Effects of the insertional and appositional forces of the canine diaphragm on the lower ribs

Authors

  • Theodore A. Wilson,

    1. Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN 55455, USA
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  • André De Troyer

    1. Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, 1070 Brussels, Belgium
    2. Chest Service, Erasme University Hospital, 1070 Brussels, Belgium
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A. De Troyer: Chest Service, Erasme University Hospital, Route de Lennik, 808, 1070 Brussels, Belgium. Email: a_detroyer@yahoo.fr

Key points

  • • Although the diaphragm causes a fall in pleural pressure, it expands the lower portion of the ribcage when it contracts.
  • • This action is conventionally considered to be the result of two mechanisms, namely, the force applied by the muscle fibres on the ribs into which they insert (insertional force) and the transmission of abdominal pressure through the zone of apposition (appositional force).
  • • In this study in anaesthetized dogs, we assessed the magnitude of the force exerted by the diaphragm on the lower ribs and the relative contributions of the insertional and appositional components.
  • • The results show that, per unit pressure, the inspiratory effect of the diaphragmatic force on the lower ribs is equal to the expiratory effect of pleural pressure and that the insertional force contributes 60% of that inspiratory effect.

Abstract  The diaphragm has an inspiratory action on the lower ribs, and current conventional wisdom maintains that this action is the result of two mechanisms, namely, the force applied by the muscle fibres on the ribs into which they insert (insertional force) and the transmission of abdominal pressure through the zone of apposition (appositional force). The magnitude of the diaphragmatic force and the relative contributions of the insertional and appositional components, however, are unknown. To assess these forces, the inspiratory intercostal muscles in all interspaces were severed in anaesthetized dogs, so that the diaphragm was the only muscle active during inspiration, and the displacements of the lower ribs along the craniocaudal and laterolateral axes were measured during quiet breathing, during occluded breaths and during passive lung inflation. From these data, the isolated effects of pleural pressure and transdiaphragmatic pressure on rib displacement were determined. Then external forces were applied to the ribs in the cranial and the lateral direction to simulate, respectively, the effects of the insertional and appositional forces, and the rib trajectories for these external forces were used as the basis for a vector analysis to obtain the relative magnitudes of the insertional and appositional contributions to the rib displacement driven by transdiaphragmatic pressure. The results show that, per unit pressure, the inspiratory effect of the diaphragmatic force on the lower ribs is equal to the expiratory effect of pleural pressure, and that the insertional force contributes 60% of that inspiratory effect.

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