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Eur RespJ 1988, 1 495-497 EDITORIAL Clinical significance of abnormal rib cage-abdominal motion J.W. Fitting* Our understanding of respiratory movements of the a certain extent from chest wall motion: the rib cage rib cage and abdomen has gained depth from a number inspiratory muscles (parasternal intercostals, scalenes, of recent studies on chest wall mechanics, enabled by stemocleidomastoids), the diaphragm, and the abdom­ the development of methods of measurement. This article inal muscles. Some of the effects produced by their evaluates how abnormal rib cage-abdominal motion can contraction are easily understood. Globally, the action be interpreted by the clinician in the light of the present of the rib cage inspiratory muscles is to lift and expand knowledge. the rib cage. The action of the abdominal muscles is to reduce the dimensions of the abdomen and of the lower part of the rib cage to which they are Measurement principles attached. The relationship between diaphragrnatic contraction A major conceptual breakthrough occurred in 1967 and rib cage-abdominal motion is far more complex. when KoNNo and MEAD proposed that, under the con­ This is due to the fact that the upper part of the dition of a fixed spinal position, the chest wall can abdomen is located within the rib cage. In this region, be considered as a system with two degrees of freedom thediaphragmatic fibres are arranged longitudinally and [1]. They stated that any change in lung volume is are apposed to the lower rib cage, hence the name accommodated by the sum of the changes in dimen­ of "area of apposition" of the diaphragm [5]. There­ sions of the rib cage and of the anterior abdominal fore, the abdominal cavity is limited by three mobile wall. When the airways are closed, the chest wall structures: the dome of the diaphragm, the area of becomes a system with a single degree of freedom, apposition of the diaphragm to the lower rib cage, so that changes in dimensions of the rib cage and and the anterolateral abdominal wall. One action of abdominal wall are equal and opposite in direction. the diaphragm, pushing upon the poorly compressible Rib cage and abdominal motion can be measured abdominal contents, is to increase abdominal pressure by two methods. As originally described, magnetometry and to expand the anterolateral abdominal wall. By can be used to measure the anteroposterior diameter of the same action, however, the diaphragm also expands the rib cage and abdomen. Respiratory inductance the lower part of the rib cage, the increment in plethysmography is now more widely used to measure abdominal pressure being transmitted through the area the changes in cross-sectional area of the rib cage of apposition. In addition, the diaphragm expands the and abdomen [2]. The signals of either method are lower rib cage by pulling directly on its costal usually displayed on an X-Y plot, with the rib cage insertions. dimension on the ordinate and the abdominal dimen­ When the diaphragm contracts, its dome moves sion on the abscissa, as in the Konno-Mcad diagram. caudally, causing an inward displacement of one part In reality, it has been shown that the chest wall, of the abdominal container. This is accommodated by in particular the rib cage, possesses a greater degree an outward displacement of the other two mobile of freedom [3, 4]. Nevertheless, the model of two structures, the lower rib cage and the anterolateral degrees of freedom has allowed deeper insight into abdominal wall, in proportions which vary according chest wall mechanics and is a practical tool for clinical to the compliance of each of these structures. It interpretation of respiratory movements. follows that the movements of the diaphragm cannot be assessed accurately from those of the abdominal wall alone. An inspiration with considerable shortening Action of respiratory muscles of the diaphragm can be performed without change in abdominal dimension. Furthermore, an isometric con­ The respiratory muscles can be subdivided into traction of the diaphragm is associated with an inward three groups, the actions of which can be inferred to displacement of the abdominal wall [6, 7]. This complex relationship does not permit simple interpretation of rib cage-abdominal mot.ion and must be considered •Division de Pnewnologic, Departement de Mcdecine Interne, CHUV, by the clinician when observing the respiratory move­ CH-1011 Lausanne, Switzerland. ments of his or her patient. 496 J.W. FITTING Abnormal motion patterns the lower rib cage. Moreover, the flattening of the diaphragm reduces the expanding force exerted by its Nevertheless, the normal pattern of rib cage and costal insertions and may even provoke an inward abdominal motion during breathing is well established movement of the lower lateral rib cage (Hoover's and any alteration of this pattern can be qualitatively sign). On the other hand, the rib cage inspiratory interpreted. Normally, both compartments expand dur­ muscles lift and expand the upper rib cage. ing inspiration in quiet breathing. The simplest The contribution of rib cage and abdomen to tidal abnormal pattern is represented by a constant para­ volume varies little in normal subjects breathing doxical movement of one compartment. Thus, a quietly. This variability in compartmental contribution paradox of the upper part of the rib cage (inward to tidal volume can be expressed by the standard displacement during inspiration) is the consequence deviation of RCNT when motion is quantified [14). of severe weakness or paralysis of the rib cage Under some circumstances, the contribution of rib cage inspiratory muscles. It is observed in quadriplegic and abdomen may change markedly from breath to patients breathing with the diaphragm, either spon­ breath. Thus, clear rib cage predominance may alternate taneously or with phrenic pacing [8, 9]. Conversely, a with clear abdominal predominance, the extreme constant abdominal paradox is observed in severe pattern being the onset of transient paradox of one weakness or paralysis of the diaphragm. In case of or the other compartment. This pattern has been obser­ paralysis, it is produced by a paradoxical movement ved in normal subjects breathing against high loads, of the diaphragm which is stretched by the fall in during protocols inducing inspiratory muscle fatigue: pleural pressure. Otherwise, an abdominal paradox may whilst the target mouth pressure was well maintained, manifest a weak diaphragmatic con-traction, which is wide variations in gastric pressure developed, as well not necessarily associated with diaphragmatic elonga­ as in rib cage and abdominal movements [15). These tion. The abdominal paradox should be looked for in subjects showed two abnormalities: a transient abdomi­ the supine position, since abdominal muscles are often nal paradox, and a cyclic alternation of breaths with recruited during expiration in the upright position and predominant rib cage or abdominal motion which may therefore mask the paradox. A less pronounced was termed "respiratory alternans". Similar abnormal abnormality is manifested by a departure from the patterns were observed in a series of patients tmder­ normal partition between rib cage and abdominal going a weaning trial from mechanical ventilation. motion, without paradox. Abdominal paradox and respiratory altemans, as judged In the seated position compared to supine the activity by inspection and palpation, developed in the patients of rib cage inspiratory muscles is higher [10], the with unsuccessful trials. Such patients showed signs compliance of the rib cage is higher, and the comp­ of respiratory muscle fatigue, manifested by a fall in lianceof the abdomen is lower [11). As a consequence, the high/low ratio of diaphragmatic EMG [16). It has rib cage expansion predominates during quiet breathing been proposed that respiratory alternans may be a in the seated position and abdominal expansion strategy to cope with respiratory muscle fatigue, predominates in the supine position. The partition bet­ allowing the diaphragm and the rib cage muscles to ween rib cage (RC) and abdominal (AB) motion can recover partially in alternation [15). be quantified by magnetometry or respiratory induc­ Such a concept implies that fatigue can develop tance plethysmography and is usually expressed by separately in different respiratory muscles. This has the index, recently been proven in normal subjects breathing RCNT=RC/(RC+AB) whereVT=ridal volume ag-ainst high loads with different ribcage-abdominal Normally, the rib cage represents approximately 70% patterns: EMG signs of fatigue appeared in the dia­ of total chest wall expansion in the seated position and phragm, or in the parasternal intercostals and sterno­ 25% in the supine position, for both sexes [12). cleidomastoids, according to the pattern of recruitment Diaphragmatic shortening cannot be quantitatively ofthesemuscles [17]. However, two other recent studies, assessed from abdominal motion, but a smaller than in which abnormal rib cage and abdominal motion normal abdominal movement (increased RCNT) implies were quantified with several indices, suggest that res­ some degree of diaphragmatic dysfunction and/or in­ piratory alternans and paradoxical motion may not creased recruitment of rib cage muscles. This is exem­ necessarily be related to fatigue [14, 18]. In normal plified by patients with chronic obstructive pulmonary subjects breathing against inspiratory resistances, an disease (COPD) who may show a predominant motion abdominal paradox and an increased variability in of the rib cage in the supine position [2, 13]. They compartmental contribution to VT was present with manifest a dysfunction of the diaphragm, which is high (fatiguing) loads, but also to a lesser degree shortened and flattened by hyperinflation. Roughly, the with low (nonfatiguing) loads. The abnormal pattern more the rib cage predominates, the less efficient is appeared early and did not worsen with time during the diaphragm. Note that, in this case, the rib cage is the fatiguing runs. The authors conclude that abnormal affected differently by the failing diaphragm and by rib cage and abdominal motion is related to the load the extra recruitment of rib cage muscles.

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