You may have noticed that when you get a dental injection, sometimes your heart feels like it's going to beat right out of your chest. You've probably just assumed that your heart is beating so hard because you're nervous.
That's partly true, but there's another reason — the dental anesthetic actually contains something that makes your heart beat faster!
As I mentioned above, nervousness may play a role in making your heart beat faster when you get a dental injection. When we get nervous, our heart beats faster because our body sends out a substance called adrenaline that increases our blood pressure and causes our heart to race.
Most of the local anesthetics used in dentistry in the United States contain epinephrine (also known as adrenaline.) Not only is your body causing your heart to speed up by releasing adrenaline, your dentist is giving you adrenaline in the local anesthetic!
Don't worry, dentists don't want you to have a double-dose of adrenaline to make you nervous. Local anesthetics contain epinephrine for another reason: The epinephrine constricts your blood vessels.
With your blood vessels constricted, the local anesthetic stays near your tooth for a long time. That way, it gives your dentist a lot of time to work on your tooth without you feeling it. Another reason that dentists want your blood vessels constricted is so that only a small amount of local anesthetic gets absorbed into your body.
You may have felt your heart pounding more during some dental injections more than others. If the dentist happens to inject the local anesthetic into a small blood vessel, it can quickly travel to your heart and cause it to beat very hard and fast. This generally subsides after 10 seconds or so and is not dangerous to you. Most of the time, the local anesthetic is not injected directly into a vessel and stays right near the nerve without affecting the heart.