Not easy to answer, a doctor may have trouble too if your symptom has passed. If you have no other symptoms, nor a recurrence of the episode, I'd think it reasonable to just make note of it, in detail on paper, and take it with you on your next visit to the doctor.
If the symptoms reoccur or others appear, make an appointment with your doctor at the earliest possible. I read you post to say you have a cardiologist, or other doctor who prescribed "tenormin for irregular heartbeat". I do not know that medication and haven't looked it up. Did your doctor define your heart condition? Has it been effective? Have you been on that medication long? Other facts that may help others "measure" your experience.
Tenormin is the generic for atenolol. I have been on it for years. The doctor who prescribed it said If I started on it I would have to keep taking it to prevent the irregular heartbeat. It has worked pretty good so far. The episode of a fast heartbeat startled me because it had never happened before. Someone told me it could have been an anxiety attack, but I wouldn't think it would last 3 hours.
Hello...
Your original question stated "I am on tenormin for irregular heartbeat" What was the nature of the "irregular heartbeat?"
It is common for anxiety attacks to cause a rapid heartbeat, but lasting 3 hours isn't really that common. Was this the one and only time you experienced this? It is really hard to say what caused you to expereince this...it could be the result of some form of SVT, but you'd need to have this event captured to be absolutely sure.
If this symptom occurs again, you should discuss with your Dr the possibility of using an event monitor to help determine what is really happening.
Good luck with this and I hope your New Year is a year of picture perfect health:)