My husband died of a cardiac arrest in March of 1993 at the age of 44. He had numerous episodes of feeling fast heart beats,  but 4 cardiiologists had all diagnosed it as "stress".  He died on his exercise bike.   His mom had problems in her early 40's but survived them.  Many cardiac tests were done at the time.  She had a full arrest at the age of 81, and the place where she had the arrest had just bought a defib.  She was their first save.  Numerous tests were performed, nothing was found EXCEPT Long Q-T.  My two daughters were diagnosed 2 years later, and this is when it was noted that their grandmaother's EKG which was taken 6 months before her arrest matched theirs bifid t-waves and all.  They have ICDs now, including grandma. (She has since had two kneww surgeries, and she is going strong...playing bingo, bridge, reading and enjoying life).   One has had shocks, the other has not.  My mother-in-law grandfather (my husbands great grand-father) died at 23.