Therapeutic aerosol: principles and practices

Delivering a drug direct to the site of disease has several advantages. In the case of aerosols, it only requires about one-twentieth of the oral dose of the drug to exert its effect, thus resulting in less or minimal systemic side-effects. The onset of action is fast and the efficacy is superior to the oral drug. Because of the anatomy of the airways which are protective against the inhalation of foreign substances, the aerosol particles must be inhaled in an optimal way in order to reach the sites of action which are the peripheral airways. The particle size must be small and the aerosol must be inhaled in a coordinated manner, especially when a pressurised metered dose inhaler is used. Because of the high pressure of the propellants used in the canister, the particles will travel at a rapid speed upon actuating, causing great impaction in the throat. Only a small percentage reaches the peripheral airways and this percentage is even smaller if the coordination between actuation and inhalation is poor. Spacers have been shown to be able to overcome this problem of incoordination and to reduce throat impaction. Alternatively, the breath-actuated dry powder inhaler can be effectively used. The nebuliser, which is another aerosol delivery system, needs proper setting of the flow rate of compressed air and an appropriate volume of solution in order to optimise the drug delivery.