Although direct pleural damage and the subsequent communication with ambient or pulmonary air can result from penetrating injury (e.g. rib fracture or accidental needlestick during line placement attempts), extra-alveolar air and pneumothorax from barotrauma during positive-pressure ventilation usually has a different pathway. M.acklln...a.Dd M§ck!Ln..I19.44.) demonstrated that a pressure gradient between the bronchovascular sheath and the alveolus is a necessary precondition for alveolar wall rupture and development of pulmonary interstitial emphysema. Accordingly, during positive-pressure ventilation air dissects into the mediastinum primarily via the bronchovascular sheaths. From there it may penetrate the parietal pleura and produce pneumothorax or dissect via fascial planes into either the subcutaneous tissue or the peritoneum. However, in a recent study the predictive value of mediastinal air for subsequent pneumothorax was only 42 per cent in all intensive care patients studied and 50 per cent in patients with acute respiratory distress syndrome ( G§mm.P.D...,e.t,,§l; 1992).
The term barotrauma suggests pressure as the predominant factor for alveolar rupture. Indeed, several studies suggest a close relationship between high peak airway pressure and occurrence of pneumothorax or mediastinal emphysema (Ga„m.mo.Q. ..el,al 1992). However, in healthy individuals airway pressures above 200 cmH 2O can occur during coughing or maximum efforts without any deleterious consequence. Furthermore, experimentally induced high-pressure barotrauma during positive-pressure ventilation in animals was prevented by strapping the chest with belts ( Dieyfyss..i.et,a/ 1988). Chest belts have the same effect as actively contracting respiratory muscles during coughing. Both mechanisms decrease the chest wall compliance as well as the transpulmonary pressure and overdistension is avoided. Therefore current concepts support the hypothesis that, rather than high airway pressure, high-volume ventilation and alveolar overdistension as well as high transpulmonary pressure are the causative determinants for barotrauma during positive-pressure ventilation. Nevertheless, occurrence of pneumothorax seems to be most closely correlated with underlying disease, specifically with acute respiratory distress syndrome. The previously noted associations between airway pressures and barotrauma might be largely related to the occurrence of high airway pressures in acute respiratory distress syndrome. The incidence of pneumothorax during mechanical ventilation is between 4 and 15 per cent, while the incidence in acute respiratory distress syndrome can be up to 60 per cent ( Gammon...,eLal, 1992).
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