What is a Metered-Dose Inhaler (MDI) in the context of treating status asthmaticus?

What is a Metered-Dose Inhaler (MDI)?

An MDI is a pressurized, portable, multi-dose aerosol delivery device that dispenses a precise amount of medication (typically 90 mcg/puff for albuterol) through a canister and actuator system, designed to deliver bronchodilators and corticosteroids directly to the airways for treating asthma and COPD. 1

Device Components and Mechanism

An MDI consists of four key components that work together to generate the therapeutic aerosol 2:

• Propellant system: Originally chlorofluorocarbons (CFCs), now replaced with hydrofluoroalkane (HFA) propellants that are non-ozone-depleting 1, 2
• Metering valve: Delivers a precise, reproducible dose with each actuation 2
• Drug formulation: Either solution or suspension of the active medication 2
• Actuator with nozzle: Shapes and directs the spray plume toward the patient's airway 2

Clinical Use in Asthma Exacerbations

Dosing for Acute Exacerbations

For short-acting beta-agonists (SABAs) like albuterol via MDI 1:

• Children: 4-8 puffs (90 mcg/puff) every 20 minutes for 3 doses, then every 1-4 hours as needed 1
• Adults: 4-8 puffs every 20 minutes up to 4 hours, then every 1-4 hours as needed 1

Efficacy Compared to Nebulizers

MDI plus valved holding chamber (VHC) is as effective as nebulized therapy for mild-to-moderate exacerbations when used with appropriate technique and trained personnel coaching 1. This equivalence makes MDIs particularly valuable in emergency settings because they are quicker to set up than nebulizers, require no power source, and are easily portable 3.

Critical Requirements for Optimal Use

Spacer/VHC Devices Are Essential

Always use a spacer or valved holding chamber with MDIs, especially for children under 4 years and when delivering inhaled corticosteroids 1, 4. Spacers provide multiple benefits 1:

• Decrease oropharyngeal deposition, reducing local side effects like thrush
• Eliminate the need for precise hand-lung coordination
• Enhance lung delivery by allowing more time for propellant evaporation
• Reduce systemic absorption of medications with high oral bioavailability

Proper Technique Requirements

For children under 4 years, use a face mask that fits snugly over nose and mouth with the spacer device 1, 4. For all patients using inhaled corticosteroids, rinse mouth and spit after each use to prevent oral candidiasis 1, 4.

Spacer Maintenance

Rinse plastic VHCs once monthly with dilute household dishwashing detergent (1:500 dilution or drops per cup of water) and let drip dry 1. This reduces static charge on plastic spacers, which can otherwise trap medication particles and reduce lung delivery 1.

Advantages Over Alternative Delivery Systems

MDIs offer distinct practical advantages 5, 3:

• Instantly available and self-contained: No assembly or power source required 5, 3
• Portable and lightweight: Easily carried by patients 5, 3
• Low cost: Most economical inhaler option available 3
• Multi-dose: Typically contains 60-200 actuations per canister 5
• Rapid deployment in emergencies: Faster to use than nebulizers in acute settings 3

Common Pitfalls to Avoid

Technique Errors

Most patients use MDIs incorrectly, which mimics inadequate dosing 4. The most common errors include 6:

• Failing to shake the canister before use
• Poor hand-lung coordination (actuating at wrong point in breath cycle)
• Inadequate inspiratory flow or breath-holding time
• Not using a spacer when indicated

Medication-Specific Considerations

Never use an unmodified MDI in children unless there is certainty about proper technique—coordination requirements are too demanding 1. For children who cannot coordinate MDI use, nebulizers remain appropriate, but large-volume spacer devices with MDI should be tried first as they are more efficient and less time-consuming than nebulizers 1.

Device Output Variability

Drug output from an MDI depends on both the MDI formulation and the specific spacer/VHC device type; data from one combination should not be extrapolated to all others 1. This means switching spacer brands may alter drug delivery even when using the same MDI 1.

Role in Stepwise Asthma Management

MDIs are used throughout the stepwise approach to asthma treatment 4:

• Step 1: SABA MDI as needed for symptom relief 4
• Steps 2-6: Inhaled corticosteroid MDIs (with or without LABA) for controller therapy, with doses ranging from low (100-250 mcg/day fluticasone equivalent) to high (>500 mcg/day) depending on severity 4

For controller medications, most ICS formulations via MDI should be administered twice daily 4.

Special Populations

Patients Requiring Mechanical Ventilation

Appropriate adapters allow MDIs to be used in patients of all ages requiring assisted ventilation 5. This makes MDIs versatile across the entire spectrum of acute care settings 5.

Very Young Children (0-2 Years)

For infants and toddlers, use MDI with large-volume spacer and properly fitted face mask rather than nebulizer when possible 1. This approach is more efficient and reduces treatment time 1.