Cranio-caudal rib cage distortion with increasing inspiratory airflow in man.

Changes in anteroposterior and transverse diameters of the chest wall, in cranial and caudal esophageal pressure (delta Pes), and diaphragm (Adi) and cranial intercostal muscle activity (Aic) were measured in supine and seated subjects during relaxation and full inspirations from FRC at constant slow or high flow rates. No distortion of the rib cage occurred during relaxation of slow inspirations. During fast inspirations cranial part of rib cage expanded relatively more than caudal one because of greater Aic for any level of Adi. Rib cage distortion during fast inspirations, as well as the occurring between seated and supine posture, was directionally consistent with flow or posture related redistribution of apicobasal lung expansion, suggesting that changes in rib cage shape affect regional lung expansion also in man. During relaxation or slow inspirations cranio-caudal difference in delta Pes (delta Pc-c) was essentially nil over all the inspiratory capacity. During fast inspirations delta Pc-c was negative (seated posture) or nil (supine posture) for small lung volume increments, positive for large volume increments. Delta Pc-c dependence on lung volume is compatible with the response of a model consisting of two parallel compartments with different degree of expansion at FRC and equal volume dependent mechanical properties.