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