Air in the Chest

The development of extra-alveolar air in the ICU patient can have serious consequences. Though abnormal air collections often result from intubation or invasive procedures, the vast majority of extra-alveolar air collections are the result of barotrauma. Five to 15% of patients receiving positive end-expiratory pressure ventilation experience some form of barotrauma. Extra-alveolar air can manifest itself as pulmonary interstitial emphysema, pneumothorax, pneumomediastinum, pneumopericardium or subcutaneous air. The chest x-ray is an important tool in the early identification of abnormal air collections.

When searching for lucencies which may represent air on the chest x-ray, the physician must be aware of the Mach effect. The Mach effect is caused by lateral inhibition of light receptors in the eye. The eye enhances the contrast between objects by increasing the brightness of objects near dark borders and decreasing the brightness of dark objects near bright borders. This optical illusion can deceive a person interpreting a chest x-ray into believing that a lucent streak exists when it does not. Look at the figure above. Notice the enhanced border between the different light densities.


1999 by the Rector & Visitors of the University of Virginia