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.