2.4 Humidification & Aerosol Therapy

Humidification & Aerosol Therapy

Conditioning inspired gas and delivering medication to the airways are everyday RT tasks the TMC Examination tests through device selection and contraindication reasoning. Whenever the body's natural humidifier is bypassed or overwhelmed, the RT must supply heat and moisture externally; whenever the airway needs medication, the RT must choose the delivery system that gets respirable particles into the lung.

Why Humidification Matters

The nose and upper airway normally bring inspired gas to 37 degrees C and 44 mg/L absolute humidity (100% relative humidity at body temperature) by the time it reaches the isothermic saturation boundary (ISB), located at or just below the carina. Bypass that conditioning and the consequences cascade.

External humidification is required when:

• The upper airway is bypassed by an endotracheal tube or tracheostomy
• Medical gas is delivered by nasal cannula above ~4 LPM (per AARC guidance)
• Secretions are thick and tenacious, or mechanical ventilation is prolonged

Consequences of inadequate humidification:

• Cilia stop beating (mucociliary dysfunction)
• Secretions thicken and inspissate, forming mucus plugs and atelectasis
• Mucosal injury and inflammation; higher ventilator-associated pneumonia (VAP) risk
• Heat loss from cold, dry inspired gas

Active Humidification (Heated Humidifiers)

A heated humidifier with a heated-wire circuit is the standard of care for invasively ventilated patients, delivering near-body humidity with minimal condensate to drain.

Passive Humidification (HME)

A heat-and-moisture exchanger (HME), or 'artificial nose,' traps exhaled heat and water vapor on a hygroscopic medium and returns them on the next breath. It needs no power or water, is lightweight, cuts circuit condensation, and is economical for short-term ventilation.

HMEs are contraindicated or inappropriate with:

• Thick, copious, or bloody secretions (the HME clogs, raising resistance)
• Hypothermia below ~32 degrees C (too little exhaled heat to recycle)
• A large air leak — cuff leak or bronchopleural fistula — so exhaled gas bypasses the device
• High minute ventilation above ~10 L/min (insufficient moisture returned)
• Aerosolized medication run through the circuit (drug deposits on the HME)

Bland Aerosol for the Bypassed Upper Airway

A spontaneously breathing patient with a tracheostomy still has a bypassed upper airway, so a tracheostomy collar with bland aerosol (heated sterile water or normal saline) supplies the humidity that prevents drying, secretion thickening, and plugging. 'Not on a ventilator' does not mean 'no humidification.'

Aerosol Drug Delivery

MDI with spacer / valved holding chamber: The spacer slows the plume so propellant evaporates before the particles reach the mouth, dropping oropharyngeal deposition from roughly 80% to 20% and raising lung deposition from about 10% to 20-40%. It is essential for children, the elderly, and acutely dyspneic patients who cannot coordinate actuation with inspiration. Because a DPI relies on the patient's own rapid, deep breath (≥30 LPM) to deaggregate the powder, it is the wrong choice for a weak or severely obstructed patient who cannot generate that flow.

Optimizing Aerosol Deposition

Several factors decide how much drug reaches the lung. Particle size is paramount: particles of 1-5 microns deposit in the small airways and alveoli, particles larger than 5 microns impact in the mouth and upper airway, and particles under 1 micron are largely exhaled. Breathing pattern matters too — a slow, deep breath with a 4-10 second breath-hold maximizes deposition, while rapid shallow breathing increases inertial impaction in the upper airway. With a nebulizer, mouthpiece delivery is more efficient than a mask, and the device should be tapped and run until sputtering to clear residual volume.

During mechanical ventilation, deposition improves when the device is placed correctly in the circuit, an in-line spacer is used for an MDI, and bias flow and humidity are accounted for; humidity in the circuit can cut delivery by roughly half, which is one reason an HME must be removed or bypassed before an in-line treatment.

Hazards and Special Considerations

Aerosol therapy carries real risks the RT must monitor. Bronchodilator overuse causes tachycardia, tremor, palpitations, and hypokalemia. Aerosolized drugs can themselves provoke bronchospasm (paradoxical bronchoconstriction), so the RT auscultates and checks peak flow before and after treatment and stops if the patient worsens. Nebulizers are a documented infection-control risk because contaminated solution or tubing can aerosolize pathogens directly into the lower airway, so sterile technique, single-patient devices, and proper drying between uses are mandatory.

Cold, dense aerosol can trigger reactive airway responses in sensitive patients. Finally, bland aerosol of hypertonic saline used to induce sputum or thin secretions can cause significant bronchospasm and must be given with a bronchodilator and close monitoring. Matching the right device, particle size, and technique to the individual patient is the recurring judgment the exam rewards.