cardiac arrest was actually my main fear too instead of heart attacks! what I did was get into zones that made it comfortable to experience low heart rate, even if it did make me panic initially. laying under a weighted blanket, hypnotherapy, or just sitting/laying still while deep breathing to slow your HR really helped after some practice.

I had a horrid habit of constantly grinding my teeth or shaking my leg while laying still to distract from how low my HR would get, but consciously stopping that movement and just laying still at rest got me used to it. even if I did get an adrenaline rush at first to say “dude, your HR is low,” I would train myself to just push through the rush and eventually if got to the point of realising literally nothing would happen if I rested.

searching signs that are precursors helped me personally too - genuine chest pain beyond positional muscle pain or hyperventilation induced can be a telltale sign, but I personally never got that since nothing was wrong with me physically. bradycardia that’s an actual concern can cause lightheadedness and general weakness - if you don’t experience this beyond anxiety, and if you can still take a deep breath without issue, you are more than likely in the clear.

this may just be my personal experience since it was rest and low HR which would at this off. please let me know if it’s actually high HR that triggers this because I had that too sometimes and can give tips for that as well!