Variation in airways resistance when defined over different ranges of airflows

In eight normal subjects airways resistance (Raw) was assessed over six ranges of airflow (about zero flow at both minimum and maximum lung volumes) using an automated whole body plethysmograph. The intervals of flow used were 21s-1 and 11s-1 spanning zero flow, and 11s-1 and 0.51s-1 measured up to and from zero flow. The wider intervals gave less variable results, the coefficients of variation being of the order 11%, 15%, and 22% for the 2, 1, and 0.51s-1 intervals respectively. In all subjects, at minimum volume Raw was some 1.5 times greater when measured over the ranges at end-expiration than at start-expiration; at maximum volume Raw was some 1.3 times greater when measured over the ranges at end-inspiration than at start-expiration. A slight increase in the slopes of the oscilloscope traces used to determine Raw was observed at minimum volume compared with those at maximum volume. In view of the substantial differences reported it is essential that the exact range of flow, and the respiratory phase used, is described when reporting measurements of Raw. Least variability is obtained by estimating slopes over a wide interval of flow, such as 1 or 21s-1.