Humidity

Air is capable of holding a certain amount water vapour, and the mass of water vapour held by a certain volume of air is the absolute humidity measured in g/metre^3 or mg/litre.

The amount of water vapour held within a sample of air, expressed as a percentage of the maximum theoretical mass of vapour that sample of air could carry, is the relative humidity.

For example 22 g/m^3 of water vapour in a sample of air that could carry 44 g/m^3 would be 50% relative humidity.

The reason this is important is because as air warms up it is capable of carrying more water vapour, so if you warm fully saturated air up, the absolue humidity remains the same, however the relative humidity will drop, as that same volume of air can now carry more vapour than before.

Equally, if you cool air down, the relative humidity will increase, as it becomes progressive less able to carry so much water vapour. This continues as the temperature drops until the relative humidity reaches 100%, beyond which point any further cooling will result in the vapour condensing back into its liquid phase, as the air cannot hold that much vapour any longer.

The temperature at which this occurs is the dew point. Dew collects on the grass on a cold morning because the air has cooled overnight, and has cooled below the temperature at which the air is able to hold that much water vapour and so the vapour condenses

What is the absolute humidity of air in room air, and in the trachea?

  • 17g/m^3 in room air and temperature
  • 44g/m^3 in the trachea at 37 degrees celsius

How can you measure humidity?

  • Relative
    • Hair hygrometer
    • Wet and dry bulb hygrometer
    • Regnault hygrometer
  • Absolute
    • Transducer
    • Mass spectrometer

How does a hair hygrometer work?

  • The length of a hair increases with humidity, so by fixing a hair at one end, and attaching a calibrated graduated scale to the other end, the hair will automatically cause the display to ‘read’ the current relative humidity
    • This works well between 30% and 90% relative humidity

How does a wet and dry bulb hygrometer work?

  • Two mercury thermometers next to each other, one with a wet bulb, and one with a dry bulb at ambient temperature
  • The wet bulb usually has a wick connected to a water bath
    • Latent heat of vaporisation causes the wet bulb to cool, in proportion to the amount of evaporation
      • The amount of evaporation depends on the humidity of the air, and therefore the difference in temperature displayed on the two thermometers correlates to the humidity, with a bigger difference indicating less humid air (and hence greater evaporation from the wet bulb)
        • The relative humidity can be found by using pre-calibrated reference tables
          • This method can only tell you relative humidity is it doesn’t directly measure water content of the air, it only measures rate of evaporation, which tells you how much more vapour the air can currently hold

How does a Regnault’s hygrometer work?

  • Air is blown through a silver tube containing ether with a thermometer in it
    • As the ether evaporates, it cools the silver plate
      • When it reaches the dew point, condensation will form on the silver plate, telling you that the air is now fully saturated with water (100% relative humidity)
        • The thermometer will tell you the temperature at which this occurs
          • Saturated vapour pressure tables can then tell you the relative humidity

How can you measure absolute humidity?

  • Mass spectroscopy
    • accurate and quick
    • bulky and expensive
  • Humidity transduction
    • Change in electrical conduction at different humidities can allow calculation of the absolute humidity

What is the interrelationship between humidity and pressure?

  • Water vapour has less atomic mass than nitrogen, so increasing humidity reduces the overall density of the gas mixture and therefore reduces pressure it generates

What is an appropriate level of humidity for the operating theatre and why?

  • 50-60% relative humidity
    • The risk of static electrical shocks increases below 40%, and this humidity also helps prevent excessive drying of the patient’s tissues

Why is humidification important and which patients are most at risk?

Humidification is important to prevent

  • thickening of mucus resulting in airway plugging
  • decreased ciliary activity and ciliary disappearance
  • keratinization and ulceration
  • heat loss (reduces loss of latent heat of vaporization)

The patients most at risk are

  • Prolonged anaesthetic
  • Intensive care
  • Respiratory disease
  • Extremes of age

What methods of humidification do you know?

  • These can be active or passive
  • Active
    • Hot water bath
      • How it works
        • Heated to 60ºC to kill bacteria
          • Fresh gas passed close to hot water and then through tubing that allows gas to cool to safe temperature
            • Can deliver fully saturated gas at 37ºC
      • Disadvantages
        • Bulky and expensive
        • Can damage patient airway if not regulated properly
        • Electric shock risk
        • Water logging can occur, making a water trap is necesary
    • Cascade humidifier
      • How it works
        • Gas bubbled through hot water via perforated plate
          • Gas fully saturated, and more efficient than a hot water bath
      • Disadvantages
    • Nebuliser
      • Uses venturi effect to entrain water along a capillary tube to form droplets that are then carried in the gas flow
        • 2 – 4 micron droplets – deposited in pharynx and upper airways
      • Ultrasonic nebulisers
        • Can deliver 100% humidify hot air and fluid overload the patient, so not commonly used
  • Passive
    • Tracheal saline instillation
    • Bottle humidifier
      • Can produce up to 40% humidity
    • Soda lime
      • Reaction of soda lime with CO2 produces water
      • Can produce up to 70% relative humidity
    • HME filter (heat and moisture exchange)
      • How it works
        • Paper or sponge filter covered by hygroscopic material
          • Warm moist gases are cooled by the filter, causing condensation, and warming of the filter
            • Inspired gas then is warmed by the filter, and humidified by the condensed water
              • This cools and drys the HME, ready for the next exhalation
                • The HME filter can achieve 25 g/m^3 , compared to 34 g/m^3 achieved by the nose and pharynx, meaning it has approximately 65% efficiency
      • Disadvantages
        • Takes 15 minutes to reach working humidity and temperature
        • Relies on minute ventilation and tidal volume to work
        • Affected by temperature and humidity of respiratory gases
        • Only efficient for 24 hours
        • Resistance to gas flow of 0.1 – 2 cmH2O
        • Can become blocked with secretions
        • Increases dead space of circuit
        • Infection risk, particularly pseudomonas