A Theoretical Comparison of Single Breath and Rebreathing Methods of Studying Soluble Gas Exchange in the Lung

Humphrey Davy (1800) was the first to show that lung volumes could be estimated from the degree to which alveolar gas was diluted in an inert and insoluble marker, hydrogen. He also measured the absorbtion of the soluble gas, nitrous oxide, and realised that its uptake would depend on “the velocity of the circulation”. Since then, other workers (e.g Bornstein, 1910; Krogh and Lindhard, 1912; Grollman, 1929a & b; Asmussen and Nielsen, 1953) have taken advantage of the solubility of biologically inert gases to measure pulmonary blood flow, and later, pulmonary tissue volume (Cander and Forster, 1959; Sackner et al, 1975). Many of these authors have used various combinations of soluble and insoluble gases to determine lung volume, ventilation, perfusion and tissue content, by various manoeuvres usually involving rebreathing or rapid exhalations after a series of breatholds of known durations. Both these techniques sacrifice much of the potential information about the inhomogenous distribution of blood and gas flows, in order to obtain a reasonably reliable functional staement about the lung as a whole.