Role of the diaphragm in chest wall mechanics

We analyzed three different assumptions about diaphragm function that determine the thoracoabdominal interaction. In the simplest case, the diaphragm is assumed to be a completely flaccid membrane serving only to partition the thorax and the abdominal cavity. In the second case, it is assumed to have a finite tension but to maintain a relatively flat surface at the base of the rib cage (i.e., a negligible zone of apposition). In the general case, it is assumed that the diaphragm has finite tension and its position may vary (i.e., permitting a zone of apposition). These possible modes of behavior are incorporated into a mathematical model of ventilatory system mechanics that distinguishes the diaphragm, lung, abdomen, and rib cage. The significance of these modes is examined with respect to data from human experiments in which gas or liquid is introduced into the pleural or abdominal spaces, causing a volume change (Vep). We show that the Vep effect on the thoracic and abdominal volumes is sensitive to diaphragm mechanics and depends on the nature of the Vep: gastric distension (with water or air) or pneumothorax. Only the behavior of the general model is consistent with physiological observations, especially the distribution of Vep. Our general mathematical model can quantitatively predict this behavior.