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Effect of diaphragmatic contraction on the action of the canine parasternal intercostals

Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, Brussels, and Chest Service, Erasme University Hospital, Brussels; and Intensive Care Unit, Saint-Pierre University Hospital, Brussels, Belgium

Submitted 21 November 2005 ; accepted in final form 2 March 2006

The inspiratory intercostal muscles enhance the force generated by the diaphragm during lung expansion. However, whether the diaphragm also alters the force developed by the inspiratory intercostals is unknown. Two experiments were performed in dogs to answer the question. In the first experiment, external, cranially oriented forces were applied to the different rib pairs to assess the effect of diaphragmatic contraction on the coupling between the ribs and the lung. The fall in airway opening pressure (Pao) produced by a given force on the ribs was invariably greater during phrenic nerve stimulation than with the diaphragm relaxed. The cranial rib displacement (Xr), however, was 40–50% smaller, thus indicating that the increase in Pao was exclusively the result of the increase in diaphragmatic elastance. In the second experiment, the parasternal intercostal muscle in the fourth interspace was selectively activated, and the effects of diaphragmatic contraction on the Pao and Xr caused by parasternal activation were compared with those observed during the application of external loads on the ribs. Stimulating the phrenic nerves increased the Pao and reduced the Xr produced by the parasternal intercostal, and the magnitudes of the changes were identical to those observed during external rib loading. It is concluded, therefore, that the diaphragm has no significant synergistic or antagonistic effect on the force developed by the parasternal intercostals during breathing. This lack of effect is probably related to the constraint imposed on intercostal muscle length by the ribs and sternum.

respiratory muscles; muscle interaction; chest wall mechanics; rib-lung coupling

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