College Answer

This question was poorly answered by candidates. Basic aspects, which were critical for a good
answer, such as definitions were often inaccurate. Terms such as ‘amount’ or ‘content’ were
commonly used without provision of units, when mass or pressure was required. The importance
of temperature was often not mentioned. Most candidates identified the importance of the upper
airway in humidification but did not describe details of this process and failed to discuss the events
occurring during expiration.
Syllabus: R2c, S2e, B1k,2d
References: Davis, Basic Physics and Measurement in Anaesthesia, pgs 145-6, Nunn’s respiratory
physiology pgs 12, 19, 166-7

Discussion

This question is identical to Question 7(p.2) from the second paper of 2009

a)

• Saturated vapour pressure is the pressure exerted by a vapour in equilibrium with liquid of the same substance. It is influenced by temperature.
  • Increasing temperature increases the saturated vapour pressure
  • Increasing pressure increases the temperature at
  • For water, at room temperature, the saturated vapour pressure is approximately 17.5 mmHg. It increases to 47 mmHg at body temperature.
• Absolute humidity is the mass of water vapour present in a given volume of air.
• Relative humidity is the percentage ratio of the mass of water vapour in a given volume of air to the mass required to saturate that given volume of air at the same temperature.
• Latent heat of vapourisation is the heat required to convert a substance from liquid to vapour at a given temperature. Latent heat of vapourisation decreases as ambient temperature increases and is reduced to zero at the critical temperature of that substance.

b)

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