Pleural pressure measured in the zone of apposition of diaphragm to rib cage in rabbits.

In 10 anesthetized adult rabbits, we studied the effect of spontaneous breathing and positive pressure ventilation on pleural pressure on the costal lung surface (Ppl) and in the zone of apposition of the rib cage to the diaphragm (Papp). Ppl and Papp were measured by rib capsules installed in the 5th or 6th rib and 11th or 12th rib, respectively. Esophageal (Pes) and gastric (Pga) pressures were measured with air-filled balloons. At end expiration (functional residual capacity), Ppl was subatmospheric (-2.5 +/- 1.4 cm H2O), decreased during spontaneous inspiration, and was in phase with Pes. In contrast, Papp was above atmospheric pressure (2.1 +/- 1.8 cm H2O), increased during inspiration, and was in phase with Pga. Papp lagged Ppl by 180 degrees during spontaneous inspiration but was in phase with Ppl during mechanical ventilation. Changes in Ppl (delta Ppl) during inspiration were greater in magnitude than either delta Papp or delta Pga. Changes in transdiaphragmatic pressure in the zone of apposition (delta Pga-delta Papp) were near zero (-0.4 +/- 0.3 cm H2O), much smaller in magnitude than those (delta Pga-delta Ppl) associated with the lung (3.0 +/- 1.5 cm H2O). These results are consistent with the concept that during breathing, abdominal pressure is transmitted to the zone of apposition of the rib cage to the abdomen. During spontaneous breathing at rest, the pleural space in the zone of apposition is mechanically independent of the pleural space associated with the lung.