Yeah, so in my frustration with all the potential that was set up in the previous episode and then pretty much ignored in the follow-up, I didn't mention one of the things that normally would have had me going 'Ohhhhhhhhhhhhhh!!! Thank you for that information so much, show!'
When Katherine and Stefan were in the tomb, after he'd refused the blood from Damon, she started talking about how he has no idea how bad it will be and one of the things she mentioned is the heartbeat slowing down, meaning ... dun dun dun ... vampires have heartbeats. Which, in a way, I suppose does make sense as they have blood pumping through their system. So that's how come neither Damon nor Stefan can tell the difference between the two because of Elena's heartbeat; Katherine has one too.
