Applied Resp – Humidification

Master answer

Humidity

Humidity

• Humidity is the concentration of water vapour present in the air.

Saturated Vapour Pressure of Water

• The water vapour pressure when the air is fully saturated.
• Depends on both pressure and Temperature
• = 47mmHg at STP

Absolute Humidity

• The amount of water vapour present in a given volume of gas (units g H2O/m3 or mgs H2O /L)
  • Room air at sea level has Absolute humidity of 10g H2O/m3
• < 100% saturation
  • Absolute humidity is temperature independent
• 100% saturation
  • Absolute humidity is temperature dependent – due to ΔSVP fully saturated air
    • at 0 °C contains 4.8 mg/L;
    • at 20 °C contains 17 mg/L;
    • at 37 °C contains 44 mg/L

Relative Humidity

• the amount of water vapour present in the gas expressed as a percentage of the amount of water vapour that would be present if the gas were saturated with water vapour.

Latent Heat of Vaporisation

• the heat required to convert 1g of a substance from the liquid phase to the gaseous phase at a given temperature (expressed in Jg^-1)

Humidification Process

INSPIRED AIR (During nose breathing)

• Air is warmed by the radiant heat from nasal blood supply.
  • ↑ing temperature → ↑ SVP → ↑’s water carrying capacity
• Moisture evaporates from the epithelia → ↑ relative humidity of the inspired air to ~90%
• Mouth breathing reduces the relative humidity of inspired air to 60-70%
• At the lungs, it reaches the isothermic saturation boundary where it achieves BTPS (body temperature and pressure, saturated with water vapour) conditions.
• This usually occurs at the second generation of bronchi.
  • Absolute Humidity @ Carina = 44 g H2O/m3
  • Relative Humidity @ Carina = 100%

EXPIRED AIR

• Expired gas transfers heat back to the cooler trachea and nasal mucosa.
• As the saturated gas cools, it can hold less water vapour (its saturated water vapour pressure falls)
• Condensation occurs on the mucosal surfaces, where the liquid water is reabsorbed.
• Reabsorption reduces potential airway water losses from 300ml/day to ~150ml/day
• Tracheal temperature and humidity fall with an increase in respiratory rate (ie, the isothermic saturation boundary moves more away from the upper airway)

Complications of non-humidified air:

• Mucosal dehydration
• Altered ciliary function
• Inspissation of secretions
• Atelectasis and V/Q mismatching (if underlying lung disease)
• ↑ heat loss (5-10%) as the inspired gases are warmed and more H2O needs to be added as vapour

Humidification Mechanisms in ICU:

• Passive
  • HME (Heat-Moisture Exchanger)
• Active
  • Bubble Humidification
  • Passover
    • Heats water in chamber
    • Evaporated water entrained by fresh gas
  • Nebulisation
    • Pressure and heat vapourises water

Gladwin 2016

Answer by Gladwin