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This Article Full Text Full Text (PDF) All Versions of this Article: 107/3/734    most recent 00470.2009v1 Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Leduc, D. Articles by De Troyer, A. PubMed PubMed Citation Articles by Leduc, D. Articles by De Troyer, A.

Mechanism of increased inspiratory rib elevation in ascites

Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, and Chest Service, Erasme University Hospital, Brussels, Belgium

Submitted 4 May 2009 ; accepted in final form 10 July 2009

The detrimental effect of ascites on the lung-expanding action of the diaphragm is partly compensated for by an increase in the inspiratory elevation of the ribs, but the mechanism of this increase is uncertain. To identify this mechanism, the effect of ascites on the response of rib 4 to isolated phrenic nerve stimulation was first assessed in four dogs with bilateral pneumothoraces. Stimulation did not produce any axial displacement of the rib (X_r) in the control condition and caused a cranial rib displacement in the presence of ascites. This displacement, however, was small. In a second experiment, the effects of ascites on the pleural pressure swing (P_pl), intercostal activity, and X_r during spontaneous inspiration were measured in eight animals. As the volume of ascites increased from 0 to 200 ml/kg body wt, X_r increased from 3.5 ± 0.5 to 7.5 ± 0.9 mm (P < 0.001), P_pl decreased from –6.4 ± 0.4 to –3.6 ± 0.3 cmH₂0 (P < 0.001), and parasternal intercostal activity increased 61 ± 19% (P < 0.001). The role of the decrease in P_pl in causing the increase in X_r was then separated from that of the increase in intercostal muscle force using the relation between X_r and P_pl during passive lung inflation. The loss in P_pl accounted for two-thirds of the increase in X_r. These observations indicate that 1) the increased inspiratory elevation of the ribs in ascites is not the result of the increase in the rib cage-expanding action of the diaphragm and 2) it is due mostly to the decrease in P_pl and partly to the increase in the force exerted by the parasternal intercostals on the ribs. These observations also suggest, however, that the rib cage expansion caused by ascites makes the parasternal intercostals less effective in pulling the ribs cranially.

chest wall mechanics; respiratory muscles; diaphragm; intercostal muscles