Measurement of lung volume by multiple indicator dilution. Differences in apparent volumes of distribution of oxygen, nitrogen and argon.

A variant of a rebreathing/indicator-dilution technique was used to measure lung volume. Two or three indicators, which should have essentially the same pulmonary volume of distribution, were used. Gas compositions were measured by respiratory mass spectrometry and an extrapolation was used to allow for oxygen consumption. Carbon dioxide output, for which the extrapolation did not allow, caused considerable discrepancies between the apparent volumes of distribution of the three indicators: oxygen, nitrogen and argon. The discrepancies varied with the change in concentration imposed between the control and rebreathing periods. An iterative arithmetical correction is described, with its limitations. The approach offers a method of lung volume measurement which seems more applicable to anaesthesia than existing methods.