Not an Embolism, Just a Bubble
The bubble the syringe left in my thigh was medication, not air — but it got me wondering what would actually happen if it had been air. Turns out that's a more interesting question than I expected.
When I wrote about the syringe leaving a hard little bubble in my thigh, that was the medication itself, sitting too close to the surface rather than spreading into the fat where it was supposed to go. Shallow injection, not a shallow problem — it dispersed on its own and left nothing but an itch.
But it got me thinking about a slightly different question. What if it hadn’t been fluid. What if there’d actually been air in there too.
The thing I pictured, half-formed, was an air embolism. Air in a vein, travelling, blocking something it really shouldn’t. That’s a genuine medical emergency, and it’s almost certainly why “air bubble” sets off an alarm for most people, me included.
It’s also not really the relevant scenario for a subcutaneous injection. An embolism is about air delivered straight into a blood vessel, as a bolus, where it can travel with the circulation. Subcutaneous means under the skin, into the fatty layer, not into a vein. There’s no free ride into circulation waiting for it there.
What actually happens to a small pocket of trapped air under the skin is considerably less dramatic than what happened in my own imagination. It occupies a tiny space between the fat cells, which would look much like the lump I actually had, just for a different reason. From there, the gases gradually diffuse out into the surrounding tissue and eventually into the bloodstream in dissolved form — not as a travelling bubble, just ordinary gas exchange — where they get carried off and breathed out same as any other gas in the body. The pocket shrinks as that happens, anywhere from minutes to a few hours depending on size and exactly where it’s sitting. Any crackling or pressure you might feel is just tissue being stretched or air moving between layers, not anything going wrong.
So it wouldn’t have been a different outcome to what I actually had. Just a different mechanism getting there — gas diffusing away instead of fluid being absorbed. Either way, the body treats a small subcutaneous pocket as something to quietly deal with, not an emergency.
Where it stops being a non-issue, though, is the golden dose, and this is the bit actually worth paying attention to. When I’m drawing up residual medication into a syringe to measure it, an air pocket in there isn’t a safety problem, it’s an accuracy problem. Air takes up volume on the barrel the same as liquid does. Draw up what looks like 0.6 mL with a visible air gap in it, and I haven’t actually got 0.6 mL of Mounjaro — I’ve got less, with the markings telling me something that isn’t quite true. That matters more with a small residual dose than a full one, because there’s less margin for the number to be wrong.
Which gives me a cleaner way to think about the whole thing than “bubble, is that bad.” Safety-wise, a small amount of air injected subcutaneously isn’t the thing to worry about. It would have dispersed on its own, same as the fluid did. Accuracy-wise, when I’m trying to measure out a specific leftover amount, it’s worth getting rid of the air before injecting — not because the air itself is dangerous, but because I actually want the number on the syringe to mean what it says.
The lump I actually had was never the emergency I’d half-filed it as, and apparently neither would the version I was imagining. Just two different things, both doing exactly what they do under skin, on their way to not being there any more.