Spirometry is the measurement of airflow into and out of the lungs. The patient is given instructions on how to perform the breathing maneuvers. To perform the procedure the nose is pinched off, and the patient breathes as instructed through a mouthpiece attached to the spirometer. The three breathing maneuvers are practiced before recording the procedure, and the highest of three trials is used for evaluation of breathing. The instrument measures air flow by electronic or mechanical displacement principles and uses a microprocessor and recorder to calculate and plot air flow.
The test produces a recording of the patient's ventilation under conditions involving both normal and maximal effort. The recording, called a spirogram, shows the volume of air moved and the rate at which it is moved into and out of the lungs. There are several lung capacities that are measured by spirometry. Accurate measurement of these are dependent upon the patient performing the appropriate maneuver properly. The most common are described below:
Vital capacity (VC): This is the amount of air in liters that is moved out of the lung during normal breathing. The patient is instructed to breathe in and out normally to full expiration for this maneuver. Vital capacity is normally about 80% of the total lung capacity. Because of the elastic nature of the lungs and surrounding thorax, a small volume of air will remain in the lungs after full exhalation. This volume is called the residual volume (RV).
Forced vital capacity (FVC): After breathing out normally to full expiration the patient is instructed to breath in with a maximal effort and then exhale as forcefully and rapidly as possible. The FVC is the volume of air that is expelled into the spirometer following a maximum inhalation effort.
Forced expiratory volume (FEV): At the start of the FVC maneuver, the spirometer measures volume of air that is delivered through the mouthpiece at timed intervals of 0.5, 1.0, 2.0, and 3.0 seconds. The sum of these measurements normally constitutes about 97% of the FVC measurement. The most commonly used FEV measurement is FEV-1, which is the volume of air exhaled into the mouthpiece in one second. The FEV-1 should be at least 70% of the FVC.
Forced expiratory flow 25-75% (FEF 25-75): This is a calculation of the average flow rate over the center portion of the forced expiratory volume recording. It is determined from the time in seconds at which 25% and 75% of the vital capacity is reached. The volume of air exhaled in liters per second between these two times is the FEF 25-75. This value reflects the status of the medium and small sized airways.
Maximal voluntary ventilation (MVV): A maneuver in which the patient breathes as deeply and as fast as possible for 15 seconds. The average airflow (liters per second) indicates the strength and endurance of the respiratory muscles.
Normal values for FVC, FEV, FEF, and MVV are dependent on the patient's age, gender and size (height).
Robert Harr Paul Johnson, The Gale Group Inc., Gale, Detroit,