How is obstruction determined in the workup of asthma?

Updated: Oct 07, 2019
  • Author: John J Oppenheimer, MD; Chief Editor: Michael A Kaliner, MD  more...
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Obstruction is defined as a ratio of less than 70% of forced expiratory volume in 1 second (FEV1) to forced vital capacity (FVC). FEV1 is normally greater than 80% of values predicted by age. However, some have suggested alternative methods of defining obstruction in pulmonary tests. [15, 16]

Young patients with a supranormal FVC can sometimes have a reduced FEV1/FVC ratio without having obstructive lung disease. Reversibility can be shown by administering a short-acting beta-2 agonist inhaler with a resultant 12% and more than 200-mL improvement in FEV1 or FVC. [17] If no response occurs, 2-3 weeks of oral or inhaled corticosteroids (20 mg twice daily for the average patient) may be required to demonstrate an improvement in airflow. Note that airflow obstruction in some patients with chronic obstructive pulmonary disease may be partially reversible.

Relative annual risk of exacerbations may be related to FEV1. A 15% drop in FEV1 after 6 minutes of running or other exercise can be diagnostic of exercise-induced bronchospasm. A 20% variation in the peak expiratory flow rate (PEFR) between high and low values is highly suggestive of asthma, but formal pulmonary function testing (as above) is recommended, because the PEFR is extremely effort-dependent.

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