I've had a heart valve replaced myself, and the GSD of a family member had an atrial tumor removed successfully.
When a heart is stressed due to a defect or disease it can stay in the mild to moderate stage for a few years, but when it progresses into the moderate to severe stage, things can change quickly.
Some tips that I learned: avoid stairs, avoid too much running, avoid extreme cold, avoid extreme heat, avoid salt and processed foods--eat natural, keep hydrated, rest often. Also, CoQ 10 is a good supplement.
Checking heart rate often at rest is good, and mark it down on a calendar.
Make note of coughing or any blue-ish colour on the tongue or gums, at which time a fast trip to the vet is in order.
My valve issues never showed up on an EKG--- only on an ultrasound.
Arrhythmias sometimes show on the EKG, and heart structural defects can show as disrupted EKG patterns.
Beta blockers can do wonders in keeping the heart calm, I still take them, and they aren't expensive.
Best wishes with this