Upward flow of pleural liquid near lobar margins due to cardiogenic motion

In anesthetized, paralyzed, supine rabbits (3-4 kg) during apnea, we injected fluorescent dye or fluorescent microspheres (2 or 6 microns diam) into the dependent pleural space and observed the arrival and movement of the dye or microspheres at superior regions. Injection was through a rib capsule located in the dependent right chest. The dye or microspheres were observed through a pleural window overlying a lobar margin. The vertical distance between the capsule and window was 3-4 cm. The movement of the dye or microspheres was recorded via a fluorescence videomicroscope, and the signals were analyzed for dye transit time and microsphere velocity. The transit time of the dye to traverse the height of the pleural space was calculated from the light intensity vs. time curve. Transit time during apnea averaged 6.0 +/- 3.4 (SD) min (n = 4). Transit time measured after the onset of mechanical ventilation was < 1 min. The direction and speed of a microsphere moving in the relatively thick pleural space adjacent to the lobar margin depended on its distance from the lobar margin. Microspheres moved upward in the pleural space that was in proximity to the lobar margin but downward at farther distances from the lobar margin. Pleural liquid recirculation occurs via the pleural space adjacent to lobar margins.