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ANTERIOR CHEST - PNEUMOTHORAX

ABSENT PLEURAL SLIDING

Absent pleural sliding on B mode and the absence of comet tails or B lines is adequate for the diagnosis of pneumothorax (1) in trauma patients. There are no B lines because fluid sinks below air due to gravity and the US cannot penetrate beyond the pneumothorax air. 

M MODE

The M mode of a pneumothorax show the barcode or stratosphere sign: static horizontal lines above and below the pleural line. 

In a trauma, B mode assessment (thin pleural line, no sliding, no comet tails or B lines) alone is adequate for the diagnosis of pneumothorax; because, in the deteriorating patient, you will not have the luxury of time and also trauma patients tend to be young and previously well. Avila et al (3) showed that adding m mode to the pleural assessment did not significantly improve accuracy for doctors who had performed >250 such assessments. However, it did help the novice improve their accuracy. 

But I would stress: the m mode should correlate with the live B mode image. Do not diagnose pneumothorax on m mode alone. 

There are too many artefacts which occur with m mode. Here are a few examples:

M mode showing barcode like sign due to off axis scanning and too little gain.

M mode on rib showing a barcode like sign

M mode of a pneumothorax showing barcode sign, but the hand movement artefact on either side could be mistaken for seashore sign or lung pulse. You know it's artefact because the disturbance occurs above and below the bright white pleural line. Anything originating from the lung originates at the pleural line. 

BILATERAL ABSENT PLEURAL SLIDING

Sometimes, there maybe bilateral pneumothoraces and it can be difficult to be confident about diagnosing absent pleural sliding. You can do one of two things in this case. The more professional one will be to translate the transducer laterally until you find the lung point. This is where normal pleural sliding comes into view in an area of no lung sliding with inspiration. Finding a lung point is 100% specific for a pneumothorax (4). The other way is to US your own lungs and compare normal sliding to what you see on the patient. But this may look a bit weird in the middle of a trauma!

LUNG POINT

The lung point allows you to quantify the size of a pneumothorax. A lung point lateral to the mid-axillary line is more likely with a moderate sized pneumothorax (5).

A lung point is created when the pneumothorax air is displaced by the aerated lung expansion on inspiration. Theoretically, adhesions, bullae etc may give the appearance of a lung point at the transition from diseased lung to sliding lung (6,7). However, in this case, the non sliding lung will not be displaced with inspiration. It will remain where it is (8). Further the diseased non sliding lung may have a thickened pleura and B lines/ comet tails.  

THIN PLEURAL LINE

Remember, the pleural line needs to be thin for pneumothorax. This is because the bright white pleural line seen on US in the presence of a pneumothorax is only the parietal pleura and the underlying air. So unless the patient happens to have a thickened parietal pleura from malignancy or adhesions, the pleural line should be very thin. 

Adhesions: Thickened pleural line and vertical comet tails

If the pleura remains thick despite angling, consider unventilated lung or adhesions and look closer for B lines/ comet tails. If in doubt do m mode: in a pneumothorax there will be barcode sign, with adhesions/ unventilated lung there will be a lung pulse as long as the patient has cardiac contractility.

SUBCUTANEOUS EMPHYSEMA

Subcutaneous emphysema is common with traumatic pneumothorax and if you're not careful, you will misdiagnose the subcut air as absent pleural sliding. Avoid making a mistake by always looking for the two hyperechoic ribs and shadows with the pleura deep to the ribs. In subcut emphysema, the static bright white line will be above the ribs. 

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REFERENCES

1. Volpicelli G, Elbarbary M, Blaivas M, Lichtenstein DA, Mathis G, Kirkpatrick AW, Melniker L, Gargani L, Noble VE, Via G, Dean A, Tsung JW, Soldati G, Copetti R, Bouhemad B, Reissig A, Agricola E, Rouby JJ, Arbelot C, Liteplo A, Sargsyan A, Silva F, Hoppmann R, Breitkreutz R, Seibel A, Neri L, Storti E, Petrovic T; International Liaison Committee on Lung Ultrasound (ILC-LUS) for International Consensus Conference on Lung Ultrasound (ICC-LUS). International evidence-based recommendations for point-of-care lung ultrasound

2. Lichtenstein, DA, Menu, Y. A bedside ultrasound sign ruling out pneumothorax in the critically ill: lung sliding. Chest. 1995; 108: 1345– 8.

3. Avila J, Smith B, Mead T, Jurma D, Dawson M, Mallin M, Dugan A. Does the Addition of M-Mode to B-Mode Ultrasound Increase the Accuracy of Identification of Lung Sliding in Traumatic Pneumothoraces? J Ultrasound Med. 2018 Nov;37(11):2681-2687.

4. Lichtenstein D, Mezière G, Biderman P, Gepner A. The "lung point": an ultrasound sign specific to pneumothorax. Intensive Care Med. 2000 Oct;26(10):1434-40

5. 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.

6. Aziz SG, Patel BB, Ie SR, Rubio ER. The Lung Point Sign, not Pathognomonic of a Pneumothorax. Ultrasound Q. 2016 Sep;32(3):277-9.

7. Steenvoorden TS, Hilderink B, Elbers PWG, Tuinman PR. Lung point in the absence of pneumothorax. Intensive Care Med. 2018 Aug;44(8):1329-1330.

8. Santos-Silva J, Lichtenstein D, Tuinman PR, Elbers PWG. The lung point, still a sign specific to pneumothorax. Intensive Care Med. 2019 Sep;45(9):1327-1328.