Question 3

Outline the anatomy and physiology of humidification during normal breathing (50% of  marks). Describe the mechanisms of humidification used within Intensive Care practice (50% of marks).

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College Answer

For a good answer candidates were expected to provide information on essential facts such 
as, normal inspired air has low water content, normal humidification provides for saturated 
water vapour 47mmHg at sea level at the alveolus, which correspondsto an absolute 
humidity of 44 g/m3 (or 100% relative humidity) at 37.C. Furthermore, candidates were 
expected to outline the basics of the anatomical features of the respiratory tractthat 
promotes humidification e.g. lining of nose and hypopharynx dissolving warm water vapour 
into the dry inhaled air, the fact that the turbinates act to increase surface area and that full 
saturation is achieved by the time air reaches the upper trachea. For the second part of the 
question candidates were expected to mention, and briefly describe, the mechanisms of 
achieving humidification, e.g. bubble systems, heat-moister exchange filters, heated water, 
ultrasonic, etc.mechanisms. Candidates generally did not have sufficient knowledge of the 
basic conepts or a structured approach to this topic.


a) Anatomy and physiology of humidification during normal breathing:

  • Inspiratory heating and humidification of the gas mixture
    • Inspired gas has a water content of around 10g/kg (50% humidity, 22° C)
    • The change in temperature and humidity occurs during respiration due to the effects of the upper airway structures on the inspired air mixture.
    • Inspired gas passes through the convoluted air passages of the nasopharynx and pharynx which generates turbulence.
    • This turbulence increases evaporative heat exchange between the air and the mucosa; such that at the posterior nasal cavity the relative humidity is already 85%
    • In the lower pharynx, the temperature is about 33° C and relative humidity approaches 100%
    • The inspired air achieves body temperature at the isothermic saturation boundary, around 5cm beyond the carina
    • Alveolar gas has a water content of around 47g/kg (100% humidity, 37° C)
  • Expiratory reclamation of heat and moisture
    • Expired gas passes over the cooler upper airway mucosa, and returns some of its heat to it
    • Expired air at the nares is usually 32° C and close to 100% humidified
    • Some of the water is also reclaimed by the process of condensation
    • This process is highly dependent on the temperature of the ambient air; the cooler the ambient air the more moisture is reclaimed.
    • In hot environments, humidity cannot be reclaimed and the net water loss increases

b) Mechanisms of humidification used within Intensive Care practice

  • The objectives of active humidification is to match the normal humidification capacity of the human respiratory tract
  • This decreases the risk from inadequately humidified gas (eg. inspissation of secretions)
  • The main types of humidifier are bubble, passover, counter-flow and inline vapourisers.
  • The main disadvantages of using an active humidifier are the risk of infection and the potential for water precipitation in the circuit tubing
  • A passive heat and moisture exhanger only produces about 20 g/m3 which is not enough for sustained protection of tracheal mucosa.


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