Hello there, thanks for the good question. Given the fact that you are a young man without symptoms, what you have described does not strike me as anything serious. It may be caused by something called deconditioning which means you need to exercise more and regularly. However, I do recommend a couple of tests to be more sure. The first one is echocardiogram to make sure structurally your heart is normal. Secondly, treadmill stress test, you walk on the treadmill and push yourself to your limit and see what happens. If these two tests are completely normal, I would not worry about it.
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When exercising it goes to 120 and stays at this constant rate (without any variation). Even when stopping it does not slow done for 20mins and then goes back to normal. Could this be a SVT