Information content of the multibreath nitrogen washout

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Abstract
The distribution of ventilation per unit volume (.ovrhdot.VA/Vol) in the lung is often studied using the multibreath N2 washout test. Most approaches have focused on the determination of a single .ovrhdot.VA/Vol distribution (consisting of from 2 compartments to a continuum) compatible with the data, but because the potential number of lung units greatly exceeds the number of breaths measured, many distributions are usually compatible with a given set of washout data. Interpretation of single distributions requires evaluation of the variability among all such compatible distributions. The technique of linear programming is well suited to evaluation of these compatible distributions and its application to the N2 washout is explained. Examples (12) of human washout data are analyzed in this way. Using the first 20 or so breaths of the washout, narrow distributions of .ovrhdot.VA/Vol are well specified. Distributions with up to 4 distinct modes are also well specified, but for broad distributions the shape cannot be adequately defined. Good resolution in the region of high .ovrhdot.VA/Vol is critically dependent on measuring the 1st few breaths of the washout.