Effect of frequency and tidal volume on pleural pressure in rabbits during high frequency ventilation.

Regional effects of the chest wall on airway pressure transmission were studied during high frequency ventilation in anesthetized rabbits. We measured airway pressure (Paw), esophageal pressure (Pes), and costal pleural pressure (Ppl) by a rib capsule and flow and volume with a calibrated pneumotachograph. Using a closed circuit, pressures and flow were measured at varying frequencies (2-80 Hz) and tidal volumes (2-20 ml). Mean Pes and Ppl increased with flow amplitude above 100-250 ml/s, whereas mean Paw decreased, consistent with air trapping. Paw, Pes, and Ppl amplitudes increased monotonically with flow amplitude except above 400-500 ml/s, where the Ppl amplitude decreased suddenly. The latter occurring simultaneously with a sudden fall in mean Paw indicated airway flow limitation in costal regions. Flow instabilities during flow limitation were consistent with the large increase in the phase difference between Paw and Ppl and its variability, with frequency. By contrast, the phase difference between Paw and Pes and its variability were relatively small. These differences in Pes from Ppl responses might be caused by a difference in the impedance of the airway-mediastinum pathway or a direct transmission of tracheal pressure oscillations to the esophagus. The former suggests that constraints offered by the mediastinum and rib cage resulted in nonuniform ventilation during high frequency ventilation.