Tissue emphysema

Under conditions of positive alveolar or positive airway pressure, i.e. positive-pressure ventilation or dynamic hyperinflation, air can also dissect into surrounding tissues. This results in pulmonary interstitial emphysema (air cysts), mediastinal emphysema (pneumomediastinum), subcutaneous emphysema, pneumopericardium, pneumoperitoneum, or even systemic air embolism.

Harmful or life-threatening effects of extra-alveolar air depend mostly on their influence on global or regional circulation. Subcutaneous or mediastinal air usually does not compromise local blood flow because the loose connecting tissue is very compliant and allows further dissection of the air along fascial planes. Thus small amounts of systemic air, which might cause cerebral or myocardial air embolism, are more likely to be hazardous than a huge subcutaneous emphysema. Although sometimes impressive, subcutaneous emphysema and pneumomediastinum are not dangerous but, rather, are markers for an apparent air leak. In both cases the clinician's suspicion should be focused on the possibility of an overlooked or threatening pneumothorax. Additionally, if air has dissected into the peritoneum, a visceral perforation must be excluded. The incidence of mediastinal emphysema in positive-pressure ventilation has been reported as up to 21 per cent ( Gammone.?...

al 1992). Subcutaneous emphysema is a common complication in mechanically ventilated patients with penetrating or blunt chest trauma. It does not usually occur alone but subsequent to pneumothorax or mediastinal emphysema.

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