Humidity and Bland Aerosol Therapy

Humidity Therapy

n    Involves adding water vapor and sometimes heat to inspired gas.


Physiological Control of Heat and Moisture Exchange

n      Nose

n     Heat and humidifies inspired gases

n     Cools and reclaims water from exhaled gases

n      Nasal mucosa

n     Very vascular

n     Actively regulates temperature change in the nose

n     Active element in promoting heat transfer

n     Kept moist by secretions from

n    Mucous glands

n    Goblet cells

n    Transudation

n    Condensation from exhaled gases

n      Mucosa lining the sinuses, trachea and bronchi also aid in heating and humidifying gases.


Physiological Control of Heat and Moisture Exchange

n      Mouth less efficient than nose because of comparatively low surface area of the mouth.

n      Inspired gas achieves BTPS 5cm below carina (Isothermic saturation boundary – ISB)

n      Under certain conditions the ISB can shift lower into the lung

n     Upper airway is bypassed

n     Mouth breathing

n     Inhaling dry/cold gas

n     High minute ventilation

n      If shift in position of ISB compromises normal heat and humidification system, humidification therapy is indicated.


Indications for Humidification and Warming of Inspired Gases

n     Primary goal

n    Maintain normal physiological conditions in the lower airways

n     Proper heat and humidity help ensure normal function of mucociliary transport system

n     Airways exposed to cold, dry gas

n    Reduced ciliary motility

n    Irritated

n    Increased mucous production

n    Inspissated secretions


Indications for Humidity Therapy

n    Primary

n   Humidification of dry medical gases

n   Overcoming the humidity deficit created when the upper airway is bypassed

n    Secondary

n   Managing hypothermia

n   Treating bronchospasm caused by cold air




Humidification Equipment

n    Humidifier

n   A device that adds molecular water to gas

n    Humidification

n   Occurs by evaporation from a water surface

n  Reservoir

n  Sphere of water in suspension


Humidifier Function

n    Humidifier performance depends on

n   Temperature

n   Surface area

n   Time in contact


Humidifier Function

n    Temperature

n   Greater the temperature the more water the gas can hold

n   Evaporative cooling makes humidifiers less efficient


Humidifier Function

n    Surface Area

n   The greater the area of contact between water and gas, the more opportunity for evaporation to occur.

n    Techniques humidifiers use to expose gas to water include

n   Passover humidification

n  Wick and membrane

n   Bubble-diffusion

n   Aerosol generation


Humidifier Function

n    Time in contact

n   The longer the gas remains in contact with waster the greater the opportunity for evaporation to occur.

n  Bubble-diffusion contact time depends on the depth of the water

n  Passover-humidifiers contact time is related to the flow of the gas

n  Aerosol suspension has the greatest contact time because the aerosol and gas travel together toward the patient.


Types of Humidifiers

n    Humidifiers

n   Can actively add humidity and/or heat to the delivered gas

n   Can passively recycle heat and humidity from the patients exhaled gases

n    Three types

n   Bubble humidifiers (active)

n   Passover humidifiers (active)

n   Heat moisture exchangers (passive)


Types of Humidifiers

n     Bubble Humidifiers

n    Goal is to raise water vapor content of the gas to ambient levels

n    Diffuses/breaks gas stream into small bubbles to increase gas contact with the water

n    Heaters can be used to increase humidification, but also increases condensation in the tubing delivering gas to the patient

n    Spring loaded pop-off valves alert caregivers of an obstructed tubing


Bubble Humidifier Efficiency


Passover Humidifier

n    Two types

n    Wick

n    Membrane


Wick Humidifiers

n      Wick placed upright in water reservoir and surrounded by a heating element

n      Capillary action draws water up

n      Dry gas enters chamber and flows around wick

n      Water is pick up by dry gas and leaves chamber fully saturated with water vapor.

n      No bubbling required



Membrane Type Humidifiers

n    A hydrophobic membrane separates the gas stream from the water.

n    Water vapor molecules can pass through the membrane, but liquid water cannot.

n    No bubbling required.


Advantages of Passover Humidifiers

n    Passover humidifiers can maintain saturation as high flow rates.

n    Add little or no resistance to spontaneous breathing circuits

n    The do not generate aerosols and therefore pose minimal risk for spreading infection


Heat-Moisture Exchangers  (HMEs)

n     Captures exhaled heat and moisture and uses it to heat and humidify the next inspiration

n     Most do not actively add heat or water to the system

n     Used for providing short term (days) heat and humidification to patients receiving mechanical ventilation.


Heat-Moisture Exchangers (HMEs)

n    Three types

n    Simple condenser

n   Contain a condenser element with thermal conductivity (usually metal).

n   On inspiration air cool the condenser element

n   On expiration, expired water vapor condenses directly on the metal surface and re-warms it.

n   On the next inspiration, cool, dry air is warmed and humidified as it passes over the condenser element.

n   Only 50% efficient

n    Hygroscopic condenser

n    Hydrophobic condenser


Heat-Moisture Exchangers (HMEs)

Hygroscopic Condenser

n     Uses a condenser element of low thermal conductivity (e.g., paper, wool, foam)

n     Impregnates the element with hygroscopic salt

n     Retains more heat than simple condenser systems can and salts help capture more of the exhaled water

n     70% efficient


Heat-Moisture Exchangers (HMEs)

Hygroscopic Condenser

n      Uses a water repellent element with a large surface area and low thermal conductivity.

n      During exhalation the condenser temperature rises because of conduction of latent heat of condensation

n      On inspiration, cool gas and evaporation cools the condenser.

n      This temperature change results in conservation of water to be used to humidify the next breath

n      70% efficient, but also is a bacterial filter


Heat-Moisture Exchangers (HMEs)

n    Characteristics of a good heat-moisture exchanger

n   At least 70% efficient (≥30 mg/L water vapor)

n   Use standard connections

n   Have low compliance

n   Add minimal weight, dead space and flow resistance to the ventilator circuit


Heat-Moisture Exchangers (HMEs)

n    Moisture output falls as minute volume increases

n    Flow resistance increases as water saturates the condenser/filter.

n    Possibly helpful in preventing nosocomial pneumonia


Heating Systems

n    Improve water output of bubble and passover humidifiers

n    Used mainly for patients with artificial airways in place and/or being mechanically ventilated

n     Have a controller that regulates the element’s heating power

n    Simplest system

n   Controller monitors the heating element to maintain a set temperature

n   In this system, the temperature at the patient’s airway has no effect on the controller

n    More sophisticated

n   Servo-controlled heating system

n   Monitors the temperature at or near the patient’s airway.

n   The controller adjusts the temperature of the heating element to maintain a prescribed temperature at the patient airway

n     Both system incorporate alarms and alarm activated heater shutdown


Heating Systems

n    Five types of heating elements

n   Hot-plate, located at the base of the humidifier

n   Wraparound, surrounds the humidifier chamber

n   Collar, sits between the water reservoir and the gas outlet

n   Immersion type, element is placed in the water

n   Heated wire, located in the inspiratory limb of a ventilator circuit

n     Active HME’s

n    Add heat and/or humidity with chemical or electrical means.

n    Works by same mechanism as hygroscopic HME, but adds heat and water to patient side of HME, so that inspired gas should reach BTPS

n    External water is delivered via a pump onto a wick and then evaporated into the inspired air by an electrical heater


Reservoir and Feed Systems

n    Heated humidifiers can evaporate > 1L/day.

n   Refilling of humidifying device can be accomplished by have a large reservoir or a continuous feed system.


Reservoir and Feed Systems

n     Simple reservoirs

n    Requires manual refilling

n    Increases risk of cross contamination, because circuit must be broken in order to fill reservoir

n    Reservoir water levels constantly change resulting in changes in the compression factor and therefore during mechanical ventilation gas volume delivery changes


Reservoir and Feed Systems

n      Automatic feed systems

n    Level-compensated reservoir

n    External reservoir aligned with humidifier maintaining consistent water level between reservoir and humidifier chamber

n    Flotation valve controls

n    Float rises and falls with water level

n    As water level falls below a set value, the float opens a feed valve

n    As water level rises above a set value, the flow closes the feed valve

n    Membrane type

n    Does not require flow control system

n    Liquid water chamber underlying membrane cannot overfill

n    Only requires open gravity feed system for proper function


Setting Humidification Levels

n    American National Standards Institute (ANSI) recommends a minimum of 30 mg/L in intubated patients

n    Little information available for recommendation on optimal humidification

n   Usual target is normal conditions for point at which gas is entering the airway

n  Carina is 35 – 40 mg/L


Problem Solving and Troubleshooting

n    Three common problems arise as the result of heating and humidifying inspired gases

n   Condensation

n   Cross contamination

n   Proper conditioning of inspired gas


Problem Solving and Troubleshooting

n    Condensation

n    Saturated gas cools as it leaves the humidifier to the delivery tubing en route to the patient

n    As the gas cools, its water vapor capacity decreases resulting in condensation/rain out

n   Condensation

n   Wastes large amounts of water
n   Can disrupt or occlude gas flow through the delivery circuit possibly altering FIO2 or ventilator function
n   Can be aspirated by the patient
n   Quickly colonized with bacterial and poses an infection risk to patient and healthcare provider.


Problem Solving and Troubleshooting

n    Condensation

n    Factors influencing volume of condensate formed

n   Temperature difference across the system

n   Ambient temperature

n   Gas flow

n   Set airway temperature

n   Length, diameter, and thermal mass of the breathing circuit


Problem Solving and Troubleshooting

n    Techniques used to reduce volume of condensate within circuit

n    Adding water traps to low point in the circuit to allow collection and removal of condensate

n    Maintain temperature within the circuit

n   Insulate the circuit

n   Heating elements/wires within the circuit


Problem Solving and Troubleshooting

n    Heated-wire circuits is complicated in neonates because of the use of radiant warmers and incubators/isoletes


Problem Solving and Troubleshooting

n    Cross contamination

n    Aerosol and condensate from ventilator circuits are a known source of bacterial colonization

n    Improved circuit and humidifier technology has reduced the risk of cross contamination

n   Wick/membrane type passover humidifiers prevent formation of bacteria carrying aerosols

n   Heated-wire circuits reduce production and pooling of condensate within the circuit

n   High reservoir temperatures are bactericidal

n    Circuit and humidifier technologies have improved to the point that changing circuits has actually become a risk factor for contaminating the circuit.


Problem Solving and Troubleshooting

n    Proper conditioning of the inspired gas

n   Using a hygrometer to measure humidity levels in the inspired gas

n  Difficult because of reaction time of the hygrometer

n   To ensure proper humidity is being delivered to the patient, a small amount of condensate (commonly referred to as vapor trail) should be seen in the artificial airway


Bland Aerosol Therapy

n    Bland aerosols consist of water/liquid particles suspended in gas.

n    The liquid can be:

n   Sterile water

n   Hypotonic / isotonic / hypertonic saline

n    The bland aerosol may or may not be driven by supplemental oxygen.


Equipment for Bland Aerosol Therapy

Large Volume Jet Nebulizers

n      Most common device used to generate aerosols

n     Pneumatically powered

n      Liquid particle aerosols generated by:

n     A high velocity gas passes through a small ‘jet’ orifice

n     Resulting low pressure at jet draws fluid from reservoir to the top of the siphon tube

n     Water is sheared off and shattered into liquid particles

n     Large, unstable particles fallout of suspension or impact baffle

n     Remaining small particles leave nebulizer through outlet and are carried by gas stream to the patient

n      A variable air entrainment port allows air mixing to increase flow rates and alter FIO2


Equipment for Bland Aerosol Therapy

Jet Nebulizer Function


Equipment for Bland Aerosol Therapy

Jet Nebulizer Function

n    Heat can be added using

n   A hot plate

n   Wrap-around

n   Collar

n   Immersion element

n    Servo control is not available

n    Humidity provided

n   Unheated (26 – 35 mg H2O/L)

n   Heated (33 – 55 mg H2O/L)


Equipment for Bland Aerosol Therapy

Ultrasonic Nebulizers (USNs)

n    Electronically powered device that uses piezoelectric crystal to generate an aerosol

n    Crystal transducer converts radio waves to high frequency mechanical vibrations

n    Vibrations are transmitted to liquid surface, where mechanical energy creates geyser of aerosol particles


Equipment for Bland Aerosol Therapy

Ultrasonic Nebulizers (USNs)

n     Properties of ultrasonic signal determines characteristics of aerosol generated

n    Signal frequency determines aerosol particle size

n   Particle size is inversely proportional to frequency

n   Frequency is set by the manufacturer

n    Signal amplitude affects amount of aerosol generated

n   Aerosol generated is directly proportional to amount of aerosol produced

n   Amplitude is set by the clinician

n     Flow an amplitude interact to determine aerosol density (mg/L) and total water output (mL/min)

n     USNs are rarely used anymore because of

n    Cost

n    Erratic reliability

n    Efficiency of disposable jet nebulizers

Equipment for Bland Aerosol Therapy

Patient-nebulizer Interface

n     Face mask

n     Face tent

n     Trach collar

n     T – piece

n     Enclosures

n    Mist tents

n    Oxyhoods


Sputum Induction

n    Short term application of high density hypertonic saline (3% - 10%) aerosols to the airway to assist mobilizing pulmonary secretions for evacuation and recovery.

n   Works by

n  Increasing volume of surface fluids

n  Stimulates irritant cough reflex

n   Must separate saliva from true sputum

n  Brush teeth?


Problem Solving and Trouble Shooting


Most Common Problems

n      Cross contamination and infection

n    Follow infection control guidelines

n      Environmental safety

n    Follow CDCP standards for airborne precautions

n    Immunocompromised patients

n    TB

n      Inadequate mist production

n    Inadequate flow

n    Siphon tube obstruction

n    Jet orifices misalignment

n    For USN

n    Is gas flowing thought device

n    The amplitude is set above minimum

n      Overhydration

n    Never us USN for continuous therapy

n      Inspissated pulmonary secretions

n    When exposed to water can swell and worsen airway obstruction

n      Bronchospasm

n    Chart review and appropriate assessment

n      Noise


Selecting an Appropriate Therapy

n    Considerations

n   Gas flow requirement

n   Presence or absence of an artificial airway

n   Characters of pulmonary secretions

n   Need for and expected duration of mechanical ventilation

n   Contraindications to using HME