"An Approach to Interpreting Spirometry" (My Synopsis)

An Approach to Interpreting Spirometry
TIMOTHY J. BARREIRO, D.O., and IRENE PERILLO, M.D., University of Rochester School of Medicine and Dentistry, Rochester, New York
Am Fam Physician. 2004 Mar 1;69(5):1107-1115.

     Spirometry is an important test to do in the office setting because a history and physical exam are poor tools to diagnosis obstructive ventilatory patterns. Spirometry measures how fast the lungs can change volume during forced exhalation. A flow volume loop is created which will show characteristic patterns depending on the disease process.  The FVC is the amount of air that the patient can forcefully exhale. The FEV1 is the portion of the air that is exhaled in one second. The ratio that is measured is the FEV1/FVC. The ratio in a normal patient is over 80%, and the results are normalized based on the height, weight, race, and gender of the patient. Spirometry can be done on patients who are current or former smokers over the age of 44 years. A baseline test may be considered in patients who are taking medications with pulmonary toxicity.  The test is also used to track treatment response. Contraindications include recent surgery of the abdomen, thoracic, or eye, recent myocardial infarction, unstable angina, pneumothorax, hemoptysis, or an acute disorder than would hinder performing the test. The spirometry should be performed three times to determine the validity of the results. The difference between the three results should not exceed 0.2L.
    In patients with obstruction, the FVC may be normal or low, the FEV1 will be low, and the FEV1/FVC ratio will be less than 0.7. In this case, a bronchodilator challenge test can be ordered.  A bronchodilator is given and the spirometry is repeated. A positive test will show a rise in 12% in FEV1, and 200 ml increase in FVC or FEV1, or a 15-25% increase in FEV25-75%. This test is positive in reversible airway disease. If the test is negative and the bronchodilation does not change the spirometry results, then the patient has an obstructive ventilatory impairment. 
     In patients with a restrictive impairment, the FVC and FEV1 will be low, but the ratio of FEV1/FVC will be normal (above 0.7).  These patients need to be referred for static lung volumes (DLCO, DLCO/VA, ERV) to determine severity (DLCO can also be done in obstruction to determine asthma vs COPD; it goes up in asthma). A maximum voluntary ventilation maneuver (MMVM) can be done to determine if the restriction is due to poor patient effort, neuromuscular disease, or airway lesion. 
     In either case, severity must be determined after the spirometry is interpreted.