Regional diaphragmatic length and EMG activity during inspiratory pressure support and CPAP in awake sheep

We examined diaphragmatic mechanics in awake sheep during quiet breathing (QB) and the randomized application of 5, 10, and 15 cmH2O continuous positive airway pressure (CPAP), inspiratory pressure support (IPS), and equal combinations of IPS and CPAP (IPS/CPAP). We measured esophageal, gastric, and transdiaphragmatic (Pdi) pressures and regional length, shortening, and electromyogram (EMG) activity of both costal and crural diaphragmatic segments. Segmental resting length normalized to QB decreased during 15 cmH2O CPAP (costal, 19.2 +/- 3.3%; crural, 7.5 +/- 2.1%; P < 0.05) and during 15/15 cmH2O IPS/CPAP (costal 25.3 +/- 4.8%, crural 9.9 +/- 2.6%; P < 0.05). Only during 15 cmH2O IPS did costal shortening fraction increase (67% QB; P < 0.05). Compared with QB, during 15 cmH2O CPAP, end-tidal CO2 increased 6 Torr, regional EMG activity increased threefold, and Pdi increased 58%. During 15 cmH2O IPS these values decreased 3 Torr, threefold, and 44% respectively, but during IPS/CPAP they remained unchanged. Expiratory gastric pressure (Exp-Pga) reached 4.3 +/- 0.4 cmH2O at 15 cmH2O CPAP, but during IPS/CPAP Exp-Pga was less (maximum of 1.7 +/- 0.4 cmH2O) than at comparable CPAP (P < 0.05), despite the shorter diaphragmatic length. We conclude that: 1) IPS alters the actions of the diaphragm during CPAP, 2) Exp-Pga is poorly coupled to diaphragmatic end-expiratory length, and 3) both IPS and the release of Exp-Pga assist active diaphragmatic shortening.