Effect of preinspiratory lung volume on closing volume determination by nitrogen method

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).