By saying cardiologist, I hope you mean you’re seeing an electrophysiologist who specializes in heart rhythm issues. I am your age and also experienced rapid heartbeat events at 140 BPM with exercise and occasionally out of the blue without activity. My heart is otherwise healthy as determined by every imaginable test. I have never fainted. These rhythm disturbances were deemed dangerous enough to require the placement of a defibrillator to prevent an episode which could cause SCD- sudden cardiac death. I take a calcium channel blocker to prevent NSVT- non-sustained ventricular tachycardia. The pill literally prevents a faster heart rate, which unfortunately prevents me from comfortably engaging in aerobic exercise. Fatigue and breathlessness ensue.
Hope you can get a thorough workup by the best EP you can find. If you’re in a small town with only community hospitals, please consider traveling to a major hospital for the best care. Good luck.