Pleural liquid pressure over the interlobar mediastinal and diaphragmatic surfaces of the lung.

Pleural liquid pressure (Pliq) was measured in the interlobar fissures and over the mediastinal apical, cardiac and diaphragmatic surface. When compared to costal values, Pliq was more subatmospheric over the cardiac region at all lung heights by about 5 cm H2O so that a costo-hilar gradient down the interlobar fissures of about 0.7 cm H2O/cm was found. No differences were found for other surfaces. Cardiac values, unlike costal ones, became more subatmospheric on increasing ventilation and heart-rate. On the basis of a Starling equilibrium a costo-hilar gradient of Pliq as well as more negative values in the cardiac surface can be explained if hydrostatic pressure in visceral pleura and/or hydraulic conductivity of parietal pleura are lower on mediastinal as compared to costal side. However the ventilation and heart-beat dependence of Pliq on cardiac region suggest a possible role of lymphatic in setting pleural liquid pressure.