Chapter 39 – Humidity and Bland Aerosol Therapy

Vocabulary flashcards summarizing essential terms and definitions from Chapter 39 on humidity and bland aerosol therapy to aid exam preparation.

Heat-and-Moisture Exchange (HME) – physiologic

Normal warming and humidifying of inspired air by the upper airway, chiefly the nose, and recovery of heat and water during exhalation.

BTPS Conditions

Body temperature (37 °C), ambient barometric pressure, gas saturated with water vapor (100 % RH at 37 °C).

Isothermic Saturation Boundary (ISB)

Point about 5 cm below the carina where inspired gas reaches BTPS; temperature and humidity stay constant below this level.

Relative Humidity (RH)

Ratio of actual water vapor in a gas to the maximum it could hold at that temperature, expressed as a percentage.

Absolute Humidity (AH)

Actual mass of water vapor present in a given volume of gas, expressed in mg H₂O /L.

Body Humidity

Relative humidity of inspired gas at body temperature; capacity is 44 mg H₂O /L at 37 °C.

Humidity Deficit

Amount of water vapor the body must add to inspired gas to reach saturation at 37 °C; 44 mg/L minus inspired AH.

Primary Indications for Humidification Therapy

(1) Humidify dry medical gases and (2) overcome humidity deficit when the upper airway is bypassed.

Secondary Indication for Humidification

Treating cold-air–induced bronchospasm.

Clinical Signs of Inadequate Humidification

Atelectasis, thick secretions, dry cough, increased RAW and WOB, infection risk, substernal pain, airway dryness.

Humidifier (device)

Equipment that adds molecular (invisible) water vapor to a gas stream.

Thermal Mass

Water volume in a humidifier; more water holds and transfers more heat, supporting stable output.

Bubble Humidifier

Active device that diffuses gas through water, forming bubbles to increase gas-water contact; has a pressure pop-off valve.

Bubble-Diffusion Principle

Breaking a gas stream into small bubbles under water to enlarge surface area and enhance evaporation.

Passover Humidifier

Directs gas over a water surface or wick or across a membrane without bubbling; suited to high flows and ventilators.

Wick Humidifier

Passover type using a porous, absorbent wick that draws water by capillary action, maximizing surface area for evaporation.

Membrane Passover Humidifier

Separates gas and liquid water with a hydrophobic membrane; water vapor crosses, liquid and pathogens do not.

Active Humidifier

Adds heat, water, or both to the gas mechanically or electrically (bubble, passover, heated nebulizer, vaporizer).

Passive Humidifier

Recycles patient-exhaled heat and moisture; typical example is the HME (“artificial nose”).

Simple Condenser HME

Metal gauze or foam core captures latent heat and water during exhalation and releases them on inhalation.

Hygroscopic Condenser HME

Condenser coated with hygroscopic salt to improve water retention and return; lowers RH of exhaled gas to <90 % to reduce clogging.

Hydrophobic Condenser HME

Water-repellent pleated filter that acts as moisture exchanger and bacterial/viral filter.

Humid-Heat

Active HME that electrically heats and adds water to boost performance for patients with moderate-high minute ventilation.

HME Booster

Electric heater placed around an HME to raise its water output for minute volumes 4–20 L /min; not for infants or pediatrics.

Heated Humidifier

Adds external heat (hot plate, wrap, immersion heater, heated wire, etc.) to raise water vapor output, mainly for intubated or ventilated patients.

Reservoir and Feed System

Large water chamber plus manual or automatic (gravity/flotation) refill mechanism to replenish water lost by heated humidifiers (>1 L/day).

Recommended Humidity for Intubated Patients

At least 30 mg H₂O/L; AARC guideline: 33 °C ± 2 °C with ≥30 mg H₂O/L at airway opening.

Condensation (Circuit Rain-Out)

Water that forms in cooled ventilator tubing; can occlude flow, waste water, or be aspirated; managed with water traps/heated circuits.

Cross-Contamination Risk

Standing water in circuits can harbor pathogens; minimizing condensate and using membrane or wick passovers reduces aerosol bacteria spread.

Bland Aerosol

Suspension of liquid water or saline particles in gas (air or O₂) without active drug content.

Indications for Bland Aerosol Therapy

Upper airway edema, bypassed upper airway, need to mobilize secretions or obtain sputum specimens.

Contraindications to Bland Aerosol

Bronchoconstriction or history of airway hyper-responsiveness.

Hazards of Bland Aerosol

Bronchospasm, infection, over-hydration, patient discomfort, caregiver exposure, airway wall edema.

Large-Volume Jet Nebulizer (LVN)

Pneumatically powered device mixing gas and liquid via a jet to generate high-output aerosol; delivers 26–35 mg/L unheated, 35–55 mg/L heated.

Ultrasonic Nebulizer (USN)

Electric device that uses a piezoelectric crystal to vibrate at high frequency, producing dense aerosol; particle size inversely related to frequency.

Aerosol Mask

Adult or pediatric facial interface designed for high-flow aerosol delivery from LVN or USN.

Face Tent

Loose-fitting device covering chin and mouth, useful when mask not tolerated or with facial dressings; connects to aerosol source.

T-Tube (Briggs Adapter)

Connector delivering aerosol to an intubated or trached patient, allowing high flows and access for nebulizer tubing.

Tracheostomy Mask

Small aerosol interface that fits over tracheostomy stoma and connects to large-bore tubing from nebulizer.

Mist Tent / Aerosol Hood

Enclosure around infant/child delivering cool aerosol; requires high fresh-gas flow to avoid CO₂ buildup and heat retention.

Sputum Induction

Diagnostic method using short-term, high-density hypertonic saline aerosol (3–10 %)—often via USN—to provoke productive cough.

Factors Enhancing Humidifier Performance

Higher temperature, greater surface area, longer contact time, and adequate thermal mass all increase water vapor output.