METHOD FOR STANDARDIZING AIRWAY-RESISTANCE FOR VARIATIONS IN LUNG-VOLUME

The inverse relationship between airway resistance and lung volume contributes to variation in measurement of resistance, as it is difficult to control precisely the lung volume at which resistance is determined each time. A correction for variation in lung volume was applied to measurement of airway resistance by using analysis of covariance with lung volume as a covariate. This method provides an assessment in each subject of degree of association between airway resistance and lung volume and uses the individual's unique slope, relating resistance to volume in order to correct resistance to a single lung volume. The corrected measurement of airway resistance was compared with conventional volume-standardized variables such as specific airway resistance, specific airway conductance and the longarithmic transformation of the latter. Eight normal male subjects had airway resistance and lung volume measured in a body plethysmograph, a mean of seven measurements made. They then inhaled histamine. Airway resistance showed only a limited association with lung volume and the slopes relating the two measurements varied between subjects. Covariance-corrected airway resistance appeared more sensitive than other methods of volume standardization in detecting broncho-constriction. Analysis of covariance adequately corrects airway resistance for variations in lung volume but further studies are required to validate its precision and sensitivity.