Jeanette Dempster was doing something she’d done countless times before — playing baseball with friends and holding down second base with the comfort that comes from being a lifelong athlete.

But in the middle of an otherwise ordinary game, as she rounded third and sprinted toward home plate, Dempster collapsed. In a moment when timing proved critical, help was unexpectedly close by — two nurses were playing in the game, one of whom had a cellphone (a rarity in 1994) and an off-duty ambulance happened to be passing by. The crew stopped and defibrillated her on the field.

Dempster, then 42, was rushed to the hospital, where she learned the cardiac arrest was the first sign of a congenital heart disease, ventricular dysplasia, that would shape the next three decades of her life.

Doctors i

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