Pleural liquid pressure at the caudal border of the lung

Model simulation indicates that pressure recorded through a capsule, when lung border is under it, is essentially equal to pressure of pleural liquid (Pliq) surrounding lung border, though only a strip of capsule membrane is exposed to this liquid (the rest is facing diaphragm and lung). At the top of the right 8th or 9th intercostal space of dogs in left lateral posture Pliq recorded through a capsule at transit of lung border during inspiration (bor. I) or expiration (bor. E) was -19.7 +/- 1.1 and -11.2 +/- 0.3 cm H2O, respectively. Pliq recorded simultaneously through a cannula on the flat surface of lung in 5th or 6th intercostal space (corrected for 1 cm lower height) was -18.7 +/- 1.4 and -12.4 +/- 0.8 cm H2O. Similar values of Pliq on lung border and on flat surface of lung, despite different kinds of deformation and different measuring devices, provide further evidence that Pliq is more subatmospheric than pleural surface pressure. Seventy percent of difference in Pliq between bor. I and bor. E was accounted for by: (a) corresponding difference in pleural surface pressure, and (b) greater tidal change of liquid than surface pressure. Possible factors accounting for the remaining 30% are discussed.