2.1 pressurised metered-dose inhalers pMDIs

These are the oldest form of modern inhaler devices,

having been introduced in the 1950s

pMDIs are still the most widely used of the inhalation devices

A common example is the Salbutamol inhaler. 

pMDIs are considered to be portable and user-friendly, consisting of three major components: 

1. The canister

2. The metering valve

3. The actuator/ mouth piece

1. canister

the canister is where the formulation resides

the canister is produced in aluminium/ stainless steel due to the inertness of these materials

with optional coatings like epoxy resin- designed to enhance compatibility of the cannister with sensitive formulations

2. the metering valve

allows a metered (measured) quantity of the formulation to be dispensed with each actuation.

3. The actuator/ mouthpiece

enables the patient to operate the device and directs the aerosol into the patient's lungs.

Formulations within pMDIs consist of

a fine powder which is dissolved/ suspended in a liquid propellant

along with surfactants + other excipients

Upon activation,

the metering valve releases a predetermined dose of the formulation as a fine spray

The spray results from volume expansion within the valve,

where the high-speed gas flow breaks up the liquid into small droplets.

what are propellants?

a gas, such as a fluorocarbon, that is used in aerosols to force the contents out of the container

Propellants in pMDIs

single largest contributor as a medicine to the NHS’s carbon emissions.

Propellant systems maintain a constant pressure

ensuring consistent spray characteristics

Historically, chlorofluorocarbons (CFCs) were used as propellants,

but they were phased out under the Montreal Protocol due to their ozone-depleting properties.

Hydrofluoroalkanes (HFAs) are now widely used,

offering similar efficacy but requiring reformulation due to differences in solubility and volatility.

These propellants are powerful greenhouse gases

and are thousands of times more powerful than CO₂

CO₂ has a global warming potential, GWP of 1

• the 2 most common propellants:

  • norflurane (HFA 134a) → GWP = 1300

  • apaflurane (HFA 227ea) → GWP = 3350

propellants are safe to the person using them,

but contribute significantly to climate change

pMDIs make up 70% of inhalers prescribed in the UK

consequently causing 3% of the NHS’s overall carbon emissions

The NHS is encouraging healthcare professionals

to give people over the age of 12 the option of changing to a lower-carbon inhaler.

pMDIs rely on accurate coordination between actuation (release into mouth) and inhalation.

without proper inhaler technique this results in sub-optimal dosing and therefore necessitates the use of spaces for patients to achieve the right dose at the hight time

Spacers help address issues like high-velocity droplet deposition in the oropharynx

and improve drug delivery to the lungs through tidal breathing. 

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