Estimation of lung volume from nitrogen washout curves
A multiple vital capacity breath nitrogen-washout technique, based on the exponential extrapolation of the volume-concentration integrals of a limited number of breaths, has been successfully employed to measure residual lung volume. The accuracy of the estimate compares favourably with that derived from helium dilution. A logical extension of the extrapolation principle to a single-breath manoeuvre necessitated the prediction of the exponential decay slope from an independent index of gas distribution, namely the slope of phase III, recorded in the course of vital capacity N2 washout. The clearcut logarithmic relationship between these respective mixing indices allows such prediction. Subject to the observance of certain limiting criteria, the modified single-breath method provides, in relatively simple fashion, results which show close agreement with helium-derived volume measurements.