Journal of Applied Physiology, Vol 50, Issue 3 487-492, Copyright © 1981 by American Physiological Society
Exchanges of oxygen and carbon dioxide alter inert gas pattern in single-breath tests
H. D. Van Liew and R. Arieli
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.
