Review of Humidity-Aerosol Therapy

Definitions of Esssential Terms Absolute Humidity – Amount of water vapour actually present in the air (expressed in mg/litre). Sometimes called water vapour content. Maximum Absolute Humidity – Amount of water vapour that the air can hold. Sometimes called water vapour capacity. Varies directly with temperature. Relative Humidity – The mathematical comparison of the above two. Content divided by capacity.

Essential Information for Doing Calculations of Relative Humidity The maximum absolute humidity at body temperature is 44 mg/litre This is sometimes called “Body Humidity” Any relative humidity less than body humidity is called the “Humidity Deficit”

Indications for Humidifying and Warming of Gases To humidify dry inspired gases To overcome the humidity deficit when the upper airway is bypassed (trached patients or those with endotracheal tubes) Less common indications Treatment of hypothermia Treatment of bronchospasm caused by cold air

Equipment Used to Humidify Gases Bubble Diffusion Humidifiers

Factors Affecting Performance of Bubble Humidifiers Time of contact between gas and the water Surface area available for evaporation to occur Temperature of the gas

How Flow Lowers Water Temp.

Typical Wick-Type Humidifier

Fisher-Paykel Wick Humidifier

Hudson-RCI “Conchatherm”

Characteristics of Heated Humidifiers Water can be heated to maintain high humidity output Temperature can be sensed “downstream” from the unit so that water temperature can be adjusted to maintain correct gas temperature (like a thermostat at home) Heated wires can be employed to prevent condensation in tubing from gas cooling

Why Wick-Type Humidifiers Are Popular Produce high vapour output even at very high gas flows (>100 l/m) Do not produce any water particles Low risk of producing nosocomial infections Have ability to utilize heated wire circuits Have continuous-feed water systems

Hazards & Problems Associated with Heated Humidifiers Overheating leading to patient airway burns Inadvertent overfilling of humidifier by therapist Increased airway resistance created by pooling of condensate in circuit Burns to caregivers from inadvertently touching heated metal surfaces

Artificial Noses Types of heat and moisture exchangers Simple condensers Hygroscopic Condenser Humidifiers (HCF) Hydrophobic Condenser Humidifiers Act by recycling exhaled heat and moisture “Ideal” unit should be able to produce at least 30 mg/l – Most produce 20 - 30 mg/l

Problems With HMEs Increase in airflow resistance (particularly when mucus enters unit) Drying and thickening of secretions can become a problem Must be removed when administering “in-line” medication aerosol treatments Lose efficiency in patients with high minute volumes (>10 l/m)

Why HMEs Have Become Popular Significant cost savings can be realized compared to a standard ventilator circuit using a heated humidifier Less therapist time needed to drain condensed water from circuit Greater simplicity However, in some cases “heated-wire” circuits may be actually cheaper than the HME

Bland Aerosol Therapy Indications for Tx of laryngotracheobronchitis (croup) Tx of sub-glottic edema Post-extubation edema Postoperative management of upper airway Presence of bypassed upper airway Need for sputum induction

Contraindications to Bland Aerosol Therapy Bronchospasm (Evidence of current disease) Asthma or chronic bronchitis patients who c/o SOB History of airway hyperresponiveness Those with a hx of asthma or other obstructive lung disease

Hazards and Complications of Bland Aerosol Wheezing associated with bronchospasm Infection Overhydration Patient discomfort Caregiver exposure to contagious aerosols Noise

Types of Nebulizers Used for Bland Aerosol Therapy Large-volume jet nebulizers Ultrasonic nebulizers

Schematic of Large-Volume Jet Nebulizer

Schematic of Ultrasonic Nebulizer (USN)

Solutions Used for Bland Aerosol Therapy Water – Most irritating, cheapest to use Normal Saline (0.9% NaCl) – Least irritating; salt crystals can condense on water intake, causing no aerosol to be produced Hypertonic Saline – 5% or 10% – Used only for sputum indutions

Delivery of Inhaled Medications Delivery systems MDIs DPIs Small-volume jet nebulizers Aka – HHNs; SVNs; wet nebs; med nebs; neb meds; acrons USNs Specialized aerosol systems

Definitions Aerosol Penetration Deposition Stability Retention Clearance

Factors for Penetration and Deposition Size of particle decides how far before depositing into the respiratory tract

Indications for Aerosols Deliver medications Humidify gases Mobilize secretions

Hazards Overhumidification Bronchospasm Infection

Examples of MDIs

Characteristics of MDIs Exact same dose each actuation High initial aerosol velocity Currently uses chloroflourocarbon (CFC) propellant. Hydroflouroalkanes (HFA) will replace CFCs pending final approval Without using a “spacer”, up to 80% of aerosol lands in the oropharynx or mouth 10% - 20% reaches small airways Administration is very technique-dependent

MDI “Accessory” Devices Holding chambers or spacers Improve ease of administration Decrease oral-pharyngeal deposition Improve distribution of the mist Flow-triggered MDIs Currently only “Maxair” (pirbuterol) is available

Optimal Technique for Using MDI Shake MDI first (Warm if cold) Actuate into chamber Inhale slowly and deeply Maintain a 10-second breath hold Allow 30 seconds between actuation

Disadvantages of MDIs Coordination can be a problem Requires use of additional spacer High oral-pharyngeal deposition Easy for patients to overuse Some medications can be quite expensive Some patients can run out without realizing

Characteristics of DPIs Always are breath-actuated since no propellant is used Easier to self-administer – No spacer needed Not very many drugs are available in this form At least as good as MDI in terms of deposition and drug response Can’t be used with young children or in ventilator circuits

Brand Names Rotahaler – Albuterol Spinhaler – Intal Turbuhaler – Terbutaline Advair – Flovent and Salmeterol

Optimal Technique for Using DPI Patient must use high inspiratory flows Best not used if patient is having severe SOB Breath-holding is not critical Medication should be stored in a low-humidity environment

Disadvantages of DPIs Some patients can’t generate the high inspiratory flows needed Assembly of unit can be difficult for some Difficult to give high doses Some pharyngeal deposition is unavoidable Not that many medications available in this form (currently)

Small-Volume Nebulizers Generally hold 2 - 6 ml of solution Can be filled using “unit-dose” preparations or multi-dose vials Generally require 4-8 l/m of flow to actuate Can be driven with either oxygen or air When using oxygen, FIO2 can be 40 - 90 %! Should be used instead of MDI or DPI if patient is tachypneic

Disadvantages of SVNs Too complex for some patients to use Requires assembly and periodic cleaning Not easily portable like the MDI or DPI Not all medications are available No steroids currently available for SNV administration

Specialized Medication Nebulizers Respigard II filters exhaled gas – Delivers very small particles (1-2 microns) Circulair – Uses a reservoir bag to conserve medication. Enhances aerosol delivery. Continuous “HEART” Nebulizers – Used for continuous drug administration (1 - 3 hours) SPAG unit – Used for administration of ribavirin

Small-Volume Ultrasonic Nebulizers Produce very dense mists at very high outputs Compact units; portable; easy to use Used for delivery of “undiluted” medications (bronchodilators; antibiotics) Some units can be powered the cigarette lighter adapter present in a car

Problems with USNs Expensive to purchase Prone to breaking down Not all medications are available in multi-dose or unit dose forms Medication must be manually added to unit prior to use