Measurement of lung volumes by plethysmography.

There are various ways of measuring absolute lung volume. These range from measurements derived from chest radiographs to the more common laboratory measurements employing either gas dilution or plethysmography. The principal difference between the latter two methods is that gas dilution techniques measure gas that is in free communication with the airway opening, whilst plethysmography measures all intrathoracic gas. Once the absolute lung volume is known, the other lung volumes can be measured from the change in volume during specific respiratory manoeuvres. Work on this article was undertaken as part of a joint European Respiratory Society/American Thoracic Society effort to develop a joint "Statement" on the measurement of lung volumes. It will review the principles, practice and limitations of plethysmography, and recommend standards for the plethysmographic measure of lung volumes. These standards will include equipment specifications and measurement techniques over the age range from infancy to adulthood. For the purpose of this review, functional residual capacity (FRC) is defined as the absolute volume of gas in the lung at the end of a normal expiration. Thoracic gas volume (TGV) is defined as the volume of intrathoracic gas at the time the airway is occluded for the plethysmographic measurement; while this is usually at FRC, in special circumstances it may not be [1, 2]. Irrespective of where in the volume cycle TGV is measured, it should be adjusted to the FRC derived from plethysmography (FRCpleth) by subtracting or adding the appropriate volume correction. In normal children and adults, there should be no difference in FRC between gas dilution techniques and plethysmography. However, in patients with lung disease associated with gas-trapping and in normal infants [3], FRCpleth generally exceeds FRC measured by gas dilution.