What determines if you need one (age, activity, family med history)? If you have had one what were your experiences?

Over the years I have read about seemingly perfectly healthy athletes of all levels that have passed due to undetected heart ailments and quite frankly it bugs me especially since I was diagnosed with an arrhythmia as a kid that I would 'grow' out of. Any thoughts?

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"In the blocks you're a prisoner, the gun releases you."