Bronchitis is a condition which is clinically defined as a chronic cough with mucus production most months of the year. The mucus secretions and inflammation in the bronchi tend to narrow the airways and provide an obstacle to airflow, thus increasing the resistance of the airways. In this manner bronchitis may cause obstructive pulmonary symptoms.
On pulmonary tests, a bronchitic may present a decreased FEV1 and FEV1/FVC. However, unlike the other common obstructive disorders, asthma and emphysema, bronchitis rarely causes a high residual volume. This is because the air flow obstruction found in bronchitis is due to increased resistance, which does not generally cause the airways to collapse prematurely and trap air in the lungs.