Effect of external negative pressure on pulmonary 99mTc-DTPA clearance in humans

We studied the mechanisms by which pulmonary solute clearance is affected by lung inflation. We examined the pulmonary clearance of inhaled technetium-99m diethylenetriaminepentaacetic acid (99mTc-DTPA) together with changes in lung volumes in healthy men after applying graded levels of continuous external negative pressure (CNP) and positive end-expiratory pressure (PEEP). The 99mTc-DTPA clearance increased from the baseline during -15 cm H2O CNP (p < 0.005) and during -20 cm H2O CNP (p < 0.001). The 99mTc-DTPA clearance increased during +15 cm H2O PEEP (p < 0.001). However, the changes during both -10 cm H2O CNP and +10 cm H2O PEEP did not differ from the baseline, indicating a threshold effect. On the other hand, changes in FRC during CNP were proportional to the applied pressures and were similar to those during PEEP with corresponding pressures. These results suggest that pulmonary vascular recruitment induced by CNP does not affect pulmonary 99mTc-DTPA clearance. This threshold effect suggests that the increased clearance is due to changes in membrane permeability rather than in the area of the alveolar-capillary interface or the lining layer thickness. We concluded that the effect of lung inflation on solute clearance may be mediated by the changes in membrane permeability.