Vertical gradients of pleural and transpulmonary pressure with liquid-filled lungs

In supine dogs with saline-filled lungs the vertical gradient of pleural surface pressure (VGPpl) was not significantly different from -1 cm H2O/cm and that of transpulmonary pressure (VGPtp) was not significantly different from zero. Hence the hydrostatic gradient of the liquid was entirely taken up by the chest wall, the ribs being rigid in the direction of gravity and the diaphragm facing an equal hydrostatic gradient on both sides. In head-up dogs VGPpl was -0.8 cm H2O/cm when the level of the liquid in the filling system was 19.5 cm below the lung top and -0.5 cm H2O/cm when this level corresponded to the top. The hydrostatic gradient of the liquid was not entirely taken up by the rib cage because of its uneven regional compliance and VGPtp was reversed with respect to that of the air-filled lung. With fluorocarbon (specific gravity 1.75) filled lungs in the supine posture VGPpl was about -1.28 cm H2O/cm and VGPtp was reversed. In the head-up posture VGPpl was about -1.2 cm H2O/cm and VGPtp was reversed.