Bronchial and Alveolar Absorption of Inhaled 99mTc-DTPA

The clearance rate of 99mTc-DTPA deposited in the lung by inhalation has been used as an index to measure lung epithelial permeability. To determine if differences exist between alveolar and bronchial absorption of 99mTc-DTPA we measured regional clearance rates in 4 beagle dogs for 30 min after preferential bronchial and alveolar deposition. Alveolar deposition was maximized by inhalation for 2 min of small 99mTc-DTPA particles (activity median aerodynamic diameter, AMAD = 0.5 micron; geometric standard deviation, GSD = 1.6) with deep slow ventilation (VT = 350 ml; f = 9 min-1), and bronchial deposition was increased by inhalation of large particles (AMAD = 4.1 microns, GSD = 2.3) with rapid shallow ventilation (VT = 50 ml; f = 65 min-1). Respective clearance rates from basal regions, which represent mainly alveolar absorption, were: for small particles, 2.29% min-1; for large particles, 1.57% min-1 (p = 0.10). Apical regions, which contain relatively more bronchial surface than do the basal regions, showed the following clearance rates: for small particles, 1.76% min-1; for large particles, 1.31% min-1 (p less than 0.05). These results indicate that in vivo alveolar absorption of 99mTc-DTPA is more rapid than bronchial absorption. Control or verification of the site of deposition of the tracer in the lung is of importance for the interpretation of the results of the 99mTc-DTPA lung permeability assay.