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Why do Doctors Flick the Needle before Shots?

by Dr.Sal MD on author

doctor flicking needle

Just before giving shots - if you're not too busy cringing or escaping off to a happy place in your mind - you'll notice that I turn the needle upside down then curiously flick it with my index finger once or twice followed by a small squirt of the medicine or vaccine released from the tip of the needle. Why do doctors and nurses do this ritual? Well the reason we tap the tip of hypodermic needles before giving them is a fine detail likely to miss your eye from your viewing distance. If you could look closer from where we stand, you'd see that we are doing is forcing the tiny bubbles sucked into the syringe while taking up the medicine to coalesce into one or two large bubbles that float to the top of the liquid that we can then flush out of the system by advancing the plunger slightly.

What's the big deal about air caught in the needle?

You might think it's to prevent introducing lethal gas bubbles into the bloodstream... but you would be wrong. The volume of gas sucked into a syringe during medication administration is orders of magnitude smaller than would be necessary to cause an air embolic stroke or embarrass your heart function. The real reason for the tapping practice is to accurately dose the medication we are giving. By the time you factor in gas bubbles that leak in through the rubber bung while drawing up liquid meds and the dead air space at the end connector of all needles, you can have an error margin of up to 0.1 or more cc's of gas needing dose correction. The other issue is to reduce subcutaneous and intramuscular injection pain as the air takes up unnecessary volume stretching the local fibers more than necessary increasing the perceived pain of the shot.

So what happens to any bubbles we do miss after flicking?

Small ones simply dissolve away into the dynamic bloodstream and any larger ones that make it to your lungs lodge in the super fine capillaries around air sacs and melt across the gas exchange membrane to meet back with the air we respire out. Whew.


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