Mechanism of the increased rib cage expansion produced by the diaphragm with abdominal support

André De Troyer, Theodore A. Wilson

MEASUREMENTS OF THORACOABDOMINAL motion in subjects with quadriplegia due to traumatic transection of the cervical cord have established that the diaphragm acting alone during breathing in humans produces an inward displacement of the upper portion of the rib cage and an expansion of the lower portion of the rib cage (4, 13, 17, 20, 21). Similarly, in dogs, isolated
contraction of the diaphragm during inspiration displaces the upper ribs in the caudal direction and the lower ribs in the cranial and outward direction (5, 6).

The expiratory action of the diaphragm on the upper rib cage is primarily caused by the fall in pleural pressure, and its inspiratory action on the lower rib cage is the result of two components of the force developed by the muscle (7). The first component, the so-called “insertional force,” is the direct cranial force applied by the muscle fibers of the costal portion of the diaphragm at the level of their insertions into the lower ribs. The second component, denoted the “appositional force,” is the lateral force due to the transmission of abdominal pressure on the lower rib cage through
the diaphragm in the zone of apposition (16). The magnitude of the rib cage displacements produced by the diaphragm in quadriplegic subjects, however, is affected by the elastance of the abdomen and by abdominal pressure. Specifically, when the abdomen in these subjects is given a passive mechanical support by a pneumatic cuff or an elastic binder, the expansion of the lower rib cage during inspiration is increased and the inward displacement of the upper rib cage is reduced (4, 20, 21).

This effect of abdominal support has traditionally been considered to be the result of an increase in the appositional force of the diaphragm. Indeed, because such a support produces an increase in abdominal pressure, the zone of apposition at end-expiration is increased. Moreover, abdominal support also causes an increase in the elastance of the abdomen. Consequently, the descent of the dome of the diaphragm in response to a given muscle activation is decreased, the zone of apposition is larger throughout inspiration, and the rise in abdominal pressure is greater. It would be expected, therefore, that the appositional force would be greater.