EFFECT OF EXTERNAL NEGATIVE-PRESSURE ON PULMONARY TC-99M-DTPA CLEARANCE IN HUMANS

We studied the mechanisms by which pulmonary solute clearance is affec ted by lung inflation. We examined the pulmonary clearance of inhaled technetium-99m diethylenetriaminepentaacetic acid (Tc-99m-DTPA) togeth er with changes in lung volumes in healthy men after applying graded l evels of continuous external negative pressure (CNP) and positive end- expiratory pressure (PEEP). The Tc-99m-DTPA clearance increased from t he baseline during -15 cm H2O CNP (p < 0.005) and during -20 cm H2O CN P (p < 0.001). The Tc-99m-DTPA clearance increased during +15 cm H2O P EEP (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 threshol d effect. On the other hand, changes in FRC during CNP were proportion al to the applied pressures and were similar to those during PEEP with corresponding pressures. These results suggest that pulmonary vascula r recruitment induced by CNP does not affect pulmonary Tc-99m-DTPA cle arance. This threshold effect suggests that the increased clearance is due to changes in membrane permeability rather than in the area of th e alveolar-capillary interface or the lining layer thickness. We concl uded that the effect of lung inflation on solute clearance may be medi ated by the changes in membrane permeability.