Chest wall shape during forced expiratory maneuvers.

Abdominothoracic shape during the forced vital capacity was studied in 10 normal subjects using magnetometers to monitor anteroposterior diameters at the level of the manubrium, xiphoid, and epigastrium, lateral rib cage diameter at the xiphoid level, and vertical motion of the rib cage. Thoracic cross-sectional area change at the xiphoid level was found to lag lung volume change, due to an early paradoxical increase (or lack of change), of lower anteroposterior rib cage diameter. To the extent that the resulting rib cage deformation can influence the pleural pressure gradient, the observed shape changes provide a potential mechanism for early preferential emptying of the upper lobes and later more homogeneous emptying in forced, compared to slow, vital capacity maneuvers. Comparisons of shape changes during Valsalva and abdominal expiratory ("expulsive") maneuvers suggest that lower rib cage deformation may not simply be due to the action of rib cage muscles affecting predominantly the lateral rib cage but rather the results of diaphragmatic activity and the influence of abdominal shape on the lower rib cage.