Humidity
What is absolute humidity?
The amount of water contained in a given volume of gas, expressed in mg/L.
What is a bland aerosol?
Liquid particles suspended in a carrier gas (oxygen or air). Solutions used include sterile water and sterile saline — hypotonic, isotonic, or hypertonic.
What is a humidifier?
A device that adds molecular water vapor to gas.
What is a heat-moisture exchanger, and why is it called the "artificial nose"?
A passive humidifier that captures the patient's exhaled heat and moisture and returns up to 70% of it on the next inspiration, mimicking what the nose does.
What is relative humidity?
The percentage of the water vapor a gas is holding compared with the maximum it could hold at that same temperature and pressure.
What are the contraindications to bland aerosol therapy?
Bronchoconstriction and a history of airway hyperresponsiveness.
How does a bubble humidifier work?
It directs a stream of gas underwater and breaks it into small bubbles.
What are the three types of HMEs?
Simple condenser, hygroscopic condenser, and hydrophobic condenser.
What is the primary site of heat and moisture exchange?
The upper airway, mainly the nasal cavity.
What are the indications for bland aerosol therapy?
Laryngotracheobronchitis, upper airway edema, subglottic edema, postextubation edema, and the need for sputum specimens or mobilization of secretions.
What are the four principles that govern humidifier performance?
Temperature, Surface area, Time of contact, Thermal mass
How much mechanical dead space does an HME add?
5 to 90 mL, depending on the device.
What are BTPS conditions?
Body temperature, pressure, and saturation (37° C, ambient pressure, 100% relative humidity, 44 mg/L).
What are the hazards and complications of bland aerosol therapy?
Wheezing or bronchospasm, infection, overhydration, patient discomfort, caregiver exposure to airborne contagions produced during coughing or sputum induction, and edema of the airway wall.
What are the three types of passover humidifiers?
Simple reservoir, wick, and membrane.
When should an HME be replaced?
When it becomes contaminated with secretions, or when flow resistance has increased enough to raise the work of breathing.
What is the isothermic saturation boundary, and where is it normally located?
The point at which inspired gas reaches BTPS conditions. Normally about 5 cm below the carina.
How does a jet nebulizer generate aerosol?
Liquid particles are generated by passing gas at high velocity through a small jet orifice. Low pressure at the jet draws fluid from the reservoir up the siphon tube, where the water is shattered into liquid particles.
What absolute humidity output should you expect from an unheated bubble humidifier?
Unheated bubble humidifiers output 15–20 mg/L.
When is an HME the appropriate choice, and when should one not be used?
HMEs are better suited to short-term use and during transport. A heated humidifier should be used instead for patients requiring long-term mechanical ventilation, greater than 96 hours, or for whom an HME is contraindicated.