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An automated method of measuring airways resistance (Raw) and thoracic gas volume (Vtg) by whole body plethysmography using a digital computer is described. The results obtained by this method are compared with those obtained by an experienced observer and the results of consistency tests on the two methods are given. There was wide variability in the values of Raw obtained using each method, and no significant differences were found between the two methods. The values of Vtg obtained by them were highly correlated, but the results obtained manually were higher than those obtained automatically by an amount proportional to the volume. The consistency of the computer measurements was shown to be greater than that of the manual method. A significant decrease between successive measurements of Raw in some subjects was noted in the manual observations but not in those derived from the automated method. The new method is suitable for the rapid determination of Raw and Vtg.

Variations in the estimation of airway resistance (Raw) and thoracic gas volume (Vtg) made by different observers using body plethysmography have been investigated. Five observers determined Raw and Vtg in normal subjects using prerecorded signals so that intrasubject variations were eliminated. The slopes of the oscillography tracings required to make the determinations were assessed using a cursor and a scale fitted to the oscilloscope. Significant and consistent differences in Raw determined by different observers were found,with a mean range of 37%. Great care should therefore be taken when comparing results obtained by different observers. The variation between observers measuring Vtg was significant and consistent but of much smaller magnitude than was the case for Raw, having a mean range of 12%.

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.