Exchanges of oxygen and carbon dioxide alter inert gas pattern in single-breath tests

Concentration of inert gas in the lung is lowered when CO₂ entrance exceeds O₂ exit and is raised when O₂ exit predominates. In air-breathing subjects who expire to residual volume, this 'metabolic gas effect' often causes a rising N₂ concentration when in fact there should be a terminal fall because of low N₂ in apical regions. In single-breath tests, we compared the dilution of resident N₂ with dilution of an inspired gas, Ne, to find the 'ideal' inert gas concentration (due only to mixing of resident gas with inspired gas). The displacement from the ideal concentration vs. volume pattern depends on the timing of the breath, because early CO₂ entrance gives way later to O₂ exit. Sometimes observed patterns are above or below but parallel to the ideal, and sometimes the observed slope of phase III is steeper than ideal for N₂ and flatter than ideal for Ne. In addition to phase III distortions, the metabolic gas effect sometimes also distorts phase IV height and the intersection between phases III and IV. The distortions depend strongly on absolute concentration of the indicator gas in the lung, so they are very small when 'closing volume' maneuvers are done in the conventional manner. However, distortions can be large and misleading when single-breath maneuvers are done in unconventional ways.