Spirometry
The information below is taken directly from:
Interpreting pulmonary function tests: Recognize the pattern, and the diagnosis will follow. Al-Ashkar et al. Cleve Clin J Med 2003;70:866
A spirometric maneuver begins with the patient inhaling as deeply as he or she can. Then the patient exhales as long and as forcefully as possible.
- FVC (forced vital capacity) is the amount exhaled in this manner. It can be conisdered a surrogate marker for TLC
- FEV1 (forced expiratory volume in 1 second) is the amount of air exhaled during the first second of the FVC maneuver. It tends to be lower in diseases that obstruct the airway, such as asthma or emphysema.
- FEV1/FVC ratio is used to determine if the pattern is obstructive, restrictive, or normal
- RV (Residual Volume) - even after one exhales as long and as hard as possible, some air remains in the lungs; this is called the residual volume.
- TLC (total lung capacity) is the RV plus the FVC. The RV (and hence TLC) cannot be measured by spirometry but by either breathing an inert gas such as helium or sitting in an airtight booth. These measurements are commonly called “static lung volumes”
For obstructive diseases, measurement of the residual volume and total lung capacity can demonstrate air trapping and hyperinflation. For restrictive diseases, the total lung capacity is needed to confirm true restriction and better quantitate the degree of restriction
Criteria for acceptability and reproducibility are established by the American Thoracic Society. In general, an FVC maneuver is acceptable if the patient has made a good effort. An effort is considered good when there is a rapid increase in air flow at the start of exhalation. A complete maneuver requires at least 6 seconds of exhalation ending with a plateau in flow (ie, no further air is being blown out). The test is repeated at least three times. To meet reproducibility criteria, all three FEV1 & FVC measurements must be within 200 mL of each other. The test with the greatest sum of the FEV1 and the FVC is reported.
A flow-time curve and flow-volume loop are usually included with the report to allow one to visualize the rapid rise and the plateau in flow over time and over lung volumes. In a proper FVC maneuver, flow rates should be at their highest near the start of exhalation and should decline through the end of exhalation. This is graphed in the volume-time curve. Similarly, flow rates are highest near the TLC (where the FVC maneuver begins) and they decline until the RV (where the FVC maneuver ends). This is graphed as the flow-volume loop.
Is the pattern obstructive, restrictive, or normal?
The first step is to assess the FEV1/FVC ratio.
- If this ratio is less than the lower limit of normal for the patient (included with the test report), an obstructive defect is present (ie, the FEV1 has fallen to a greater degree than the FVC).
- If this ratio is greater than the lower limit of normal, then either the spirometry test is normal or a restrictive defect is present.
The next step is to look at the FVC (if only spirometry is available).
- If the FVC is less than the lower limit of normal, a restrictive defect is suggested. The TLC (if this was measured) can confirm the presence of restriction if this value is less than the predicted lower limit of normal.
- If the FVC is normal then no specific defect pattern is found.
