What is the role of lung volume determination in pulmonary function testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Lung volumes provide useful information that confirms the presence of restrictive lung disease suggested by a low vital capacity on a spirometry test. Hyperinflation, elevation of the RV and TLC can be demonstrated by this test. The test is dependent first on an accurate measurement of the volume of gas in the lungs at a resting end-expiration, known as the FRC, which represents the balance of the elastic recoil properties of the lung and the chest wall.

FRC can be measured by one of three techniques, inert gas dilution, nitrogen washout, or whole-body plethysmography. Both gas dilution techniques are subject to error by leaks at the mouthpiece or nose clip or, occasionally, even small leaks from the eardrum. When measured by whole-body plethysmography, resting end-expiratory volume is known as the FRCpleth and will include the volume of gas contained in noncommunicating spaces such as blebs or bullae that the FRC measured by gas dilution techniques will not measure. In addition to this advantage, body plethysmography allows multiple determinations of lung volumes to be made rapidly.

When measured by inert gas dilution or nitrogen washout, premature termination of the procedure before adequate demonstration of equilibrium or washout results in underestimation of FRC, RV, and TLC. Repeat measurements should allow a recovery period of 1.5 times the wash-in or wash-out time to prevent residual helium or oxygen from affecting the new measurement. Body plethysmography is performed rapidly, allowing multiple determinations in minutes. Ideally, each measurement of lung subdivisions should be linked to each FRC or ITGV measurement (patient should remain on the mouthpiece).

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