The way 16-year-old Amelia Hennessy throws the word “tachycardia” around, you would think she was talking about an annoying homework assignment. But when the high school runner and nordic skier felt her heart beating abnormally hard to the point of severe dizziness, she knew her reoccurring problem – supraventricular tachycardia – was worse than the average teenager’s. The issue came frighteningly front and center in August, when Hennessy collapsed at the finish line of the...