I was diagnosed with Afib about 3 years ago. I questioned my cardiologist about my heart rate as shown on the Garmin because I was going well beyond what was supposed to be my max rate based on my age. I was told to go by perceived effort and leave the HR monitor at home. Had surgery last year to repair a leaking valve, relocate a congenital issue with a cardiac artery, and the ablation which so far all is good. Would suggest that you work with your cardiologist with any of your concerns.