My cardiologist who runs moderately keeps advising to reduce intensity which I have. On the other hand, a lot of literature proposes to keep the intensity up as we age. Of course, on a personal level, how intense, how often, and age should all be part of it. On my next ride I am going to use a different heart rate monitor to see if the above mentioned condition is repeated.
I have also arrange for a stress test with my cardiologist.