Determining lung volumes is useful to doctors in diagnosing, directing treatment and assessing response to treatment of a variety of conditions. Patient knowledge of them builds health literacy and promotes patient engagement.
Alveolar volume (VA) – Is the surface area of the lungs available for gas exchange. It is not quite equivalent to TLC because it does not include anatomical dead space. In general, it is approximately 95% of TLC. Another difference is TLC is a measurement made by lung plethysmography whereas VA is a calculation made from a single breath during a DLCO test. The breath for the measurement begins with VT and ends with TLC.
Functional residual capacity (FRC) – Is the volume of air remaining in the lungs following a normal expiration.
Inspiratory reserve volume (IRV) – Is the maximal volume of additional air that one can breathe into the lungs with a conscious effort after a normal inspiration.
Total lung capacity (TLC) – The volume of air in the lungs at the end of a maximum breath inward. It is equal to the functional residual capacity and the inspiratory capacity. Lung plethysmography is the means of measuring it. It is below normal when restrictive lung disease is present.
Forced vital capacity (FVC) – Is the maximum volume of gas that one can forcefully expel from the lungs during spirometry. It is often low when restrictive lung disease is present.
Forced expiratory volume in one second (FEV1) – Is the volume of air that one can expel from the lungs during the first second of a forced expiration. It is often an indicator of airway obstruction when it is low.
Volume of inspired gas – Is the maximal volume of a gas mixture a patient inhales during a DLCO test. The inhalation is from RV to TLG. It is not a part of regular spirometry or lung plethysmography, but is a part of spirometry performed during DLCO testing. It is one of the parameters for deciding if the DLCO test is valid.