rate or rhythm abnormalities.

If you throw a cholesterol plaque or a thrombus into a coronary vessel, you'll have ischemia shortly thereafter where there was no ischemia prior to the embolism. So yes, that's true; people can have a normal ECG until they don't.

We can pick up on chronic changes as that occur secondary to other conditions like longstanding hypertension or sleep apnea. A really thick walled heart gets thicker with poorly controlled hypertension, and because it's thicker it's more sensitive to ischemia. And you can pick the wall thickening with an ECG.

Without getting into a very detailed discussion that would bore most of y'all and distract me from my boards studying, a normal ECG is great but it doesn't mean that it will remain normal. An abnormal one isn't great, but isn't the end of your world either.