The relationship between the pulmonary distribution of inhaled fine aerosols and tests of small airway function

Gamma-camera lung images obtained after aerosol inhalation (2.2 micron mean aerodynamic diameter) were compared with those obtained during inhalation of krypton-81m gas. The penetration of the aerosol to the periphery of the lung appeared in both smokers and nonsmokers to be similar to the penetration of krypton-81m. An index of the inhomogeneity of deposition of the aerosol (aerosol distribution index, ADI) was calculated by dividing the variance of the cell by cell radioactivity counts by the mean count. This index was expressed as the mean value for both lungs (ADIM) or the value for the lung with the higher value (ADIH). ADIM and ADIH were correlated with the FEV1 and maximum expiratory flow rates at 50% and 25% of vital capacity and on average showed higher values for smokers than non-smokers. ADIH, forced expired time and partial expiratory time from 50% to 0% of vital capacity were correlated with pack-years of smoking. It is concluded that the results of tests of small airway disease are related to inhomogeneities of airflow in the periphery of the lung and that measurements of aerosol deposition may provide a sensitive indicator of disease of small airways. The future potential of the test is discussed.