The depth of the injection is, if you have a good supply closet, dictated by the length of the needle you choose for the syringe. Subcutaneous injections are done with a needle that’s 1/2 to 5/8 of an inch long - the fatter patients get the longer needle. IM injections are done with 3/4 inch (for a very thin person in the deltoid) 1 inch (for a normal weight person in the deltoid or a thin person in the hip) or 1 1/2 inch (for a fat person in the hip). You choose the correct length needle so that you can insert it all the way to the hub and know that it’s more likely than not in the correct tissue. Medical IV’s are usually done not with a rigid needle, but a flexible catheter that stays in place. (It’s rare that you need to give a single one-and-done IV injection; keeping the IV cath in means I only have to find your vein once, and I don’t have to worry as much about “blowing out” your vein by poking a bunch of holes in it over a couple of days for various meds and fluids.) Drug abusers, of course, will often use rigid needles for frequent IV injections because it’s what they have, and that’s one reason their veins tend to be damaged.
We used to teach nurses that when doing an IM injection, they should draw back on the syringe a little bit to see if blood appeared in the syringe. If it did, then they were considered “in a vein” and should withdraw the needle and change it before trying again. We don’t do that anymore, because it wasn’t found to actually be effective at decreasing risk, and it’s stressful on both the nurse and the patient.
If you mess up an IV stick, either nothing happens (you didn’t get into the vein) or bruising happens or the person bleeds a lot. I can’t fathom anyone without a clotting disorder losing a dangerous amount of blood from a blown IV stick, but blood does spread a lot, so it can look scary. I was taught to do IV sticks by my husband, a retired paramedic. The first one went in like butter, smooth and perfect. The second one, I blew the vein - the needle went in one side of the vein and right out the back…and he’s on blood thinners. Bled like a stuck pig and I was mortified and horrified and swore I was never going to become a nurse, I’d just go live under a bridge somewhere. He laughed, helped me clean up the blood and then made me do another one, which went okay.
I can’t remember where I found the information, but the question of air bubbles in IV tubing and in syringes has come up before on the Dope. Turns out you need about 60cc, or 1/4 cup, of air inside a vein before it becomes life threatening. That rarely happens with IV tubing these days now that we use pumps. It mostly happens during an accident or surgery, if an open vein isn’t clamped off and it sucks air in. The little bubble in the syringe won’t kill you. It can, however, mess up the dose of the medication, because it takes up room in the syringe that should be filled with medicine. So if you see a bubble in a syringe prefilled by the manufacturer, don’t worry about it; they fill them by weight, not volume, so it will be accurate. But do knock the bubble out of your insulin syringe when you’re filling it from a vial!