I use the term atelectasis frequently when describing chest x-ray or CT scan findings to patients and families.  There often is a look of consternation on their faces, and I need to go further to explain this phenomenon.  I thought it might be helpful to get this term out to more folks who may be reading their x-ray reports and wonder what this word means.  The term atelectasis is derived from Greek words that mean incomplete expansion.

Visualize a sponge.  A sponge has many small air spaces when it is expanded but if you squeeze the sponge and compress the air spaces it appears to be more solid.  This, conceptually, is what happens when lung tissue undergoes atelectasis.  The tiny air spaces in the lung collapse, and that portion of the lung becomes more solid.

Atelectasis may occur in two ways.  The first way is for an obstruction to occur in an airway.  The lung beyond this obstruction collapses when all the air is absorbed.  The blockage of the airway may occur because of mucous plugging, foreign body aspiration or tumor, among others.  An inspection of the airway is needed to determine the nature of the obstruction if the atelectasis is not relieved.

The second way atelectasis can occur is when the lung is compressed when patients develop a pleural effusion (fluid collection in the chest). If the amount of fluid is great enough it compresses and squeezes the lung, creating atelectasis.  Fluid removal and analysis is required in most circumstances.