Obstructive Lung Disease
- Obstructive Lung Diseases are pathologies of the lung characterized by chronic or episodic increases in airflow resistance. The source of increased resistance to airflow is ultimately caused by chronic or transient reductions in the luminal diameter of the airways; however, the source of luminal narrowing is different for each disease.
- Airway resistance cannot be directly measured in a patient easily; however, the ratio of Forced Expiratory Volume (FEV) to Forced Vital Capacity (FVC) provides an excellent proxy variable that tracks changes in airway resistance. In general, obstructive lung diseases are characterized by a reduction in both FEV and FVC. However, when resistance to airflow is increased the FEV decreases more than the FVC, resulting in a lower total FEV-to-FVC ratio. A normal FEV/FVC ratio is approximately 80% and diseases which reduce this ratio below 70% are considered to display an "Obstructive Pattern". This means that less than 70% total vital capacity is able to be expired in the first second of forced expiration.
- As mentioned, while increased airflow resistance is the common theme of all obstructive lung diseases, the physical source of increased resistance is different for each pathology. In asthma airways are episodically narrowed due to secretion of mucous by the airways whereas in emphysema the airways are narrowed during expiration due to reduced elastic traction of the lung parenchyma. In contrast, airway narrowing in chronic bronchitis arises from thickening of the alveolar wall.