I n the chaotic swirl of events after President John F. Kennedy’s assassination, doctors feared that Vice President Lyndon B. Johnson had suffered a heart attack upon arrival at Parkland Memorial Hospital in Dallas. The signs were ominous: Johnson’s face was ashen, and he was clutching his chest. “There was the real possibility that the No. 3 in the line of succession would become president,” the historian Michael Beschloss told me. Johnson was reportedly examined and a heart attack ruled out —