Sounds like you have lots of concern and rightfully so as we all hear of a "friend of a friend" who had a dangerous arrhythmia that they didn't even know about.
Any beat that comes in too early or takes over the natural rhythm of the heart beat is an arrhythmia. Not all arrhythmias are dangerous. As we get older, just like we get wrinkles on our face, our heart gets "wrinkles" or irregularities that just come with age.
The cardiology group I was with, followed the general concensus adopted in the last 15 or so years, that single PVC's (not strung together in multiple beats) that did not produce symptoms (light headedness, drop in BP etc)no matter how many times they occurred, did not warrent medication ot treatment.
To help visualize the heart and evenything it does, I like to use the example of a house. Your heart is a house. The walls, floors, and beams are the "structure" or muscle walls that form the chambers of the heart. The wiring is the electrical system or the pathways in the heart that transmit electrical impulses that regulate heart beat and have to do with extra beats and arrhythmias, and the plumbing is the coronary artery system that carries blood to the muscle. The windows are the heart valves.
Sometimes there is a valve disorder that can cause the PVC's. Since your echo was normal, that is not the cause. That said, just because you had a normal echo does not mean it translates to a healthy "electrical system". The walls and windows can be sound, but you can still have a short in the wiring.
Without catching that episode of "terrifying palpitations" we just don't know what is going on. You may want to ask your doctor for an "event monitor". Unlike a holtor, that has you wired up for 24 hours, and event monitor is the size of a credit card and you carry it with you. When you feel something weird, you hold it to your chest and push a button and record the episode like an ekg. You can store many epiosodes as digital data in the card. Then, when you have something on the card, you call a number and transmit the info over the phone. They transmit it to your doctor for him/her to read. You can keep it up to 30 days (I believe that is what most insurance comapnies pay for) This would be a good way to know what your heart is doing during those feelings.
Sometimes an exercise test is done aslo to see what happens when the heart is made to speed up. Less serious PVC's go away with exercise.
I hope this helps! Please let me know whow you do!
PS. I'm sure your doctor checked your potassium level as this could be the cause of PVC's, but sometimes, it's just the way you were "wired" when you were built!