Fall in specific airway conductance at residual volume in small airway obstruction.

A technique of body plethysmography has been developed for measuring specific airway conductance (SGaw) at residual volume (RV). Box pressure variations were recorded as difference between chest volume change and air volume respired at atmospheric pressure, delta(V1--V2). They could easily be related to alveolar pressure change per liter of lung volume and SGaw could directly be calculated without breathing against a closed shutter. To eliminate 'temperature-humidity artifact', calculations were made near end-tidal points where air movement in and out of lungs is minimal. For that purpose airflow changes were related to box pressure changes over a period of 0.2 sec from the beginning of inspiratory effort. SGaw was measured at FRC as well as at RV. Excessive air trapping at RV in some cases resulted in some chest expansion unaccompanied by any airflow at mouth. This has been termed pre-flow work (PFW). Any abnormal fall in SGaw at RV or the presence of PFW could be used as an indicator of small airway obstruction. Using this method obstruction in small airways was found in all smokers with smoking history of 5 to 30 packs years (mean 13.1 pack years) and in 4 patients who had a history of cough and/or dyspnoea or exertion. Simultaneous measurements of SGaw at FRC and SGaw 0.2 sec and PFW at RV could be of great help in determining the degree and site of airflow obstruction.