Interpretation of “24 Hour Lung Retention” in Studies of Mucociliary Clearance

To determine if clearance from ciliated airways is complete in 24 hrs, we measured whole lung and regional clearance of radiolabeled aerosols over 24 hrs in a large number of normal and diseased individuals. Using a gamma camera and computer analysis, we found small but significant amounts of aerosol in the central airways of normal subjects, and greater central retention in some patients with obstructive lung disease. In patients, a significant factor related to the central retention of aerosol was the presence of chronic flow limitation during tidal breathing. Whole lung retention in normal lungs correlated with the initial pattern of aerosol deposition, but not in disease. We conclude that the 24 hr image often contains a significant amount of deposited aerosol in central airways and not just alveoli. In addition, the presence of flow limiting segments in central airways is associated with prolonged retention of aerosol in patients with obstructive lung disease.