Dorothy Lee and her husband of 40 years were driving home from a Bible study group one wintry night when their car suddenly hit the curb. Mrs. Lee looked at her husband, who was driving, and saw his head bob a couple of times and fall on his chest.

In the ensuing minutes, Mrs. Lee recalls, she managed to avoid a crash while stopping the car, called 911 on her cellphone and tried to revive her husband before an ambulance arrived. But at the hospital, soon after learning her husband had died of a heart attack, Mrs. Lee's heart...