The inhalation of aerosols, their penetration and deposition into the lungs, their absorption and activity is affected by the severity of pulmonary disease. Bronchoconstriction or obstruction of airways will lead to diversion of flow to non-obstructed airways. In advanced disease the remaining airways and alveoli may be increasingly exposed to inspired particles. Disease-induced structural changes, such as the increased resistance to airflow seen in patients with obstructive airway disease, leads to a more central deposition of aerosol. Asymptomatic smokers also tend to deposit aerosol particles centrally, possibly due to airway goblet cell hypertrophy.
Narrowing of airways by mucus, inflammation or bronchial constriction can increase linear velocities of airflow, enhance inertial deposition and cause more deposition in the central airways. In adult respiratory distress syndrome, characterized by acute inflammatory oedema, the lung permeability to proteins increases and accumulation of fluid occurs. Lung deposition from MDI's was not found to be significantly different in asthmatics when compared to normals7 32, however a greater proportion of the dose was located more centrally in asthmatic subjects. This resulted in faster clearance of the drug as penetration into the lung is lessened33.
A large number of other diseases and conditions can lead to altered respiratory flow. These include microbial infections, pneumoconiosis, carcinoma and obstructive pulmonary disease. In the majority of cases these lead to increasing lung rigidity, a decrease in tidal volume and an increase in respiratory rate, together with mechanical obstruction of parts of the airway. It is unfortunate that all these factors reduce deposition in exactly the region in which treatment is needed, so that the dose may target only the healthy regions of the lung.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.