Had 24hr ecg now having bloods done?

I was feeling unwell and fainted a few months ago. GP said heart rate was high and sent me to get and ecg. Results were high and had prolonged QT. Had a 24 hour ecg last week and they said results would be 3 weeks but had a call today saying Dr needs bloods done following ecg results but couldn’t tell me why. 

What bloods would they be testing following a 24 hr ecg? 

They want to test if there is heart damage