Key Takeaways
- The nose normally warms inspired gas to 37C and humidifies to 44 mg/L absolute humidity (100% relative humidity at body temperature)
- When the upper airway is bypassed (ETT, tracheostomy), an external humidification system MUST be used
- Heated humidifiers (passover, wick, cascade) deliver 33-44 mg/L absolute humidity and are preferred for intubated patients
- Heat and moisture exchangers (HMEs) are passive devices that capture exhaled heat and moisture and return it on inspiration
- HMEs are contraindicated in patients with thick secretions, hypothermia, large air leaks, or high minute ventilation (>10 L/min)
- Bland aerosol therapy (sterile water or normal saline) is used for patients with bypassed upper airways (tracheostomy collar)
- Aerosol drug delivery systems include small volume nebulizers (SVN), metered-dose inhalers (MDI), and dry powder inhalers (DPI)
- MDIs require a spacer/valved holding chamber for optimal drug delivery, especially in patients with poor coordination
Humidification & Aerosol Therapy
Adequate humidification of inspired gases is essential for maintaining mucociliary function, preventing airway damage, and mobilizing secretions. When the natural humidification system (nose and upper airway) is bypassed or overwhelmed, the respiratory therapist must provide external humidification. Aerosol therapy delivers medication particles to the airways for treatment of bronchospasm, inflammation, and secretion clearance.
The Need for Humidification
The nose and upper airway normally condition inspired gas to 37 degrees C and 44 mg/L absolute humidity (100% relative humidity at body temperature) by the time it reaches the carina. This is called the isothermic saturation boundary (ISB).
Humidification is REQUIRED when:
- The upper airway is bypassed (endotracheal tube, tracheostomy)
- Oxygen or medical gas is delivered at flows >4 LPM by nasal cannula
- The patient has thick, tenacious secretions
- Prolonged mechanical ventilation is in use
Consequences of inadequate humidification:
- Mucociliary dysfunction (cilia stop beating)
- Thickening and inspissation of secretions
- Mucus plugging and airway obstruction
- Airway mucosal damage and inflammation
- Increased risk of ventilator-associated pneumonia (VAP)
- Hypothermia from inspired cold, dry gas
Active Humidification Systems (Heated Humidifiers)
| Type | Mechanism | Output | Advantages |
|---|---|---|---|
| Passover humidifier | Gas passes over the surface of heated water | 30-40 mg/L | Simple design, low resistance |
| Wick humidifier | Gas passes through a water-saturated wick | 33-44 mg/L | Efficient, no risk of water aspiration |
| Cascade (bubble) humidifier | Gas bubbles through heated water | 33-44 mg/L | High output, widely used in critical care |
| Heated wire circuit | Supplemental heating wire in the ventilator circuit | Prevents condensation | Reduces circuit condensation (rain-out) |
Key point: Heated humidifiers with heated wire circuits are the gold standard for mechanically ventilated patients, providing 100% body humidity with minimal condensation.
Passive Humidification (HME — Heat and Moisture Exchanger)
HMEs are also called artificial noses. They capture exhaled heat and moisture on a special material and return it to the patient on the next inspiration.
Advantages of HMEs:
- No external water or power source needed
- Lightweight and portable
- Reduce circuit condensation
- Lower cost than heated humidifiers for short-term use
Contraindications/Limitations of HMEs:
- Thick, copious, or bloody secretions (HME becomes clogged)
- Hypothermia (body temperature <32 degrees C — insufficient exhaled heat)
- Large air leak (cuff leak, bronchopleural fistula — exhaled gas bypasses HME)
- High minute ventilation (>10 L/min — insufficient moisture return)
- Aerosolized medications being delivered through the circuit
Aerosol Drug Delivery Systems
| Device | Mechanism | Particle Size | Advantages | Disadvantages |
|---|---|---|---|---|
| SVN (Small Volume Nebulizer) | Compressed gas nebulizes liquid medication | 1-5 mcm (respirable) | No coordination required; can deliver multiple drugs | Longer treatment time; requires power source |
| MDI (Metered-Dose Inhaler) | Propellant (HFA) aerosolizes medication | 2-5 mcm | Portable, fast, consistent dosing | Requires coordination; needs spacer for best results |
| DPI (Dry Powder Inhaler) | Patient's inspiratory effort disperses powder | 1-5 mcm | Breath-actuated, no propellant, portable | Requires adequate inspiratory flow (>30 L/min) |
| Vibrating mesh nebulizer | Vibrating mesh generates fine aerosol | 1-5 mcm | Very efficient, quiet, faster than SVN | Higher cost; requires cleaning |
MDI + Spacer/Valved Holding Chamber:
- A spacer slows particle velocity and allows propellant to evaporate
- Reduces oropharyngeal drug deposition from ~80% to ~20%
- Increases lung deposition from ~10% to ~20-40%
- Essential for patients with poor hand-breath coordination (children, elderly, acute dyspnea)
A mechanically ventilated patient with thick, tenacious secretions is currently using an HME. The respiratory therapist should recommend:
At what nasal cannula flow rate does the NBRC recommend adding humidification to supplemental oxygen?
Which of the following is an advantage of using an MDI with a spacer compared to an MDI alone?
The isothermic saturation boundary requires inspired gas to be warmed to _____ degrees Celsius and humidified to 44 mg/L absolute humidity.
Type your answer below
Which aerosol delivery device requires the patient to generate an inspiratory flow of at least 30 LPM?
Which of the following are contraindications for using a heat and moisture exchanger (HME)? (Select all that apply)
Select all that apply
A patient with a tracheostomy is breathing room air without mechanical ventilation. What is the MOST appropriate method to provide humidification?