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Pronounced "at-el-ECK-tah-sis." You are likely to hear this one when your doctor is discussing a chest xray. This refers to partial collapse of a part of the lung. This happens when there is plugging (usually with thick mucus) of a bronchial tube. If air cannot get into and out of the section of lung tissue supplied by a bronchial tube, it will rapidly collapse.
This temporary and basically harmless phenomenon is due to the fact that oxygen makes up 20% of the air we breathe. Oxygen is very rapidly absorbed into the bloodstream. A pocket of lung tissue that is closed off from the outside air, usually by a mucus plug, will thus rapidly lose the volume of the oxygen as it is absorbed into the bloodstream. This volume loss causes collapse.
On a chest xray, the collapsed segment will naturally appear more dense than the surrounding airy lung tissue. This appearance may mimic pneumonia, but usually we can differentiate the two based on the clinical setting.
Atelectasis is most often observed in children having an asthma attack. This xray finding does sometimes lead to the overdiagnosis of pneumonia in these patients, especially in the emergency room setting.
The segmental collapse of atelectasis of course looks completely different from the generalized collapse of the much more serious pneumothorax.