Effects of acute hyperinflation on chest wall mechanics in dogs

We studied chest wall mechanics at functional residual capacity (FRC) and near total lung capacity (TLC) in 14 supine anesthetized and vagotomized dogs. During breathing near TLC compared with FRC, tidal volume decreased (674 +/- 542 vs. 68 +/- 83 ml; P less than 0.025). Both inspiratory changes in gastric pressure (4.5 +/- 2.5 vs. -0.2 +/- 2.0 cmH2O; P less than 0.005) and changes in abdominal cross-sectional area (25 +/- 17 vs. -1.0 +/- 4.2%; P less than 0.001) markedly decreased; they were both often negative during inspiration near TLC. Parasternal intercostal shortening decreased (-3.0 +/- 3.7 vs. -2.0 +/- 2.7%), whereas diaphragmatic shortening decreased slightly more in both costal and crural parts (costal -8.4 +/- 2.9 vs. -4.3 +/- 4.1%, crural -22.8 +/- 13.2 vs. -10.0 +/- 7.5%; P less than 0.05). As a result, the ratio of parasternal to diaphragm shortening increased near TLC (0.176 +/- 0.135 vs. 0.396 +/- 0.340; P less than 0.05). Electromyographic (EMG) activity in the parasternals slightly decreased near TLC, whereas the EMG activity in the costal and crural parts of the diaphragm slightly increased. We conclude that 1) the mechanical outcome of diaphragmatic contraction near TLC is markedly reduced, and 2) the mechanical outcome of parasternal intercostal contraction near TLC is clearly less affected.