The Nuances of the Pneumothorax

(first published on blog.5minsono.com July 26, 2015)

There are three bedside tests we have to evaluate for pneumothorax: the physical exam, chest X-ray (CXR) and ultrasound (US).  The best one we have is undoubtedly US.  Learning the basics of this exam is fairly easy, with studies showing that it takes a small amount of didactics and hands-on, and has a very high retention rate. ^1-3 That being said, there are a few nuances that you should know before basing your clinical management off of your US.

Oh, and if you need review on how to do the exam, check out this video:

There are 4 signs you can use to help you in the evaluation of a PTX:

Lung sliding

[showhide type=”PTX” more_text=”If you need a review on how lung sliding works, click here” less_text=”Click here to hide text on PTX”]

With the probe in the sagittal orientation, identify the ribs in cross section, and look for a white line underneath them. This white line will always be there, and represents the visceral-parietal pleural interface (if there is no PTX) or just the parietal pleura (if there is a PTX). By merely looking at the line itself you won’t be able to tell if there is a PTX, because it’s appearance looks identical in both a PTX and without a PTX. What you’re evaluating is the back and forth movement of that white line. If you see that white line moving, the visceral and parietal pleura are touching (no PTX).  If that white line is not moving, you may have a PTX.  Why do you always have that same white line even if you do or do not have a PTX?  Well, when the pleura are opposed the distance between the two lines is too small for the machine to pick up their signals individually, but what it can detect is the relative motion between the two.  Now, when you have a PTX, the visceral and parietal pleura will be separated by air, which blocks the sound waves from reaching the visceral pleura.  So you will be seeing the parietal pleura (which is stuck on the ribs) without the motion of the visceral pleura just underneath it.  Even though the visceral pleura might be 1 mm or 100 mm below the parietal pleural there will be a layer of air between them, making that visceral pleura invisible to the ultrasound machine.

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Lung sliding is most useful when you see it. When lung sliding is seen, it has been reported to have a negative predictive value of 100% for ruling out pneumothorax at that interspace.^4,5 When you don’t see lung sliding, it’s not quite as useful.  If you have a 32 year old triathlete that was in an MVC and has a seatbelt sign that has no lung sliding on one side, you can be fairly certain that lack of lung sliding is due to a PTX.  That being said, in most of the patients we will be seeing, the absence of lung sliding might not be good enough to use in clinical practice. The table below shows other conditions that can cause there to be a lack of lung sliding. ^5,6

Where to scan to look for lung sliding.

Most of the current literature teaches that the evaluation for PTX should begin between the 2nd-4th ribs at the mid-clavicular line.⁷ If you have a hemodynamically unstable patient in whom you suspect a PTX as the cause of said hypotension, you can pretty much look anywhere in the anterior chest wall.  A PTX large enough to cause shock is very unlikely to not be identified there.⁸ However, if you want to increase your sensitivity, you might want to look a bit lower.  Let me explain: if you have a patient that isn’t in extremis, you should probably make sure to look at whatever part of the chest is most anterior.  In supine patients, you should start your evaluation around rib spaces 5-8.  One study found that in supine trauma patients, most pneumothoraces were seen in between rib spaces 5-8 bilaterally, not the commonly taught 2nd-4th IC spaces.⁷ Lichtenstein found that 38% of supine patients with PTX had the PTX localized in the lower half of the lung (the rest had it over the entire anterior wall, including the lower half).⁹ Make sure you look at all of the lungs, including laterally and superiorly, since you can only rule out a PTX in the rib space you see lung sliding.

 What about M-mode?

[showhide type=”mmode” more_text=”If you need a review on how m-mode in PTX works, click here” less_text=”Click here to hide text on m-mode”]

M-mode is an alternative way for the ultrasound machine to interpret what it hears.  Instead of playing a real-time image, what it does is it graphs out time, in the x-axis, over movement and depth in the y-axis.  So to get the proper m-mode picture, you’ll place the m-mode marker over the pleural line in an intercostal space. If you get a “stratosphere” or “barcode” sign, it means that the US machine isn’t detecting any movement of the pleural line, and implies PTX.  If you get a “sandy beach” or “seashore” sign, you’re seeing movement of the pleural line, implying the absence of a PTX. That gravely appearance of the “beach” represents movement of the pleural line.  No movement looks like straight lines.

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M-mode (or motion mode), is a technique that previously was taught to be useful in the evaluation of a PTX.  In fact, many publications that speak on the topic report that m-mode is a useful adjunct in the evaluation of a PTX. ^5,10-13 (Update: 7/3/18).  Previously, I was pretty much a hater for using m-mode in the evaluation of a pneumothorax.  So much so, in fact, that I did a study on it.  I sent a quiz to 140 physicians and randomized them to get just the ultrasound clips or the ultrasound clips and the corresponding m-mode image of patients with and without a traumatic pneumothorax.  Turns out if you’re inexperienced (defined as having done <250 total ultrasounds in your life), the addition of m-mode increases your specificity and overall accuracy.  Take that information with a grain of salt, though.  This was essentially a convenience sample of residencies and people that I knew, and is only image interpretation, not image acquisition (which is a completely different skill set). Here’s the link.

[showhide type=”Lung_study” more_text=”If you need a review on the data from the study, click here” less_text=”Click here to hide text on Lung sliding study”]

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B-lines and PTX

[showhide type=”Blines” more_text=”If you need a review on what b-lines are and how they’re generated, click here:” less_text=”Click here to hide text on b-lines”]

B-lines are hyperechoic (white) vertical artifacts that start at the pleural line, extend all the way to the bottom of the screen, and move with respiration (assuming that the pleural line is moving). Whereas it can be normal to see one or two in any particular rib space (especially in the lung bases if we see more than 3, it is considered to be pathologic of increased lung water. We know that they form when there is increased water content, but we’re not 100% sure exactly how they form. The theory is that when you have small amount of water surrounded by air, sound waves bounce around in that droplet of water. Each time they hit the border of the water molecule, they let out a sound that is picked up by the ultrasound transducer.  This happens so fast, and so frequently that it creates a vertical reverberation-type artifact that is translated by the ultrasound machine as a b-line.¹⁵

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B-lines are thought to be generated from the visceral pleura, so it makes sense that visualization of B-lines rules out PTX. ¹⁶  For the most part that is true, but there are 2 things that you should keep in mind when using this clinically.  First, make sure they’re actual b-lines and not comet-tail artifacts.  B-lines extend all the way to the bottom of the screen and move with respiration.  Comet-tail artifacts look similar to b-lines except they don’t extend all the way to the bottom of the screen.  One study found that 84% of patients with a pneumothorax had comet-tail artifacts in the area of a pneumothorax.⁹ Second, although b-lines have been reported to be absent in all pneumothoraces, there is one bit of information that may contradict that.  There have been a few published articles that report b-lines being present in patients with pneumonectomies.  While no definitive explanations exist, there is a prevailing hypothesis that those patients with pneumonectomies have inflammation or other changes within and around the parietal pleura that mimic b-lines ^17,18

 Lung point

The lung point, which is when you have lung sliding and lack of lung sliding in the same rib space, has been reported to be 100% specific for a pneumothorax. That sounds amazing, but as with most things, these findings need to be taken with a grain of salt. The only studies published that looked at the accuracy of the lung point for ruling in a PTX are both by the same author. In one publication, it was reported that out of 43 ICU patients with a PTX, the lung point was 100% specific for a PTX,⁹ and in the other, they looked at 66 ICU patients with known PTX, and also found it to be 100% specific for a PTX. ¹²  That’s all we have on lung point. (There is one other study that implies that the lung point is very accurate, but they include absence of b-lines, lack of lung sliding and lung point as their criteria with no mention of how each of those signs did individually ¹⁷)
There have, however, been a few case reports of false-positives. One study reported a pseudo-lung point on the left side of the chest that was due to the lung coming into contact with the heart. ²⁰ A study by Soldati et al ¹⁹ reported that in trauma patients with suspected PTX, their one false-positive lung point was due to the same issue. Besides the border of the heart and lung, the border between the diaphragm and the lung can also create a false-positive lung point ²¹ Oh, and the sensitivity of the lung point is really bad so it can’t be used to rule out a PTX. ^9,12  Now, I’m not saying that the lung point is useless. I look for it in all of my patients in whom I suspect a PTX.  What I’m saying is that we need more data behind the lung point before we can say that it is 100% specific for a PTX.

 Lung Pulse:

The presence of what has been called the “lung pulse” also has been reported to help rule out a pneumothorax.  This is most helpful when a patient is apneic, or if for whatever reason they’re only ventilating one lung (like in a mainstem intubation). The theory is that if the visceral and parietal pleura are connected (i.e. when there isn’t a PTX) when the heart beats, it will transmit movement through the lung to the visceral parietal pleural interface (VPPI), showing a pulse at the level of the VPPI.²² Physiologically this makes perfect sense, but you also have to take this with a grain of salt, since there’s only one study that has evaluated the accuracy of this.  The study included 15 purposefully main-stemmed patients and to 30 appropriately intubated patients.⁶

 When does ultrasound fail?

If you have a patient with subcutaneous emphysema, all bets are off. Air in the subcutaneous tissue will cause a false pleural line with occasional “b-lines” to be generated above the pleural line, and obscure the actual pleural line.  If you see what looks like a pleural line above the ribs, there is subcutaneous air obscuring your actual pleural line.  Those “b-lines” that are generated above the pleural line have been dubbed “E-lines”.  If you see this finding, your ultrasound is pretty much useless in the eval of subcutaneous emphysema. ²² (In the gif, the highlighted blue line is the subcutaneous air) Also, it should go without saying, but if you there’s a dressing or bandage covering the chest, you won’t be able to use US either.

 Summary:

There are 4 signs you should know to evaluate for a PTX, three rule it out, and one rules it in:

1. Lung sliding – Rules out PTX
2. Lung point – Rules in PTX
3. B-lines – Rules out PTX
4. Lung pulse – Rules out PTX

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References:

1. Bloch AJ, Bloch SA, Secreti L, Prasad NH. A porcine training model for ultrasound diagnosis of pneumothoraces. J Emerg Med. 2011;41:(2)176-81. [pubmed]
2. Abbasi S, Farsi D, Hafezimoghadam P, Fathi M, Zare MA. Accuracy of emergency physician-performed ultrasound in detecting traumatic pneumothorax after a 2-h training course. Eur J Emerg Med. 2013;20:(3)173-7.
3. Krishnan S, Kuhl T, Ahmed W, Togashi K, Ueda K. Efficacy of an online education program for ultrasound diagnosis of pneumothorax. Anesthesiology. 2013;118:(3)715-21.
4. Lichtenstein DA, Menu Y. A bedside ultrasound sign ruling out pneumothorax in the critically ill. Lung sliding. Chest. 1995;108:(5)1345-8.
5. Lichtenstein DA. Ultrasound in the management of thoracic disease. Crit Care Med. 2007;35:(5 Suppl)S250-61.
6. Lichtenstein DA, Lascols N, Prin S, Mezière G. The “lung pulse”;: an early ultrasound sign of complete atelectasis. Intensive Care Med. 2003;29:(12)2187-92.
7. Mennicke M, Gulati K, Oliva I, et al. Anatomical distribution of traumatic pneumothoraces on chest computed tomography: implications for ultrasound screening in the ED. Am J Emerg Med. 2012;30:(7)1025-31.
8. Noble VE. Think ultrasound when evaluating for pneumothorax. J Ultrasound Med. 2012;31:(3)501-4.
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13. Lichtenstein DA, Mezière G, Lascols N, et al. Ultrasound diagnosis of occult pneumothorax. Crit Care Med. 2005;33:(6)1231-8.
14. Adhikari S, Zeger W, Wadman M, Walker R, Lomneth C. Assessment of a human cadaver model for training emergency medicine residents in the ultrasound diagnosis of pneumothorax. Biomed Res Int. 2014;2014:724050.
15. Agricola E, Bove T, Oppizzi M, et al. “Ultrasound comet-tail images”: a marker of pulmonary edema: a comparative study with wedge pressure and extravascular lung water. Chest. 2005;127(5):1690-5.
16. Lichtenstein D, Mezière G, Biderman P, Gepner A. The comet-tail artifact: an ultrasound sign ruling out pneumothorax. Intensive Care Med. 1999;25:(4)383-8.
17. Sperandeo M, Varriale A, Sperandeo G, et al. Characterization of the normal pulmonary surface and pneumonectomy space by reflected ultrasound. J Ultrasound 2011;14:22-7.
18. Cavaliere F, Zamparelli R, Soave MP, Gargaruti R, Scapigliati A, De Paulis S. Ultrasound artifacts mimicking pleural sliding after pneumonectomy. J Clin Anesth. 2014;26:(2)131-5.
19. Soldati G, Testa A, Sher S, Pignataro G, La Sala M, Silveri NG. Occult traumatic pneumothorax: diagnostic accuracy of lung ultrasonography in the emergency department. Chest. 2008;133:(1)204-11.
20. Zhang Z, Chen L. A physiological sign that mimics lung point in critical care ultrasonography. Crit Care. 2015;19:155
21. Gillman LM, Alkadi A, Kirkpatrick AW. The “pseudo-lung point” sign: all focal respiratory coupled alternating pleural patterns are not diagnostic of a pneumothorax. J Trauma. 2009;67:(3)672-3.
22. Volpicelli G. Sonographic diagnosis of pneumothorax. Intensive Care Med. 2011;37:(2)224-32.

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