Automatisation of the single-breath nitrogen washout test

The paper presents a computer-based system for performance and analysis of the single-breath nitrogen washout test (Sb-N₂ test). A pneumotachometer and a rapid gas analyser are used for volume flow rate and nitrogen fraction measurements. The data are acquired and analysed by a PDP-11/34 computer. Several sources of measurement errors are considered. An algorithm is proposed to reduce errors in flow measurements due to viscosity variation in respired air. The algorithm for closing volume (CV) detection and calculation of other Sb-N₂ parameters is developed from 50 tracings of 18 subjects. The CV is obtained by an iterative linear fitting of the later half of the Sb-N₂ curve. Comparisons between computer and hand measurements of three trained readers show good correlations (r=0·87, 0·93, 0·91). The variability of computer CV measurements was comparable with that of the hand reader with the lowest variance. The coefficient of variation of residual volume and slope of alveolar plateau measurements are apparently reduced. The results demonstrate the adequacy and convenience of using the computer to analyse the Sb-N₂ test and indicate that the automation allows for reduction of experimental errors and of test parameter variabilities.