RIB CAGE DEFORMATION DURING STATIC INSPIRATORY EFFORTS

Patterns of rib cage (RC) deformation were studied in six normal subjects during moderate static inspiratory efforts such that esophageal pressure (Pes) as an index of transthoracic pressure fell to between -30 and -60 cmH2O during each maneuver. At lung volumes below 50% inspiratory capacity (IC), static inspiratory efforts deformed RC to a more elliptical shape; RC anteroposterior (AP) diameter became smaller and RC lateral diameter became larger. However, at high lung volumes (>50% IC) the opposite change in RC dimensions occurred despite similar changes in Pes, i.e., the RC became more circular. These differences in RC deformation did not appear to be a passive consequence of increased lung volume because the RC could be voluntarily deformed to a more circular shape at low lung volumen when subjects performed static inspiratory efforts mainly with their intercostal and accessory muscles rather than their diaphragm as judged by a smaller change in transdiaphragmatic pressure for the same Pes; or subjects statically contracted their diaphragm with it held in a relatively flattened configuration as assessed by a large abdominal AP dimension. It is suggested that deformation of the RC during static inspiratory efforts is not as predictable as has previously been suggested but depends on the pattern of contraction and configuration of the respiratory muscles.