Effect of negative abdominal pressure on regional lung volumes in supine dogs

Using 133Xe we measured regional lung volumes from apex to base in supine dogs during the application of negative abdominal pressure (-50 cm H2O). Changes in rib cage shape were monitored with magnetometers. Negative abdominal pressure caused a decrease of 5% in the cross-sectional area of the upper rib cage and a decrease of 19% in the cross-sectional area of the lower rib cage. These changes were similar to those observed when the dog was tilted to the erect position. Over the 10 cm of lung examined, control measurements showed a mean apex-to-base gradient of 1.4% TLC/cm, apical regions being more expanded than basal. Negative abdominal pressure increased this gradient to a mean of 2.1% TLC/cm. This change was in qualitative agreement with measurements of costal pleural surface pressure made in similar experiments by others. However, the changes we observed tended to be smaller than those predicted on the basis of costal pleural surface pressure. Since regional volumes represented the average of the pressures operating on lung regions, it was suggested that during negative abdominal pressure this average differed from pressure measured at the costal surface.