Let’s look at interpretation of lung ultrasound.
1) Lung Sliding
Lung sliding describes the shimmering movement of the pleura, synchronous with respiration (Ants Marching). The short vertical artifacts that originate from the pleural line and fade rapidly are known as comet tails or Z lines. They are present in normal lung.
Lung sliding occurs due to movement of the parietal pleura against visceral pleura during respiration, when both surfaces are in apposition. It will also be seen in pathologic conditions that do not affect ventilation.
M Mode demonstrating the sea shore sign, seen with normal lung sliding. The lower part of the image resembles sand, while the upper part resembles the sea and horizon.
2) Absent Lung Sliding
Lung sliding is absent in pneumothorax, where the visceral and parietal pleura are separated. Pneumothorax is best detected by scanning the anterior chest, with the patient in the supine position, as air in the pleural space rises to the anterior part of the chest.
The size of the pneumothorax can be estimated by looking for the lung point. A lung point seen at the base of the lung would indicate a larger pneumothorax than if the lung point were to be seen in the lateral aspect of the chest.
Lung sliding is also absent or reduced when visceral pleura does not slide against parietal pleura. This may occur in the following scenarios:
- Apnoea
- Endobronchial Intubation – Absent lung sliding in non ventilated lung
- Collapsed Lung – Atelectasis or Airway Obstruction
- Inflammtory Adherences
- Lung Overdistention, Severe Bullous Disease
In the absence of lung sliding, imaging with M-Mode gives rise to an image resembling a barcode, as shown above.
3) Lung Pulse
Subtle rhythmic movement of lung parenchyma caused transmission of heartbeat. Lung pulse can be seen in normal lung, especially in the left hemothorax. It is absent or reduced when there is lung hyperinflation or bullous disease.
It is absent in pneumothorax.
Lung pulse is present and increased when there is increased lung density (eg-atelectasis, consolidation) as the collapsed or fluid/pus filled lungs will transmit cardiac pulsation to the pleura better than an air filled lung.
Identification of lung pulse without lung sliding is indicative of lack of ventilation, which may occur in apnea, endobronchial intubation or airway obstruction (eg mucous plugging / foreign body) causing atelectasis.
Key points:
Absent lung sliding = Suspect pneumothorax >>> Look for a lung point to confirm pneumothorax.
Absent lung sliding but lung pulse present >>> Unlikely to be pneumothorax. Consider atelectasis.
4) Lung Point
Lung Point is highly specific for pneumothorax, and is almost 100% diagnostic for the presence of pneumothorax. It is described as the presence of lung sliding on one side of the image and absence of lung sliding on the other. It occurs at the junction of pneumothorax and normally aerated lung.
It’s presence RULE’s IN a pneumothorax.
This sign should be actively sought when absent lung sliding is seen. In a very large pneumothorax, with complete collapse of the lung, the lung point sign may not be present. Further imaging modalities should be used to make a diagnosis.
Decision Tree For Pneumothorax. Adapted from Lichtenstein et al.