Thank you for your question.
I am fellow expert in cardiology and will try to help you with your questions.
A person who is having palpitations after a heart attack
is most likely suffering from abnormal heart rhythms.
These can be atrial or ventricular in origin.
ECG can miss them.
We need a test called holter or event monitor for this.
These tests have high chances of picking up abnormal rhythm that other tests can miss.
I hope this helps.
Let me know if you have more questions.