Measurement of lung volume by multiple indicator dilution. Testing a modelled hypothesis with on-line computation.

Respiratory mass spectrometry and on-line computation were used to estimate lung volume in normal subjects by a rebreathing technique, using oxygen, nitrogen and argon as indicators. The computation had previously been tested in a numerical model. In real normal lungs, it performed well enough for practical purposes, despite recognizable oversimplifications in the modelling assumptions. The differences in estimate to be expected from different composition changes between control and rebreathing were less than 100 ml, and those from different indicators within a composition change were less than 50 ml. The use of two indicators in lung volume measurement allows a correction for volume changes attributable to carbon dioxide and nitrous oxide, and a third indicator provides a degree of quality control.