Effective respiratory system elastance during positive-pressure breathing in supine man.

Two healthy males relaxing supine on a ballistobed were mechanically ventilated at positive end-expiratory pressures (PEEP) from 0 to 19 cmH2O. Pressures at the airway opening, middle esophagus, and stomach were monitored, together with tidal volume (VT) and ballistobed displacement. The effective elastance (i.e., sum of active and passive components) of the respiratory system (E'rs) and its components--abdominal muscle (E'ab), diaphragm (E'di), and rib cage (E'rc)--were calculated. With increasing PEEP, lung compliance increased slightly, E'rc and E'di decreased linearly, and E'ab increased linearly. The combined effective elastance of abdomen and diaphragm (E'ab+di) first decreased and then increased again. The abdomen-diaphragm contribution to VT during mechanical ventilation was approximately half that of spontaneous breathing.