Analysis of nitrogen multibreath washout curves through a statistical approach

A new method for analyzing nitrogen multibreath washout curves, founded on a statistical multicompartment model is presented. This method consists of two phases: (i) the preliminary conversion by mathematical means of the nitrogen end-tidal concentration curves - as usually available in routine practice - into the corresponding lung nitrogen decay curves normalized for ventilation rate and function al residual capacity and (ii) the determination of distribution function G(k) in the Laplace integral y(t) = integral of infinity,G(k)e-ktdk (where y(t) represents the experimental curve, k is the compartmental rate constant and the random variable of the statistical set). G(k) values are approximated by an accurate calculation of the distribution moments and the use of Edgeworth-Cramér series expansion. The clinical usefulness of the method was verified in normal subjects and in patients with chronic obstructive pulmonary disease. An excellent separation of the two groups was achieved by calculating the following parameters: the first moment (m1), the standard deviation (sigma) and the coefficient of dispersion (sigma/m). Besides, further information about the lung ventilation pattern could be obtained by a skillful inspection of the approximated distribution function curves of the rate constant k.