Lung volumes, which play a major role in breathing, are subdivisions of total lung capacity (TLC). TLC represents the total volume of air present in the lungs during maximal inspiration. These volumes are reported in liters and can be categorized as either volumes or capacities. Capacities consist of two or more volumes. A spirometer is commonly used to measure many lung volumes. In this method, the patient breathes normally into the spirometer, allowing measurement of the tidal volume (VT), which is the volume of air moved in and out of the lungs with each quiet breath. The patient then performs a maximal inhalation followed by a forceful and complete exhalation, allowing measurement of the vital capacity (VC), which is the total volume of exhaled air from maximal inspiration to maximal exhalation. The residual volume (RV) refers to the air remaining in the lungs after a complete exhalation. TLC is the sum of VC and RV, representing the total volume of air in the lungs at the end of maximal inspiration, including both the air that can be moved (VC) and the air that is always present (trapped) in the lungs (RV). The functional residual capacity (FRC), also known as the resting volume of the lungs, refers to the volume of air in the lungs at the end of exhalation during quiet breathing. FRC is composed of RV and the expiratory reserve volume (the volume of air that can be exhaled from FRC to RV).

Measurement of RV and TLC can be done using two methods: helium dilution and body plethysmography. Both techniques provide valuable information about lung function and diseases. Helium dilution, the older and simpler method, is often less accurate compared to body plethysmography, which requires advanced and costly equipment.

Pulmonary function tests are frequently utilized for diagnosing lung function abnormalities and assessing the progression of lung diseases. These tests can differentiate between obstructive lung diseases and restrictive lung diseases. In healthy individuals, the ratio of RV to TLC is typically less than 0.25, indicating that around 25% of the total lung volume is trapped air. Obstructive pulmonary diseases cause an elevation in the RV/TLC ratio due to an increase in RV without a proportional increase in TLC. Conversely, restrictive lung diseases lead to an elevation in the RV/TLC ratio as a result of a decrease in TLC.

In normal individuals, the FRC measured by helium dilution and the FRC measured by plethysmography are equivalent. However, this is not the case for individuals with lung disease. The FRC measured by helium dilution represents the volume of gas in the lungs that communicates with the airways, while the FRC measured by plethysmography is the total volume of gas in the lungs at the end of a normal exhalation. If a significant amount of gas becomes trapped in the lungs due to premature airway closure, the FRC determined by plethysmography will be considerably higher than that determined by helium dilution.
and play a major role in the work of breathing (see ). All lung volumes are subdivisions of total lung capacity (TLC), the total volume of air that is contained in the lung at the point of maximal inspiration. Lung volumes are reported in liters either as volumes or as capacities. A capacity is composed of two or more volumes. Many lung volumes are measured with a spirometer. The patient is asked to first breathe normally into the spirometer, and the volume of air (the tidal volume [VT]) that is moved into and out of the lungs with each quiet breath is measured. The patient then inhales maximally and exhales forcefully and completely, and the volume of exhaled air is measured. The total volume of exhaled air, from a maximal inspiration to a maximal exhalation, is the vital capacity (VC). Residual volume (RV) is the air remaining in the lungs after a complete exhalation. TLC is the sum of VC and RV; it is the total volume of air contained in the lungs at the end of maximal inspiration, and it includes the volume of air that can be moved (VC) and the volume of air that is always present (trapped) in the lungs (RV). Functional residual capacity (FRC) is the volume of air in the lungs at the end of exhalation during quiet breathing and is also called the resting volume of the lungs. FRC is composed of RV and the expiratory reserve volume (the volume of air that can be exhaled from FRC to RV).
Measurement of Lung Volumes
RV and TLC can be measured in two ways: by helium dilution and by body plethysmography. Both methods are used clinically and provide valuable information about lung function and lung disease. The helium dilution technique is the older and simpler method, but it is often less accurate than body plethysmography, which requires sophisticated and expensive equipment.
21.1
Pulmonaryfunctiontestsareoftenusedtodiagnoseabnormalitiesinlungfunctionandtoassesstheprogressionoflungdisease.Theycandistinguishthetwomajortypesofpulmonarypathophysiologicprocesses:obstructivelungdiseasesandrestrictivelungdiseases.Forexample,innormalindividuals,theratioofRVtoTLCislessthan 0.25.Thusinahealthyindividual,approximately25%ofthetotalvolumeofairinthelungistrapped.Inobstructive pulmonary diseases, anelevationinRV/TLCratioissecondarytoanincreaseinRVoutofproportiontoanyincreaseinTLC.Incontrast,inrestrictive lung diseases, theelevationintheRV/TLCratioiscausedbyadecreaseinTLC.
In normal individuals, the FRC measured by helium dilution and the FRC measured by plethysmography are the same (
Table 21.1 ). This is not true in individuals with lung disease. The FRC measured by helium dilution is the volume of gas in the lung that communicates with the •Fig. 21.4 Measurement of Lung Volume by Helium Dilution. C1,knownconcentrationofaninertgas;C2,new(previouslyunknown)concentrationofthegas;V1,knownvolumeofabox;V2,lungvolume(initiallyunknown).
airways, whereas the FRC measured by plethysmography is the total volume of gas in the lungs at the end of a normal exhalation. If a significant amount of gas is trapped in the lungs (because of premature airway closure; see ), the FRC determined by plethysmography is considerably higher than that determined by helium dilution.