Action of the isolated canine diaphragm on the lower ribs at high lung volumes

Key points: Subjects with chronic obstructive pulmonary disease and hyperinflation commonly have an inward displacement of the lateral walls of the lower rib cage during inspiration. This paradoxical displacement, conventionally called 'Hoover's sign', is traditionally attributed to the pull by radially oriented diaphragmatic muscle fibres. In this study in anaesthetized dogs, we measured the displacement of the lower ribs during isolated spontaneous diaphragm contraction at different lung volumes, and determined the separate effects on rib displacement of pleural pressure and of the force exerted by the diaphragm. The results show that diaphragm contraction at low lung volumes causes an inspiratory displacement of the lower ribs, but this is progressively reversed into an expiratory displacement as lung volume increases. However, the effect of the force exerted by the diaphragm on the ribs remains inspiratory at all lung volumes. These observations suggest that Hoover's sign is usually caused by the dominant expiratory effect of pleural pressure on the lower ribs, rather than an inward pull from the diaphragm. The normal diaphragm has an inspiratory action on the lower ribs, but subjects with chronic obstructive pulmonary disease commonly have an inward displacement of the lateral portions of the lower rib cage during inspiration. This paradoxical displacement, conventionally called 'Hoover's sign', has traditionally been attributed to the direct action of radially oriented diaphragmatic muscle fibres. In the present study, the inspiratory intercostal muscles in all interspaces in anaesthetized dogs were severed so that the diaphragm was the only muscle active during inspiration. The displacements of the lower ribs along the craniocaudal and laterolateral axes and the changes in pleural pressure ({increment}Ppl) and transdiaphragmatic pressure were measured during occluded breaths and mechanical ventilation at different lung volumes between functional residual capacity (FRC) and total lung capacity. From these data, the separate effects on rib displacement of {increment}Ppl and of the force exerted by the diaphragm on the ribs were determined. Isolated spontaneous diaphragm contraction at FRC displaced the lower ribs cranially and outward, but this motion was progressively reversed into a caudal and inward motion as lung volume increased. However, although the force exerted by the diaphragm on the ribs decreased with increasing volume, it continued to displace the ribs cranially and outward. These observations suggest that Hoover's sign is usually caused by the decrease in the zone of apposition and, thus, by the dominant effect of {increment}Ppl on the lower ribs, rather than an inward pull from the diaphragm.SCOPUS: ar.jFLWIN