Airway Heat and Moisture Exchange

• Airway heat and moisture exchange occur under normal conditions.
• Dry gases affect the respiratory tract.
• Knowing when inspired gas should be humidified and warmed.
• Various types of humidifiers work differently.
• It is possible to enhance humidifier performance.
• Humidifier heating and feed systems should be selected and used safely.
• Recognize the indications, contraindications, and hazards associated with humidification during mechanical ventilation.
• Patients receiving humidity therapy should be monitored.
• Common issues with humidification systems can be found and addressed.
• Identify when to use bland aerosol therapy.
• Huge aerosol generators function in a specific way.
• Delivery systems for bland aerosol therapy are available.
• Recognize and address frequent issues with aerosol delivery systems.
• Sputum induction can be performed.
• The best therapy for conditioning a patient's inspired gas can be determined.

Physiologic Control of Heat and Moisture Exchange

• The upper airway, primarily the nose, is essential for heat and moisture exchange.
• The nose warms and moistens air during inhalation, then cools and reclaims water from exhaled gas.
• Body temperature reaches 37°C, with barometric pressure, saturated with water vapor [100% relative humidity at 37°C] under BTPS conditions.
• Achieved by moving inspired gas into the lungs
• The isothermic saturation boundary (ISB) is typically around 5 cm below the carina.
• Temperature and relative humidity decrease during inspiration and increase during exhalation above the ISB.
• Temperature and relative humidity remain constant below the ISB.
• The ISB shifts distally when someone breathes cold, dry air, when the airway is bypassed, or when minute ventilation increases more than normal.
• Shifts in the ISB can impair the body's natural heat and exchange processes.
• Humidity therapy may be utilized.

Relative Humidity

• The ratio between the amount of water in a given volume of gas and the maximum amount it can hold at that temperature.
• It is expressed as a percentage and measured using a hygrometer.
• Relative humidity = Content/Capacity × 100%

Absolute Humidity

• The amount of water in a given volume of gas, measured in mg/L.
• Saturated air at 20°C can hold 17.3 mg/L of water vapor.
• If the absolute humidity is 12 mg/L, the RH = 12 mg/L/17.3 mg/L x 100% = 69%
• Increasing capacity without changing content heats a saturated gas.
• Cooling a gas reduces capacity.

Body Humidity

• Relative humidity at body temperature, expressed as a percentage.
• The capacity of water at body temperature is 44 mg/L.
• Body humidity is calculated as absolute humidity/44 mg/L × 100.

Humidity Deficit

• When inspired air is not fully saturated at body temperature.
• The body's humidification system corrects the deficit.
• Humidity deficit is calculated as 44 mg/L minus absolute humidity.

Indications for Humidification

• Maintain normal physiological conditions in the lower airways.
• Flows greater than 4 LPM cause immediate heat and water loss, prolonged exposure can cause structural damage to the epithelium
• Potential problems from breathing cold dry air include reduced ciliary motility, more sensitive airways, increased mucus production, and thickened pulmonary secretions.
• Primary indications include humidifying dry medical gases and overcoming humidity deficits when the upper airway is bypassed.
• Secondary indications include treating bronchospasm caused by cold air.

Clinical signs of inadequate airway humidification

• Atelectasis is one such sign.
• Dry, nonproductive cough.
• Increased airway resistance.
• Increased rate of infection.
• Increased effort of breathing.
• Patient complaints of substernal pain and airway dryness.
• Thick, dehydrated secretions.

What is a Humidifier

• A device that adds molecular water to gas through evaporation of water.

Physical Principles Governing Humidifier Function

• Four variables affect the quality of humidifier performance: temperature, surface area, contact time, and thermal mass.
• The higher the temperature of the gas or ambient air, the more water vapor it can hold.
• The greater the area of contact between water and gas, the more possibility there is for evaporation to occur.
• Contact time refers to the amount of time air is in contact with water allowing evaporation to occur.
• Bubble humidifiers utilize contact time based on the depth of the water column.
• The deeper the column, the greater the time of contact as the bubbles rise to the surface
• Gas flow rate through Passover and wick-type humidifiers affects contact time inversely.
• High flow rates reduce available evaporation time.
• Thermal mass refers to a material's ability (water, in this example) to absorb and store heat energy.
• The greater the quantity of water in the humidifier, the greater its thermal mass.
• Greater thermal mass enhances the capacity to hold and transfer heat and moisture to therapeutic gas.