There are many designs of inhaler presently available they range from pressurised metered dose inhaler (pMDI, or puffer), through dry powder inhalers, to soft mist inhalers and nebulisers.
For each design or type, the user will be inhaling an aerosol (a mixture of air and the medication), and the speed of inhalation (the inspiratory flow) will influence where the medication contained within will be deposited or absorbed.
Some inhalers (e.g. dry powder inhalers) also depend on the inspiratory flow being sufficiently high enough to create an aerosol. If the airflow through the inhaler is too low, then only a small amount of drug will be exit the inhaler.
Inhaler devices - how they work
Drug delivery from the various types of inhaler devices is produced by different methods.
Inhaler devices are designed to deliver drug particles of a certain size to the small airways during inhalation. Particles of this size (generally agreed to be approximately between 2 and 5 microns) are known as the 'respirable fraction'. The particles are either in aerosol (in a suspension or a solution) or dry powder form.
Metered dose inhalers (MDIs)
The most commonly used inhaler, the MDI consists of a plastic housing and a metal aerosol canister. The canister contains a mix of propellant and medication when pressed, a precise quantity of this mixture is released. As the propellant vaporises almost instantaneously, a cloud of medication is directed out from the inhaler.
With most MDIs, the aerosol is delivered under pressure at high speed (often over 70 miles/100 kilometers per hour). The inhalation should be timed with actuation of the device and should be slow and steady. Inhaling too fast may cause a greater proportion of the aerosol to impact at the back of the throat and subsequently be swallowed, thus reducing the beneficial clinical effect and increasing the potential for local and systemic side effects.
With the majority of MDIs, manual depression of the drug canister actuates the drug delivery. However, with breath-actuated metered dose inhalers (e.g. AutohalerŽ), the aerosol is released by mechanical actuation, triggered by the patient breathing in through the device at any flow rate above a minimum level. At inspiratory flows below this, the patient will not receive any of the medication, because a dose will not be released.
An example of the common pressurised metered dose inhaler (pMDI).
MDIs with Holding Chamber / Spacer
A container that holds the aerosol cloud produced by a pMDI is known as a spacer, or chamber. There are differed types, some contain a valve, some contain a whistle that provides a warning if the inhalation is too fast. They remove the need to coordinate inhaling and pressing the canister, (many people find this difficult), and also helps to reduce the number of large particles in the aerosol cloud (which are too big to reach the parts of the lung that would benefit).
Autohaler is an example of a breath-actuated metered dose inhaler.
Dry powder inhalers (DPIs)
Drug delivery from DPIs is triggered by inhaling through the device. Whilst each design of dry powder is very different, the basic principle remains the same. A metered quantity of powdered medication is drawn into the airflow, and follows a specific route within the inhaler towards the mouth of the patient. The twists and turns within the inhaler create a resistance that is designed to break up the medication into particles of a respirable size.
Some DPIs rely on patients placing a new capsule of drug into the device each time it is used, others have a gravity-fed hopper that allows a certain amount to be released into the airflow, and others have individually packaged doses that are released each time the inhaler is used.
These two devices are both Dry Powder Inhalers.
Different designs of inhalers
Whatever the mechanism of generating the aerosol, the patient must inhale the medication into their lungs. Breathing in through an inhaler requires air to travel through the device before it travels through the mouth, throat and windpipe, finally reaching the lungs.
When breathing in through an inhaler, the patient will encounter a resistance. The resistance is different for each of the various inhaler designs, and is dependent on the design of each inhaler how it generates the aerosol cloud, the formulation of the medication, and how easily air can pass within.
This internal resistance of the inhaler has a significant effect on how fast a patient can inhale through it, which in turn affects the quantity and quality of medication the patient receives, and where in the respiratory tract it is deposited.
