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Arterial gas embolism: the injury that starts with a held breath
Why the first rule of diving is never to stop breathing
Of all the rules of diving, there is exactly one that allows no nuance, no exceptions, and no personal judgment: never stop breathing. The reason is that just a few meters of ascent with the airway closed are enough to cause the fastest and most serious injury in the sport.
In this article you'll see how gas expansion tears lung tissue, the four distinct conditions that overexpansion can produce, how arterial gas embolism presents on reaching the surface, how it differs from decompression sickness, and why its prevention is almost entirely a matter of technique.
This is the diving injury that leaves the least room for time: it appears within minutes and can threaten life immediately. It is also, in the vast majority of cases, preventable with a behavior taught on the first day of any course. This article is educational material: it does not replace specific dive first-aid training or medical judgment, and if you suspect this injury, the correct action is to activate emergency services and consult a hyperbaric medicine service.
Boyle's Law versus the lung
The mechanism is nothing sophisticated, and that's exactly the unsettling part.
Underwater you breathe gas at ambient pressure. If you fill your lungs at ten meters and ascend to the surface without exhaling, that gas doubles in volume under Boyle's Law, just as you saw in the article on the gas laws. The lung is elastic, but its elasticity has a limit, and once internal pressure exceeds what the tissue can tolerate, the alveoli rupture.
The most surprising fact is the scale involved. A deep dive isn't necessary: the relative pressure changes are greater near the surface, which is why pulmonary overexpansion can occur from ascending just a few meters with the airway closed. The final stretch of the ascent, the part that seems harmless, is precisely the most demanding.
There are two ways to get there. The first is voluntarily holding your breath, something that almost always happens as a reflex: during an unplanned rapid ascent, in response to a scare, when losing the regulator, or during a poorly executed exercise. The second is air trapping: an area of the lung that, due to a pre-existing respiratory condition or a localized obstruction, doesn't empty its gas at the same rate as the rest even though the person is breathing normally.
When the alveolus ruptures, the gas escapes toward whatever path it finds, and that path determines which of the four possible conditions results.
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