Pneumonia versus atelectasis: the differentiation can be difficult
As mentioned before, hepatization of the lung i.e., lung looking like liver tissue can occur in both pneumonia and atelectasis. Dynamic air bronchograms, when present, point toward pneumonia but static air bronchograms can be seen in both conditions. The differentiation can be very difficult at times and the management should be guided by the clinical picture.
Dynamic air bronchograms are not always as obvious as previously demonstrated. The probe angle needs to be adjusted as necessary to differentiate out of plane lung motion from moving air bronchograms. Other things to note:
- More pleural effusion and less consolidated tissue suggest collapsed lung i.e., relaxation or passive atelectasis.
- More consolidated tissue with less effusion, especially when the patient has fever or signs suggestive of infection points toward pneumonia even in the absence of dynamic air bronchograms.
- Fibrin strands or loculated pleural effusion suggest infectious etiology.
- Increased color flow in the consolidated tissue favors pneumonia while little to no flow suggests atelectasis. Low flow in atelectasis is possibly due to compression as well as ischemic pulmonary vasoconstriction.
Here is another example of pneumonia with dynamic air bronchograms that require careful observation. Color flow is overall increased though there is some interference from tissue motion while the patient is breathing rapidly.
Also note that small subpleural consolidations do not always show typical dynamic air bronchogram pattern. For example, this lung ultrasound image obtained with the straight linear array probe (5–13 MHz) over right anterior chest demonstrates lung sliding and a small subpleural hypoechoic area with ragged margin separating it from the surrounding normal lung. This is described as the “shred sign” because of its distinctive irregular boundary with the normal lung.