A heart attack is a sudden and serious condition in which blood flow to the heart is interrupted, though the heart itself may still function. This is in contrast to a cardiac arrest, in which the heart itself stops functioning. Depending on the severity, a heart attack may lead to death if not treated immediately.
In February of 2154, Major J. Hayes suffered a heart attack after being shot through the heart by a Xindi-Reptilian while being beamed away. Dr. Phlox tried to revive him, but was unsuccessful. (ENT: "Countdown")
In 2287, Captain James T. Kirk, Captain Spock and Dr. Leonard McCoy climbed a turboshaft on the USS Enterprise-A to evade Sybok's followers. Kirk told them "look at it this way, we'll get a good workout" to which McCoy replied "yeah, or a heart attack." (Star Trek V: The Final Frontier)
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 possible heart attack or cardiac arrest and collapsed on the spot. She had to be given a new heart. (DS9: "Profit and Lace")
In 2376, the Doctor asked Lieutenant B'Elanna Torres to delete his medical database. Torres sarcastically asked him if he was sure about doing that, in case he gave his Qomar fans a heart attack and couldn't revive them. (VOY: "Virtuoso")