Section 3

Humidity

• Water in a gaseous state behaves like gas molecules, exerting pressure and moving randomly.
• When calculating the partial pressure of a gas with humidity, the partial pressure of water vapor must be considered due to Dalton's Law of Partial Pressures.

Aerosols

• Particulate matter suspended in a gas.

Measuring Humidity

• Water content or absolute humidity: Measured in grams or milligrams of water.
• Relative humidity: Expressed as a percentage of water capacity.
  • Formula: Actual Humidity / Capacity
• Evaporation: Water transitioning from a liquid to a gaseous state.
  • Molecules escape the liquid and become gas, reducing the overall energy level and temperature.
  • Higher temperature leads to more energy and more molecules escaping.
• Humidity deficit: The difference between the inspired gas content and the content of gas at body temperature and pressure saturated (BTPS) conditions (37°C, 43.9 mg/L, 47 mmHg).
  • Clinically significant if large and sustained, leading to retained secretions, airway plugging, obstruction, and infection.

Humidity Therapy Equipment

• Humidifier: Adds water vapor to a gas.
• Factors influencing humidifier efficiency:
  • Temperature: warmer liquid results in more molecules entering the gaseous state and a higher water-holding capacity.
  • Surface area: larger surface area leads to more humidity.
  • Time for contact: longer contact time results in higher humidity.
  • Important note: Gas flow cannot be slowed if the patient requires it!

Clinical Application of Humidity Therapy

• According to the AARC Clinical Practice Guideline for Humidification during Mechanical Ventilation:
  • The humidifier should provide a minimum of 30 mg H2O/L of delivered gas at 30°C.
  • When using a heated humidifier, monitoring should include temperature probes at the humidifier and airway to monitor heat output.
    • Condensation on temperature probes can cause overheating.

Humidifiers

• Room humidifier: An aerosol generator with a high potential for infection.
  • Moves large volumes of air, and the large water reservoir provides a good environment for bacterial growth.
  • Must be kept clean!
• Bubble humidifier: Delivers 33-40% relative humidity at 37°C.
  • Efficiency is influenced by flow rate (high flows are better), ambient temperature, and duration of operation.
  • Built-in pop-off valve at 2 psi, sometimes with a "whistle alarm".
  • Diffuser breaks gas into small bubbles, increasing surface area.
• Pass-over humidifier: Gas flow passes over a water reservoir or water-saturated material, allowing water to evaporate into the gas.
  • Higher temperatures increase water-holding capacity.
  • Types include:
    • Simple reservoir model
    • Wick unit
    • Membrane device

Types of Pass-Over Humidifiers

• Simple reservoir model: Gas flows over the surface of a volume of water.
  • Typically used as a heated system for mechanically ventilated patients.
• Wick humidifier: Gas passes over or through water-saturated material.
  • Usually a heated system used with mechanical ventilation.
• Membrane humidifier: Water is allowed to pass through a membrane, leaving behind contaminants.
  • Example: Vapotherm

Condensation

• Gas cools as it leaves the humidifier, leading to a lower carrying capacity because of decreased temperature.
• Condensate collects in tubing.
• Factors affecting condensate formation:
  • Temperature difference across the system.
  • Ambient temperature.
  • Gas flow.
  • Set airway temperature.
• Minimizing condensation can be achieved through:
  • Placing water traps at the lowest point in the circuit.
  • Using a heated-wire circuit.
  • Drainage bag.

Heated Circuit with Humidifier

• Heated circuits aim to maintain temperature and minimize condensation (also known as "rainout").
• Servo heater: Digitally controlled using feedback from sensors.

Heat & Moisture Exchangers (Artificial Nose)

• Absorbs heat and moisture from exhaled gas and returns it to inhaled gas.
  • Provides 70-90% relative humidity at 30-31°C.
• Performance is significantly affected by mucus.
• Short-term usage is recommended.

Humidity and Aerosol Therapy

• Humidity is water in a gaseous state.
• It behaves like a gas molecule, exerting pressure and moving randomly.
• When calculating the partial pressure of a gas containing humidity, account for the partial pressure of water vapor.

Aerosols

• Particulate matter suspended in a gas

Measurement of Humidity

• Water Content or Absolute Humidity: Measured in grams or milligrams of water.
• Relative Humidity: Expressed as a percentage of water capacity. Formula: Actual Humidity/Capacity.
• Evaporation: Water transitioning from liquid to gaseous state. Higher temperature increases evaporation.
• Humidity Deficit: The difference between the inspired gas content and the content of gas at BTPS (37°C, 43.9 mg/L, 47 mmHg). Significant if large and sustained, leading to retained secretions, airway plugging, obstruction, and infection.

Humidity Therapy Equipment

• Humidifier: Adds water vapor to a gas
• Factors influencing humidifier efficiency:
  • Temperature: Warmer liquid increases evaporation.
  • Surface Area: Greater surface area increases humidity.
  • Time for contact: Longer contact increases humidity.
• AARC clinical practice guideline for humidification during mechanical ventilation: Provides a minimum of 30 mg H2O/L of delivered gas at 30°C.
• Heated Humidifier Monitoring: Monitor temperature at the humidifier and airway using probes to avoid overheating.

Common Types of Humidifiers

• Room Humidifier: Functions as an aerosol generator, can lead to infection due to large air movement and potential bacterial growth in the reservoir. Requires regular cleaning.
• Bubble Humidifier: Delivers 33-40% relative humidity at 37°C, best for low flow, ambient temperature, and short-term use. Equipped with a pop-off valve at 2 psi for safety. Diffuser breaks gas into bubbles, increasing surface area for humidification.
• Pass-Over Humidifier: Gas flows over water reservoir or saturated material facilitating water evaporation into the gas.
  • Types:
    • Simple reservoir: Gas flows over water surface.
    • Wick: Gas flows over or through water-saturated material.
    • Membrane: Water passes through a membrane, filtering contaminants.

Condensation

• Gas cools as it leaves the humidifier, decreasing its carrying capacity and leading to condensate collection in tubing.
• Contributes to condensation: Temperature difference between system, environmental temperature, gas flow, and airway temperature.
• Minimizing condensate:
  • Place water traps at the lowest point in the circuit.
  • Use a heated-wire circuit.
  • Drainage bag.

Heated Circuits with Humidifiers

• Maintains temperature to minimize condensation ("rainout").
• Uses a servo heater, digitally controlled by feedback from sensors.

Heat and Moisture Exchangers (HME)

• Absorbs heat and moisture from exhaled gas and returns it to inhaled gas.
• Produces 70-90% relative humidity at 30-31°C.
• Moisture exchange is significantly reduced by mucous.
• Suitable only for short-term usage.