Optoelectronic Plethysmography for Measuring Rib Cage Distortion

The pressure acting on the part of the Rib Cage that is apposed to the costal surface of the lung is quite different from that acting on the part apposed to the diaphragm. The non uniformity of pressure distribution led Agostoni and D’Angelo (1985) to suggest that the rib cage could be usefully regarded as consisting of two compartments mechanically coupled to each other (Agostoni & D’Angelo, 1985; Jiang et al., 1988, Ward, 1992): the pulmonary rib cage (RCp), and the abdominal rib cage (RCa). The magnitude of the coupling determines the resistance to distortion and is an important parameter in the mechanics of breathing. Unitary behaviour of the rib cage was thought to be dictated by rigidity and the restrictive nature of rib articulations and interconnection. Nonetheless, important distortion of the rib cage from its relaxation configuration has been described in asthma (Ringel et al., 1983) quadriplegia (Urmey et al., 1981) and also in health individual during a variety of breathing pattern (quiet breathing, hyperventilation, single inspiration, involuntary breathing acts, such as phrenic nerve stimulation); (Crawford et al., 1983; McCool et al., 1985; Ward et al., 1992; D’Angelo, 1981; Roussos et al., 1977). In summarizing these results Crawford et al., (1983) and more recently McCool et al., (1985) concluded that the maintenance of rib cage shape needs not be attributed to inherent stiffness but may be the consequence of apparently coordinated activity of the different respiratory muscles. Under circumstance such as lung hyperinflation or when mechanical coupling between the upper rib cage (RCp) and the lower rib cage (RCa) is very loose rib cage muscle recruitment is essential to prevent paradoxical (inward) rib cage displacement. (Ward et al., 1992). Moreover the deformation of the chest wall (CW) occurring during hyperventilation and while breathing through a resistance implies that the work of breathing in these conditions is slight larger than that calculated only the basis of the volumepressure diagram. And indeed part of the force exerted by the respiratory muscles is expended to change the shape of the chest wall relative to that occurring at the same lung volume during relaxation (Agostoni & Mognoni 1966). Most of what is known about the kinematics of the chest wall i,e., the thoraco-abdomen compartment comes from studies (Sackner, 1980; Gilbert et al., 1972) using RIP (Respitrace®). However, the RIP method is subject to error, the volume being inferred from cross-sectional area changes. Also, evaluation of the breathing pattern with RIP is reliable only when the rib cage and abdomen behave with a single degree of freedom such as during