WebMD has an easy to read introduction to "bradycardia" and the article's links bring one quickly up to spead in a simple way about what can be a symptom of "an aging heart." "Brady" comes from the Greek word bradys for "slow" and "cardia" from the Gr. word kardia for "heart." So an aging heart can be a slow heart, slow in the sense that it beats slower than the normal 60 beats per minute or 60 bpm. Symptoms of a slow and aging heart include fatigue and "syncope," a word used by Greeks for a "fainting spell or swoon."
(I have these derivations of medical terms from a helpful little book at hand by William S. Haubrich, M.D., entitled Medical Meanings: A Glossary of Word Origins (Second Edition 1997) I found it in the bookstore at the UR medical school, when Mary moved there from Byrn Mawr. We were first in that store because that is where the med school kept its sales inventory of new white coats for its first year students, and there Mary tried her's on for the first time. But I digress.)
A slow heart can be a sign of a very fit heart. But in my case, we surely have an aging heart. It may be a relatively fit heart, but aging trumps fit sometimes, and it does in my case. So, hello, aging heart bradycardia.