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Real Impact: Some particles may not reach the target in the airways

Posted Aug 06 2012 6:14am

One of the mechanisms
There are several mechanisms that impact the transport and deposition of aerosols in the lungs. These mechanisms may differ from each other. One of them is the impaction due to inertia. It occurs when aerosol particles travel so fast that they hit whatever surface they find first.

Fast and big
Some inhaler devices may generate speeded aerosols and this needs to be taken into consideration when patients are prescribed some medications. It is important to educate patients on this aspect of therapy so the put some additional effort to make the best of their treatment.

What really happens is that particles with a bigger size and fast flow will try to preserve their trajectory. Even when there are changes in the airflow, the line of movement ahead stays the same. What can be expected is that the particle will hit any wall.

A real change of trajectory
In the airways there is some risk to favor this mechanism of particle deposition. For instance, in the upper airways (nose, mouth, throat) there are natural changes in the path of air. If the aerosol particles of the inhaler device are inhaled too fast or they are big (bigger than 5 μm) they will tend to stay in the upper airway and only few will reach lower airways. As a result of this, the expected effect in the lungs may not occur.

Stokes Number
There is a number o express the probability of impaction due to inertia. That number is called Stokes Number. If the Stokes Number is high, there is increased risk that impaction due to inertia will occur.

In order to avoid upper airway impaction of aerosols due to inertia, it is better to consider particles with lower Stokes number.

In summary, fast inhalations, speeded aerosols with bigger particles might translate into less lung airways deposition and some effects in the upper airways. In another post, other mechanisms will be reviewed.

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