Lung Volumes. Definition Clinical assessment of lung function is aided by the determination of several parameters derived from usage of a spirometer. This device allows for accurate measurement of the rate and volume of air flowing into and out of a patient's lungs during particular maneuvers.
Lung Volumes Tidal Volume The Tidal Volume is the volume of air breathed in and out by a person during normal breathing at rest. Residual Volume The Residual Volume is the volume of gas remaining in the lung after a person has breathed out to his or her maximum capacity. Naturally, the residual volume cannot be determined directly using spirometry since it is not possible to measure the remaining air volume after maximum expiration.
Consequently, more advanced techniques must be used to determine this parameter. Lung Volumes The above graph shows the volume of air inspired and expired by an individual over time using a spirometer. After a period of initial quite breathing the individual takes a maximal inspiration and a maximal expiration. An important measurement taken during spirometry is the forced expiratory volume FEV , which measures how much air can be forced out of the lung over a specific period, usually one second FEV1.
In addition, the forced vital capacity FVC , which is the total amount of air that can be forcibly exhaled, is measured. Patients exhale most of the lung volume very quickly. In this instance, it is difficult for the patient to get the air out of his or her lungs. It takes a long time to reach the maximal exhalation volume. In either case, breathing is difficult and complications arise.
The lung capacities are measurements of two or more volumes. The vital capacity VC measures the maximum amount of air that can be inhaled or exhaled during a respiratory cycle.
It is the sum of the expiratory reserve volume, tidal volume, and inspiratory reserve volume. The inspiratory capacity IC is the amount of air that can be inhaled after the end of a normal expiration. It is, therefore, the sum of the tidal volume and inspiratory reserve volume. The functional residual capacity FRC includes the expiratory reserve volume and the residual volume. The FRC measures the amount of additional air that can be exhaled after a normal exhalation.
The total lung capacity TLC is a measurement of the total amount of air that the lung can hold. It is the sum of the residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume.. Volumes and capacities which include the RV are estimated using alternative techniques such as whole body plethysmography or helium dilution.
What can be measured by spirometry? Common measurements obtained by spirometry include see figure below , Vital capacity VC or forced vital capacity FVC — the maximum amount of air that can be expelled from the lungs after a full inhalation Forced expiratory volume in 1 second FEV1 — volume of air which can be forcibly exhaled in one second Tidal volume TV — the volume of air inhaled and exhaled during restful breathing The inspiratory and expiratory reserve volumes IRV and ERV — the volume of additional air that can be forcible inhaled IRV or exhaled ERV outside of a normal tidal breath.
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