Fainting (syncope) and near-syncope
Overview: Fainting, also known as syncope, is the temporary and sudden loss of consciousness.
Syncope (pronounced "sin-ko-pea") is caused by low oxygen levels (hypoxia) in your brain. Pre- or near-syncope is just what it sounds like: a person feels as if they are going to faint, but does not lose consciousness.
There are a number of things that can cause this. Not all of them have to do with your heart. But fainting is never a good thing, and neither is nearly fainting. Even if it isn't caused by something terribly dangerous, you can certainly hurt yourself fainting. And since usually you can't be sure what the cause is, and some of the causes are serious, you should always take it seriously!
Kidd, Stephen K., Christopher Doughty, and Samuel Z. Goldhaber. "Syncope (fainting)." Circulation 133, no. 16 (2016): e600-e602.