Transdiaphragmatic pressure and rib motion in area of apposition during paralysis

Changes in pleural surface pressure in area of apposition of diaphragm to rib cage (delta Ppl,ap), changes in abdominal pressure (delta Pab), and redial displacement of the 11th rib have been recorded in anesthetized, paralyzed dogs during lung inflation or deflation. Above functional residual capacity (FRC) changes in transdiaphragmatic pressure in area of apposition (delta Pdi,ap) were essentially nil in intact (INT) dogs either in lateral or supine posture, and in partially eviscerated (EVS) dogs in lateral posture, either in the 10th or 11th intercostal space. Below FRC delta Pdi,ap could be positive (INT lateral and EVS), nil (EVS), or negative (INT supine and EVS); it could be different in the 10th and 11th intercostal spaces. Hence, with stretched (like with contracted) diaphragm, delta Ppl,ap measured at one site often differs from delta Pab and is not representative of average pressure acting on area of apposition. With volume increase above FRC, the 11th rib moved slightly in and then out in EVS and linearly out in INT. With volume decrease below FRC it moved out progressively in EVS, and it moved in and eventually reversed in INT. In paralyzed dogs in lateral posture the factor having the greatest influence on displacement of the abdominal rib cage is Pab. Mechanical linkage with pulmonary rib cage becomes relevant at large volume, whereas insertional traction of diaphragm becomes relevant at low volume.