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PNEUMOTHORAX

Features of pneumothorax are:

1. Thin pleural line

2. Absent pleural sliding

3. Lung point

4. M mode: barcode sign

Features which make pneumothorax unlikely are:

1. thickened pleural line

2. b lines or comet tails

3. M mode: lung pulse 

One of the first indications for lung US was the diagnosis of pneumothorax (1). In 2005, Blaivas et al (2) showed that US has a much higher sensitivity to supine CXR and since then several meta-analyses have confirmed this (3,4). Lung US has an accuracy approaching CT for pneumothorax diagnosis (5).

Diagnosis of pneumothorax is easiest in the supine patient. Air rises to the surface. Thus placing the transducer at the highest point in the supine patient's chest will make the diagnosis easier. This is usually at the 3rd or 4th rib space parasternal. 

ABSENT PLEURAL SLIDING

Pneumothorax: a collection of air between the parietal and visceral pleurae. As you know, air causes a reverberation artefact, thus causing A lines deep to the parietal pleura. The main difference between normal lung and pneumothorax is pleural sliding: Present in normal lung; absent in pneumothorax. Most pneumothoraces are unilateral and comparison with the contralateral side makes the absence of pleural sliding more obvious.

Pleural sliding looks like shimmering of the pleural line. When there is no sliding, the whole lung may move especially if the patient is dyspnoeic, but there will be no shimmer. The pleural line will be static.

absent pleural sliding

pleural sliding present

BARCODE SIGN

Most guidelines recommend diagnosis of absent pleural sliding in 2D (6). You can save a clip to prove this. M mode of the pleura will show a barcode sign when there is no pleural sliding, saving a still image of this will save storage space. 

To get a good m mode image, once you get a good on axis image of the pleura with lovely A lines, keep your hand still, place the m mode on the pleura. Hand movement can create artefacts which will make the image difficult to interpret.

 M mode: barcode sign (absent pleural sliding)

M mode: seashore sign (pleural sliding present)

The lung is connected to the heart by blood vessels. Thus, cardiac pulsations are transmitted to the lung: causing the lung to pulse with each heart beat. This may be seen in 2D sometimes. But it is seen more clearly in M mode. It is a vertical deflection originating from the perietal pleura which occurs at rhythmic intervals in time with the QRS. 

When there is air at the parietal pleura (pneumothorax) lung pulsations are absent. 

This is a useful sign when it is pleural sliding is difficult to discern. For instance, this occurs in the presence of adhesions, absent ventilation, pleural blebs. All of these conditions still have lung at the parietal pleura and so there will always be a lung pulse.

LUNG PULSE on M mode

LUNG POINT

The other feature specific to pneumothorax is the lung point. Presence of lung point dramatically increases the accuracy of US diagnosis of pneumothorax (7). Lung point is seen in 2D as the transition from absent pleural sliding to present pleural sliding. The lung point occurs because the expanding lung comes into the US field of view with each inspiration. A lung point posterior to the mid axillary line indicates a moderate volume pneumothorax (8).

Lung point: absent pleural sliding (right of screen); pleural sliding (left of screen)

M mode of the lung pulse will be sea-shore sign interspersed with barcode sign.

M mode of lung pulse

DIFFERENTIATING LUNG POINT FROM ADHESIONS

In a comment to an article disputing lung point as specific for pneumothorax, Santos-Silva explains the difference between the transition points from absent to present pleural sliding seen in pneumothorax and adhesions(9). 

In a pneumothorax, as the lung expands with inspiration, the air is displaced and sliding pleura takes its place. So when you're looking at the lung with the typical bat sign view, at a lung point on expiration you will see static pleura and on inspiration pleural sliding will come in to the space occupied by air (absent pleural sliding). 

the pleural sliding displaces the static pleura

Adhesions also have a similar transition from absent pleural sliding to pleural sliding. The difference is that the adhesed lung will remain in place. The normal lung can never expand to displace it. 

the expanding (pleural sliding) lung nudges the adhesed (absent pleural sliding) lung but is unable to displace it

Whenever you assess a patient for lung point try and get them to take a deep breath to get optimum lung expansion. This exaggerates the difference between pneumothorax and adhesions. 

Also, adhesed lung usually has B lines and/or comet tails  (not seen in pneumothorax) and often thickened pleura (only seen in pneumothorax if the parietal pleura is thick for some reason).

ARTEFACTS THAT EXCLUDE A PNEUMOTHORAX

Some artefacts when present preclude the diagnosis of pneumothorax. Water in the lung tissues gives rise to comet tails and B lines (see B lines section). Pleural air rises to the surface and will always be above  water. Thus a pneumothorax will only have A lines. The presence of comet tails and B lines means the presence of lung. Similarly small pleural effusion give rise to the quad sign (see pleural effusion section). Again, pleural air will be higher than fluid and so a quad sign will never be see in the presence of a pneumothorax. 

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REFERENCES

1.Wernecke K, Galanski M, Peters PE, Hansen J. Pneumothorax: evaluation by ultrasound preliminary results. J Thorac Imag. 1987; 2:76-8

2. Blaivas M1, Lyon M, Duggal S. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med. 2005 Sep;12(9):844-9.

3. Alrajab S, Youssef AM, Akkus NI, Caldito G. Pleural ultrasonography versus chest radiography for the diagnosis of pneumothorax: review of the literature and meta-analysis. Crit Care. 2013 Sep 23;17(5):R208.

4. Alrajhi K1, Woo MY2, Vaillancourt C Test characteristics of ultrasonography for the detection of pneumothorax: a systematic review and meta-analysis.Chest. 2012 Mar;141(3):703-708

5. Soldati G, Testa A, Sher S, Pignataro G, La Sala M, Silveri NG (2008) Occult traumatic pneumothorax: diagnostic accuracy of lung ultrasonography in the emergency department. Chest 133:204-211

6. Volpicelli, G., Elbarbary, M., Blaivas, M. et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med 38, 577-591 (2012). https://doi.org/10.1007/s00134-012-2513-4

7. Lichtenstein D, Mezière G, Biderman P et-al. The "lung point": an ultrasound sign specific to pneumothorax. Intensive Care Med. 2001;26 (10): 1434-40.

8. Volpicelli G, Boero E, Sverzellati N, Cardinale L, Busso M, Boccuzzi F, Tullio M, Lamorte A, Stefanone V, Ferrari G, Veltri A, Frascisco MF. Semi-quantification of pneumothorax volume by lung ultrasound. Intensive Care Med. 2014 Oct;40(10):1460-7.

9. Santos-Silva, J., Lichtenstein, D., Tuinman, P.R. et al. The lung point, still a sign specific to pneumothorax. Intensive Care Med 45, 1327–1328 (2019).