Regional pleural filtration and absorption measured by fluorescent tracers in rabbits

In prone anesthetized rabbits, we used Evans blue-dyed albumin (EBA) to study regional pleural filtration and FITC dextran to study regional pleural absorption. Evans blue was injected intravenously, and the animals were ventilated for 6 h at either of two levels of ventilation. Postmortem the right rib cage was frozen and thawed before study. EBA fluorescence emitted from the rib cage surface was measured along the cranial-caudal axis near the mid chest with fluorescence videomicroscopy. Fluorescent light intensity increased from the third to the eighth rib in a cyclic fashion, with peaks at the ribs and troughs at the intercostal spaces. This increase was greater at the higher ventilation. Fluorescent images of cross sections of a frozen rib cage verified a cranial-caudal gradient in filtration. Fluorescent images of FITC dextran absorbed from the pleural space into the rib cage surface indicated major areas of absorption at the ventral, caudal, and cranial regions adjacent to the lung margins and areas of absorption scattered in the intercostal spaces. Simultaneous measurements of EBA filtration and FITC absorption showed sites of maximal filtration that were different from sites of maximal absorption. Pleural uptake of fluorescent microspheres (2-microm diameter) located lymphatic stomata distributed randomly within clusters in the intercostal spaces and channels of lymphatic lacunae parallel to blood vessels. Diaphragmatic uptake of microspheres was confined mainly to the ventral surface of the central tendon. Visceral pleural absorption was minimal.