of airway burns and water intoxication.
  • ↑ Work of Breathing
    • Humidified gas
      ↑ Resistive work of breathing due to ↑ gas viscosity.
    • Device
      HME adds resistance.
  • Infection
    Risk of culture of organisms from water bath.

References