Social media - specifically the "tactical medicine" space that has exploded on social media in recent years - loves chest seals. In part, this is for much the same reason as tourniquets - they're associated with a product that a company can sell you, and a piece of "gear" that you can carry - and therefore makes for a much sexier topic.
But what this has led to is a confused jumble of opinions from people who seem more confident than they perhaps should be.
Intuitively, chest seals makes sense - there is a hole, so patch it up. The adage "neck to navel" gets bandied around a lot. "It's a sucking chest wound!" advocates will say.
But is it?
To properly understand the distinction between an "open pneumothorax" and a "sucking chest wound" we must first leave behind the humdrum world of gunshots and thoracic trauma, and enter the action-packed world of physics.
First - let's understand how normal breathing works
Normal inspiration - the inward flow of fresh air from the atmosphere into our lungs - is a function of suction. The negative pressure that allows this suction is generated by expanding our thoracic volume. To understand this, you have have to understand Boyle's law.
P1V1 = P2V2 (assuming a fixed temperature)
When we increase our intrathoracic volume (V) from V1 (small) to V2 (large), there must be a proportional decrease in our intrathoracic pressure (P) from P1 (higher) to P2 (lower). This is why pulling back the plunger on a syringe will cause it to draw up fluid. Increasing the size of your chest cavity by contracting your diaphragm sucks in air. Not too complex.
When we expand our intrathoracic volume under normal circumstances, air will enter down this pressure gradient via the one path it can: the trachea. Now - bear with me here, because this will come in useful later - the rate that air can flow through it is determined by another law: Poiseuille's law.
Ignoring most of the terms in that equation for the time being - this shows that flow (Q) is proportional to the fourth power of the radius (r) for the pipe that the flow is occurring through. This makes intuitive sense - the bigger the pipe, the greater the flow. Flow is also inversely proportional to length (l) - the longer the pipe, the lesser the flow.
How is this related to our open pneumothorax?
Enter our penetrating chest wound. The projectile has created a defect in the thoracic wall. Now when our intrathoracic volume expands, air will be sucked down the trachea AND potentially through the new hole.
The only way that pneumothorax will get bigger is if the rate of air entry through it exceeds the rate of air entry down the trachea. So if we go back to our formula - you can see that the main determinants of flow that are relevant are width and length of the defect.
The good news is that most chest wounds are tiny - I've seen a fair number of gunshot wounds in my time, and they're almost always surprisingly small (at least for handguns and their entry wounds, in any case). Shrapnel and stab wounds can be similarly small, further reinforcing the importance of a thorough secondary survey. As far as their size goes, I can even give you exact figures, since it seems people with unusual predilections do perform research on this - my brief search of the literature found articles predominantly authored in Russian, make of that what you will. The maximum radius for entry wounds inflicted by (9mm) handgun rounds to the anterior trunk was about 3mm in one German study [1] (though there are some caveats to this, including the fact that the study was performed on pigs). The adult trachea on the other hand has a radius of about 10mm [2].
But, while our gunshot wound has a narrower width than that of the trachea, it's also shorter. Mean chest wall thickness will vary depending on location, but cadaveric studies show it's in the neighbourhood of 40mm [3]. The trachea, on the other hand, is 120mm in length [2].
If we run the numbers - the flow through the trachea will be many many times greater than that of the defect in the chest wall (all other things being equal).
As you can see, despite being much shorter, the relatively narrower radius of the gunshot wound means that it's very difficult for a chest wound to preferentially entrain air into the pleural space. Don't get too hung up on the numbers, they're there to demonstrate the concept through a worked example, and shouldn't be taken as gospel.
If you solve for X in the equation above to see at what point the width of a chest wound would make flow through the wound (QGSW) exceed that of the trachea (QTrachea) you'll notice the wound has to have a radius of about 7.5mm (15mm diameter). So the critical mass (or indeed, the critical width) that a chest wound need be in order to overcome tracheal flow is when it is ~75% the width of the patient's trachea. Sure enough, if you open up your copy of the Definitive Surgical Trauma Care manual (which I'm sure is by your bed for nighttime reading, as mine is) it does indeed suggest that open pneumothoraces should be considered significant if the wound's diameter is greater than or equal to two thirds that of the patient's trachea, as this can be sufficient to cause air to preferentially be sucked into the open wound as intrathoracic pressure falls during inspiration. It's not often I have the time nor the inclination to start measuring the diameter of patient tracheae in the heat of resuscitation, so perhaps in adult wounds we should use a ballpark figure of 1.5cm for the width of a wound. Obviously in patients with a slimmer chest wall, this number will be smaller, and for the larger amongst us, this number will be greater.
The key defining characteristic really is the "sucking" part. If you can see or hear the movement of air and frothy blood at the site of the injury, then your diagnosis is made. Not only will the pneumothorax become larger and larger as more air is entrained through the wound, there will also be a reduction in flow through the trachea - to the obvious detriment of ventilation and gas exchange. These wounds should be immediately sealed with an occlusive dressing. Commercially available options can be vented or unvented, which we will come to shortly. Improvised dressings should be secured on three sides so as to create a valve, because there is likely to be underlying injury to lung parenchyma, and any bronchopleural fistula that is also contributing to the pneumothorax will cause it to tension if the wound (and therefore the mechanism for air to escape from the chest) is fully occluded.
So what does this mean for me treating open chest injuries?
This brings us back to the distinction between open pneumothoraces (a pneumothorax where there is a communication between the pleural space and the outside world) and a sucking chest wound (a pneumothorax associated with an open wound that causes the pneumothorax to become larger with each breath due to preferential movement of air through the wound into the pleural space).
Putting a chest seal on the latter is a life-saving intervention, putting a chest seal on the former could theoretically convert a stable pneumothorax into a tension - even more so if the patient is positive pressure ventilated.
Interestingly, the Committee on Tactical Combat Casualty Care (CoTCCC) don't seem to make any distinction [4].
"All open and/or sucking chest wounds should be treated by immediately applying a vented chest seal to cover the defect. If a vented chest seal is not available, use a non-vented chest seal".
It does seem as if chest seals that employ a one-way vent as part of their design do reduce the rate of tension pneumothorax occurring as a complication of the seal, at least in animal models [5]. While unvented seals were previously common [6], the use of vented seals is preferred for this very reason. I would argue one should exercise even more caution when only unvented seals are available to hand, especially when sealing wounds that aren't clearly exhibiting suction physiology. We're not going to go down the rabbit hole of the various brands, as all the major players seem to be similarly effective [7].
In summary:
Apply a chest seal if air movement can be seen or heard through the wound or the wound is large enough that air is likely to be preferentially entrained into the pleural space through it. Treatment of suction physiology is potentially life-saving, though risks conversion to tension physiology. Avoid their use in patients receiving positive pressure ventilation, and if there is a deterioration in respiratory function or haemodynamic stability in any patient at any point after application, burp or remove the chest seal and re-evaluate.
References:
[1] Geisenberger D, Große Perdekamp M, Pollak S, et al. Differing sizes of bullet entrance holes in skin of the anterior and posterior trunk. Int J Legal Med. 2022;136(6):1597-1603.
[2] Furlow PW, Mathison DJ. Surgical anatomy of the trachea. Ann Cardiothorac Surg. 2018;7(2):255-60.
[3] Laan DV, Vu TD, Thiels CA, et al. Chest wall thickness and decompression failure: a systematic review and meta-analysis comparing anatomic locations in needle thoracostomy. Injury. 2016;47(4):797-804.
[4] Butler FK, DuBose JJ, Otten EJ, et al. Management of open pneumothorax in tactical combat casualty care: TCCC guidelines change 13-02. J Spec Oper Med. 2013;13(3):81-6.
[5] Kuhlwilm V. The use of chest seals in treating sucking chest wounds: a comparison of existing evidence and guideline recommendations. J Spec Oper Med. 2021;21(1):94-101.
[6] Schauer SG, April MD, Naylor JF, et al. Chest seal placement for penetrating chest wounds by prehospital ground forces in Afghanistan. J Spec Oper Med. 2017;17(3):85-9.
[7] Kotora JG, Henao J, Littlejohn LF, Kircher S. Vented chest seals for prevention of tension pneumothorax in a communicating pneumothorax. J Emerg Med. 2013;45(5):686-94.