Thoracoabdominal mechanics during relaxed and forced vital capacity.

Thoracoabdominal configuration, intrathoracic (esophageal), intra-abdominal (gastric), and transdiaphragmatic pressures were studied in six normal upright subjects during relaxed (RVC) and forced vital capacity (FVC). Chest wall configuration showed substantial departure from its relaxation characteristics during FVC. Paradoxical (outward) movement was recorded for a low lateral diameter of the rib cage (on the costal margin) at high volume during RVC and during most of FVC, while the other rib cage dimensions were decreasing. Transdiaphragmatic pressure was positive during most of the FVC, particularly toward RV, reflecting active contraction of the diaphragm. We conclude that diaphragmatic activity modulates forced expiration and that the chest wall may influence the FVC maneuver.