Assessment of changes in airway calibre using the body plethysmograph

We set out to assess the contribution of errors in measurement of thoracic gas volume and the relationship between volume and conductance to the estimation of airway resistance and conductance. In three subjects, errors in thoracic gas volume were smaller by a computerized than a manual method, but were not great enough to have much effect on the variability of resistance and conductance. The conductance-volume relationship was very variable in 17 subjects; from this relationship, it was possible to predict whether conductance or specific conductance would be the more variable measurement in 16 out of the 17. Conductance-volume relationships changed markedly after bronchoconstriction with histamine in one out of three subjects. Overall we found specific conductance to be the most satisfactory measurement. This measurement can be made without estimation of thoracic gas volume by closure of the mouth shutter and can, therefore, be used to follow acute changes in airway calibre.