Interpretation of Pulmonary Function Tests
Thomas Gross, MD
Peer Review Status: Externally Peer Reviewed by the Department of Internal Medicine Virtual Hospital Editorial Board
Spirometry
Spirometry with flow volume loops assesses the mechanical properties of the respiratory system by measuring expiratory volumes and flow rates. This test requires the patient to make a maximal inspiratory and expiratory effort. The patient in a sitting position breathes into a mouthpiece, and nose clips are placed to prevent air leak. To obtain interpretable results from spirometry, it is essential that the patient give full effort during testing. At least three tests of acceptable effort are performed to ensure reproducibility of results.
Flow volume loops provide a graphic illustration of a patient's spirometric efforts. Flow is plotted against volume to display a continuous loop from inspiration to expiration. The overall shape of the flow volume loop is important in interpreting spirometric results. The volume versus time curve is a an alternative way of plotting spirometric results and is another useful illustration of patient performance.
Spirometry is a versatile test of pulmonary physiology. Reversibility of airways obstruction can be assessed with the use of bronchodilators. After spirometry is completed, the patient is given an inhaled bronchodilator and the test is repeated. The purpose of this is to assess whether a patient's pulmonary process is bronchodilator responsive by looking for improvement in the expired volumes and flow rates. In general, a > 12% increase in the FEV1 (an absolute improvement in FEV1 of at least 200 ml) or the FVC after inhaling a beta agonist is considered a significant response. However, the lack of an acute bronchodilator effect during spirometry does not exclude a response to long term therapy.
Similarly, spirometry can be used to detect the bronchial hyperreactivity that characterizes asthma. By inhaling increasing concentrations of histamine or methacholine, patients with asthma will demonstrate symptoms and produce spirometric results consistent with airways obstruction at much lower threshold concentration than normals.
Definitions and Terms
Spirometry is typically reported in both absolute values and as a predicted percentage of normal. Normal values vary depending on gender, race, age, and height. It is therefore not possible to interpret PFT's without such information. There is no single set of standard reference values, however, and "normal" varies with the reference value used in each laboratory. It is therefore important to ensure that the reference formulas in a PFT lab are applicable to the patient population being tested. At the University of Iowa Hospitals and Clinics, reference values have been obtained from Morris et al.