Page 1 of 5

Aerosols

It is important to remember that an aerosol is not the same as humidity. Humidity is water in a gas in molecular form, while an aerosol is liquid or solid particles suspended in a gas. Examples of aerosol particles can be seen everywhere: as pollen, spores, dust, smoke, smog, fog, mists, and viruses.

Aerosols can be created for therapeutic uses by physically shattering or shearing matter or liquid into small particles and dispersing them into a suspension. This can be accomplished by a variety of ways, including using gas jets, spinning disks, or ultra high frequency sound.

The particle size of an aerosol depends on the device used to generate it and the substance being aerosolized. Particles of this nature, between 0.005 and 50 microns, are considered an aerosol. The smaller the particle, the greater the chance it will be deposited in the tracheobronchial tree. Particles between 2 and 5 microns are optimal in size for depositing in the bronchi, trachea and pharynx.

Aerosol therapy is designed to increase the water content delivered to the pulmonary tree, and to deliver drugs to this area. Deposition location is of vital concern, and factors that affect aerosol deposition are aerosol particle size and particle number (see Table below).

Table: Particle size and area of deposition.

Deposition of Particles is also affected by:

Gravity - Large particles are deposited before smaller particles; and gravity affects large particles more than small particles, causing them to rain-out.

Viscosity - The viscosity of the carrier gas plays an important role in deposition. For example, if a gas like helium, which has a low viscosity and molecular weight, is used as a carrier gas, gravity will have more of an effect upon the aerosol. Helium is very light and hence can't carry these particles well, leading to rain-out and early deposition.

Kinetic activity - As aerosolized particles become smaller, they begin to exhibit the properties of a gas, including the phenomenon of "Brownian movement." This random movement of these small (below lmm) particles causes them to collide with each other and the surfaces of the surrounding structures, causing their deposition. As particle size drops below 0.1m, they become more stable with less deposition and are exhaled.

Particle inertia - Affects larger particles which are less likely to follow a course or pattern of flow that is not in a straight line. As the tracheobronchial tree bifurcates, the course of gas flow is constantly changing, causing deposition of these large particles at the bifurcation.

Composition or nature of the aerosol particles - Some particles absorb water, become large and rain-out, while others evaporate, become smaller and are conducted further into the respiratory tree. Hypertonic solutions absorb water from the respiratory tract, become larger and rain-out sooner. Hypotonic solutions tends to lose water through evaporation and are carried deeper into the respiratory tract for deposition. Isotonic solutions (0.9% NaCl) will remain fairly stable in size until they are deposited.

Heating and humidifying - As aerosols enter a warm humidified gas stream, the particle size of these aerosols win increase due to the cooling of the gas in transit to the patient. This occurs because of the warm humidified gas cooling and depositing liquid (humidity) upon the aerosol particles through condensation.

Ventilatory pattern - RCPs easily control this by simple observation and instruction. For maximum deposition, the patient must be instructed to:

• Take a slow, deep breath.

• Inhale through an open mouth (not through the nose).

• At the end of inspiration, use an inspiratory pause, if possible, to provide maximum deposition.

• Follow with a slow, complete exhalation through the mouth.

In many cases, aerosols are superior in terms of efficacy and safety to the same systemically administered drugs used to treat pulmonary disorders. Aerosols deliver a high concentration of the drugs with a minimum of systemic side effects. As a result, aerosol drug delivery has a high therapeutic index; especially since they can be delivered using small, large volume, and metered dose nebulizers.