Measurement of lung volume by multiple indicator dilution. A simple model and its ramifications

A simple, idealized rebreathing manoeuvre was simulated numerically on a Hewlett-Packard 9825A desk-top calculator. Approximate imitations were obtained of real-life observations of differences between apparent volumes of distribution of different indicators in the same rebreathing manoeuvre. The simulated patterns of discrepancy depended heavily on the uptake or output profiles which were assumed for carbon dioxide and nitrous oxide during the hypothetical rebreathing manoeuvres. The simulations pointed to the analytical algebraic expressions which explained the simulated patterns of discrepancy. A procedure was devised to correct for the effects of these uptakes or outputs. It depended on considering the mixing of oxygen, nitrogen and argon as if it were occurring in a separate part of the rebreathing system occupied only by these gases. The volume of this notional part would decrease linearly with time at a rate equal to the oxygen consumption.