Humidification

Define humidity and give an outline of the importance of humidification

Humidification describes the amount of water vapour present in air:

  • Absolute Humidity is the amount of water vapour in a given volume of air (g.m-3)
  • Relative Humidity is the ratio between the amount of water vapour in a sample of air (absolute humidity) and the amount of water required to fully saturate that sample at its current pressure and temperature
  • Moisture is the water produced by condensation when relative humidity exceeds 100%.
  • Humidification of inspired air is important to avoid drying out mucosa and sputum, which leads to tissue damage and failure of the mucociliary elevator
  • Optimal function requires a relative humidity of greater than 75%

Mechanism

The nose is:

  • Optimised for humidification
    The septum and turbinates increase contact of air with mucosal surfaces by:
    • Increasing surface area
    • Generating turbulent flow
  • The preferred orifice for breathing unless airways resistance becomes a significantly limiting factor
    This is relevant in:
    • Airway obstruction (e.g. polyps)
    • At high minute ventilations (> 35L.-1)
  • Humidifies inspired gas to 90%, compared to 60% for the mouth

Method of humidification:

  • Fluid lining the airway acts as a heat and moisture exchanger
  • In inspiration:
    • Relatively dry air is evaporates water from the airway lining
    • Relative humidity is increased to 90% in the nasopharynx and 100% BTPS by the second generation of bronchi
      This gives a water vapour pressure of 44mmHg at BTPS, with an absolute humidity of 44g.m-3.
  • In expiration:
    • Air cools in the upper airway
      As cooler air has a lower saturated vapour pressure, moisture contdenses on the airway.
    • Moisture is reabsorbed
      This reduces potential water losses from the airway from 300ml.day-1 to 150ml.day-1.

References

  1. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.
  2. WeatherFaqs. Absolute and Relative Humidity.
  3. CICM September/November 2012
Last updated 2017-09-17

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