A heart attack, also referred to as a arterial occlusion by the Doctor, is a sudden and serious heart condition that leads to death if not treated immediately.
The Clown, a manifestation of fear produced by the Kohl Viorsa's artificial reality, liked to behead people as a means of killing them. As he was not real, the actual deaths came in the form of a heart attack caused by unmanageable stress levels resulting from intense fear. (VOY: "The Thaw")
When Grand Nagus Zek was deposed because he gave females equal rights, Ishka was set to meet with Nilva, an influential member of the Ferengi Commerce Authority, to convince him that women can live as equals. However, during an argument with Quark, Ishka suffered a massive heart attack and collapsed on the spot. (DS9: "Profit and Lace")