Why do some injections hurt more than others?

[moriah]

On a TV show about painless dentistry techniques, they showed a mechanized hypodermic needle for novacaine injections. Once the needle was injected, the novacaine was slowly injected by the machine. It seems that one of major causes of pain from novacaine injections (other than hitting a nerve) is that the too-quick injection of the novocaine into the gum tissue was painful because it swelled the gum tissue.

Peace.

[wonky]

I get a Depo-Provera shot in the muscle of my upper arm every three months. Sometimes, it burns quite unpleasantly during the shot. Sometimes I can barely feel it. Sometimes my arm will ache for two days afterwards. Sometimes there’s not discomfort at all.

Thus my conclusion: I don’t know.

[picunurse]

The reason having blood drawn didn’t hurt is because blood vessels don’t have pain receptors. Injections given in a muscle hurt more because the muscle moves, and is dense. Subcutaneous injections are given in the less dense tissue below all the layers of skin, but above the muscle. Its not a very deep space, so large volumes can’t be injected there. Intradermal injections are actually in the skin, like the TB test, there is very little space so the fluid raises a “weal” when injected properly. Only tiny amounts are tolerated.
So, injection volume is a factor, as is the tissue being injected. The make up of the solution plays a part.

A run down:

1. Intravenous injection (IV)= Large volume of fluid quickly. A needle carefully put into a vein. A little pain as needle passes through the skin, then no pain, because there are no pain receptors in blood vessels. Drugs like broad spectrum antibiotics given for very serious infections, blood products, anesthesia, pain meds after surgery, emergency drugs like NaHCO[sub]3[/sub] lidocaine for heart arrhythmias.
   2.Intramuscular injection(IM) =1 to 5 milileters rarely over 7mls. Hurts worst due to density, and drug’s make up. Drugs like pennicillin, gamma globulin given for exposure to some serious diseases, antiemetics, tranquilizers, serious pain relievers, like morphine. IM injections are used less and less these days. If one is sick enough to need and IM injection,they are sick enought to get an IV.
2. Subcutaneous (SQ or SubQ) = 0.1to 1.0 mls. hurts less due to small volume, less caustic ingredients. Drugs like insulin, heparin (a blood thinner) some vaccines, epinephrine for bee stings.
3. Intradermal = 0.1ml or less used mainly for skin test, as in allergies, TB testing.

Of course there can be overlapping , and obviously IV must be used with medical supervision, so are rarely used outside the hospital setting.

[norinew]

@picunurse

Intravenous injection (IV)= Large volume of fluid quickly. A needle carefully put into a vein. A little pain as needle passes through the skin, then no pain, because there are no pain receptors in blood vessels.

I’ve always known you to be quite knowlegeable in matters of medicine, but I can’t quite believe this, as certain things given intravenously burn pretty bad; saline solution comes immediately to mind (like when they’re flushing the tube), but I know I’ve had other IV drugs that burn. If vessels have no pain receptors, whence the burn?

[Dr_Paprika]

As in my above post, an IV injection can be painful if the injection is acidic, or the volume of infusion is rapid. Blood vessels have a nerve supply (that helps them dilate and constrict) but comparitively few “light touch” receptors.

[picunurse]

@norinew

I’ve always known you to be quite knowlegeable in matters of medicine, but I can’t quite believe this, as certain things given intravenously burn pretty bad; saline solution comes immediately to mind (like when they’re flushing the tube), but I know I’ve had other IV drugs that burn. If vessels have no pain receptors, whence the burn?

You know, that was something I learned in nursing school and never questioned, but you’re right that something causes the pain you refer to.
I’ll go exploring and see what answer I can come up with. I did a quick look at google for blood vessel physiology, and found thousands of medical school class sites, so, I’ll sift through and see if there is any science behind something I’ve taken as truth for 40 years. And I’m the one who always questioned everything.

[Mirasawa]

@Ferret Herder

He meant that med students learned how to give injections by using saline in a syringe. If they were told to inject a drug named, say, Aba, they had to pick out a bottle labeled “Aba” with a non-expired date on it. Bottles with expired dates or different names, like “Abe”, had pure water in them. That way you get taught about how to pay attention without having fatal consequences result, like might happen in real life when they’re using the real thing.

Oh LAWD !!! That’s GREAT!!! HAHAHAHA

Another question like the OP…

A needle to the fingertip is more likely to hurt because of a greater concentration of nerve endings, right?

Is there anywhere on the body where the needle can actually bypass all nerve endings because they are so sparse? Or am I hoping too much?

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[Dr_Paprika]

No, but not too many nerves in the back. Hope y’all got long arms.