Definition: Alveoli devoid of air (not replaced)

Types of Atelectasis:

Resorptive atelectasis

When airways are obstructed there is no further ventilation to the lungs and beyond. In the early stages blood flow continues and gradually the oxygen and Nitrogen get absorbed, resulting in atelectasis.

Relaxation atelectasis

The lung is held close to chest wall because of the negative pressure in the pleural space. Once the negative pressure is lost the lung tends to recoil due to elastic properties and becomes atelectatic. This occurs in patients with pneumothorax and pleural effusion. In this instance, the loss of negative pressure in the pleura permits lung to relax, due to elastic recoil. There is common misconception that atelectasis is due to compression.

Adhesive Atelectasis

Surfactant reduces surface tension and keeps alveoli open. In conditions where there is loss of surfactant the alveoli collapse and become atelectatic. In ARDS this occurs diffusely to both lungs. In pulmonary embolism due to loss of blood flow and lack of  CO2, the integrity of surfactant gets impaired. 

Cicatricial Atelectasis

Alveoli gets trapped in scar and becomes atelectatic in fibrotic disorders.  

Round Atelectasis: An instance where the lung gets trapped by pleural disease and is devoid of air. Classically encountered in Asbestosis.

Signs of Loss of Lung Volume

Generalized 

• Shift of Mediastinum: The trachea and heart gets shifted towards the atelectatic lung.  

• Elevation of Diaphragm: The diaphragm moves up and the normal relationship between left and right side gets altered.

• Drooping of Shoulder  

• Crowding of Ribs: The interspace between ribs is narrower compared to the opposite side. 

Movement of Fissures

• You need a lateral view to appreciate the movement of oblique fissures. Forward movement of oblique fissure in LUL atelectasis. Backward movement in Lower lobe atelectasis.

• Movement of transverse fissure can be recognized in the PA film

Movement of Hilum

• The right hilum is slightly lower than left normally. This relationship will change with lobar atelectasis. 

Compensatory Hyperinflation

• Compensatory hyperinflation as evidenced by increased radiolucency and splaying of vessels can be seen with the normal lobe or opposite lung. 

Alterations in proportion of Left and Right Lung

• The right lung is approximately 55% and left lung 45%. In atelectasis this apportionment will change and can be a clue to recognition of atelectasis. 

Hemithorax asymmetry

In normals right and left henithorax are equal in size. The size of hemithorax will be asymmetrical and smaller on the side of atelectasis.

In a given case one or more of these features will alert you to the presence of atelectasis. 

Etiological considerations.

• It is difficult to ascertain the etiology of atelectasis unless there are other clues to suggest an etiology 
  • Mass or nodes in hilum  Foreign body (Coin in trachea-bronchial tree) Lytic lesion in bone 
• Clinical history and physical findings should help in determining the most probable etiology for a given atelectasis.

Special radiological signs

Bowing Sign 

In LUL atelectasis or following resection, the oblique fissure bows forwards in the lateral view. Bowing sign refers to this feature. 

Double Lesion Sign 

If you encounter atelectasis of RUL and RLL sparing RML, it is difficult to comprehend a single endobronchial lesion to account for both lesions. There has to be two independent endobronchial lesions, hence it was proposed that it is unlikely to be due to primary bronchogenic cancer. The concept is based on knowledge of Anatomy. 

Caution: Do not rule out lung cancer when you see this phenomenon, you just have to find an alternate explanation for the second lesion. 

S curve of Golden

The transverse fissure is "S" shaped. The proximal portion of the fissure is convex because the tumor mass prevents the fissure from moving towards hilum. Whenever you see this shape to a fissure consider a mass causing atelectasis.

Open bronchus sign

When air bronchogram is visible in an atelectatic lung, it implies that there is no airway obstruction. It is more a trapped lung with patent airways.

Inverted comma sign

Do not mistake a Azygous lobe to movement of transverse fissure. Azygous lobe gives the appearance of an inverted comma.