The vulnerable period is crucial to understanding why normal hearts can self-electrocute. For example, when a healthy young athlete drops dead after getting a blow to the chest from a baseball or hockey puck, it is because the heart was hit during its vulnerable period. Scientists have confirmed the presence of the vulnerable period in mammals by slamming a baseball mounted at the end of an aluminum shaft into the chests of eight- to twelve-week-old anesthetized piglets at various times in the cardiac cycle. They found that when the impact occurs within a narrow window 10 milliseconds long and
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