Pleural pressure between diaphragm and rib cage during inspiratory muscle activity

We measured the changes in pleural surface pressure (delta Ppl) in the area of apposition of the rib cage to the diaphragm (Aap) in anesthetized dogs during spontaneous breathing, inspiratory efforts after airway occlusion at functional residual capacity, and phrenic stimulation. Intact dogs were in supine or lateral posture; partially eviscerated dogs were in lateral posture. delta Ppl,ap often differed significantly from changes in abdominal pressure (delta Pab); sometimes they differed in sign (except during phrenic stimulation). Changes in transdiaphragmatic pressure in Aap (delta Pdi,ap) could be positive or negative and were less in eviscerated than in intact dogs. delta Pdi,ap could differ in sign among respiratory maneuvers and over different parts of Aap. Hence average delta Pdi,ap should be closer to zero than delta Pdi,ap at a given site. Since delta Ppl,ap = delta Prc,ap, where Prc,ap represents rib cage pressure in Aap, delta Pdi,ap = delta Pab - delta Prc,ap. Hence, considering that delta Pab and delta Prc depend on different factors, delta Pdi,ap may differ from zero. This pressure difference seems related to the interaction between two semisolid structures (contracted diaphragm and rib cage in Aap) constrained to the same shape and position.