WHY DO DOCTORS FLICK THE NEEDLE BEFORE GIVING SHOTS?
Just before getting shots - if you're not too busy cringing or escaping to a happy place in your mind - you'll notice that I, and other doctors, turn the needle upside down then curiously flick it with our index fingers once or twice followed by a small squirt of the medicine or vaccine release from the tip of the needle. Why do doctors and nurses perform this ritual? Well the reason we tap the tip of hypodermic needles before giving injections 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 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.
Can air caught in the needle kill?
You might think we tap off the air in shots to prevent introducing lethal gas bubbles into your 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 needles?
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.