Humidifiers

Humidifiers add water vapour to inspired gas, taking the place of normal body mechanisms which are bypassed or impeded by invasive and non-invasive ventilation. Maintaining adequate humidity of inspired gas is important in:

• Reducing metabolic load
  Humidification of inspired gas accounts for ~15% of basal heat expenditure.
• Maintaining function of the mucociliary elevator
  Inspiration of dry gas increases viscosity of mucous.
• Reducing water loss
  Water will be absorbed from mucosa to humidify gas.

Humidifiers can be classified into active or passive.

Passive Humidifiers

Passive humidifiers:

• Do not require power
• Do not require water

The Heat and Moisture Exchange (HME) filter is the classic passive humidifier:

• Placed between the patient and the patient Y-piece
• Consists of:
  • A moisture exchange layer
    Pleated, hygroscopically coated foam or paper.
    • Expired gas cools as it passes, condensing onto the foam, with condensation promoted by hygroscopic coating (usually this is NaCl)
    • The latent heat of vapourisation results in a decreased temperature of expired gas
  • A filter layer
    Typically a electrostatic or hydrophobic material.
• Expired gas is cooled and dried
• Inspired gas is then heated and humidified
• An HME takes up to 20 minutes to be fully effective, and can achieve a relative humidity up to 70%
• Efficacy depends upon the patient's core temperature and the condition of the airway

Pros

• Cheap
• Lightweight
• Straightforward
• May contain anti-bacterial filter

Cons

• May be blocked with vomit and secretions
• Increase airway resistance
• Increase dead space
• Not as effective as powered active systems
• Only last 24 hours
• Takes 15-20 minutes to become fully effective

Active Humidifiers

Active Humidifiers:

• Require either:
  • Power
    Unpowered humidifiers are typically less effective, and only operate well at lower flow rates.
  • Water
  • (Or both)
• Consist of:
  • A water bath
    Typically sterile water.
  • A heating element
    To heat the water bath.
  • A gas pipe
    Inspired gases are bubbled through the water bath to humidify them.
  • A water trap
    To trap condensed water. Should be changed regularly to minimise infection risk.

Pros

• Greater humidification
• Appropriate for long-term ventilation

Cons

• Bulky
• Expensive
• Require power
• Infection risk from water bath

References

1. McNulty G, Eyre L. Humidification in anaesthesia and critical care. Continuing Education in Anaesthesia Critical Care & Pain, Volume 15, Issue 3, 1 June 2015