Journal of Applied Physiology, Vol 38, Issue 1 10-15, Copyright © 1975 by American Physiological Society
Effect of preinspiratory lung volume on closing volume determination by nitrogen method
K. Kaneko, J. Mohler and O. Balchum
The effect of preinspiratory lung volume on the N2 closing volume (CV) was
studied in simulated CV determination in a lung model. The model analysis
supports our hypothesis that inspiration of O2 initiated at the "closing
capacity" (CC = CV ;V) will improve the resolution of the inflection point
between phases III and IV. It further indicates that if the inflection
point is located by extrapolating the terminal portion of phase IV, then
the original N2 method could underestimate CV systematically as much as 5%
VC because of the relatively small initial rise in N2 concentration.
Conversely, the CV obtained by the modified method should be closer to the
value obtained by the "bolus method." In practice, the modification can be
done simply by adding a dead space (DS) with its capacity equal to CV
(600-700 ml in male adults). The theoretical lung model analysis was
confirmed by the experimental study, which showed that CV with DS was
consistently larger than CV without DS (a mean difference of 4.7% VC).
