A radiographic method for measuring steady-state functional residual capacity in the supine patient. A method suitable for sleep studies.

We have devised a method to measure functional residual capacity (FRC) in the recumbent, spontaneously breathing patient. Simultaneous anteroposterior and lateral chest radiographs were exposed at the end of an exhalation, as determined by tracings of flow sensed by a nasal thermistor. The volume of the lungs was then planimetrically measured. Functional residual capacity was sequentially measured in 20 supine subjects, both by a helium dilution technique and by the radiographic technique. Planimetric measurement of FRC correlated well with the helium dilution technique, with a range of FRC measurements from 1.53 to 6.41 L (r = 0.94). This method of measuring lung volume should be useful in evaluating changes in FRC associated with different sleep stages and in explaining the mechanisms causing nocturnal oxygen desaturation in patients with chronic obstructive lung disease.