I have an ICD. Actually on my second unit. I am not sure how yours is programmed but mine does basically nothing unless there's a really bad arrhythmia. I've got HCM, with arrhythmias and family history of sudden death... at least thankfully not afib that needs to be controlled. I have had no sense that the ICD is inhibiting my heart rate. I don't feel it pacing, anyhow, which is a very distinctive feeling. It basically leaves me alone unless something goes wrong. But I seldom ever leave zone 2. I'm very limited by my heart condition which causes angina (much less so since surgery).
I've never heard that they have accelerometers that adjust their settings for activity. I'll have to read up on that.
If you are taking medications like metoprolol consider that they are also inhibiting your heart rate.
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Genesis 49:16-17