Heart attacks are guaranteed to occur if your hero reaches the maximum value of 200 on their Stress Bar while Afflicted. Heart attacks immediately drop a hero's Health down to 0, placing that hero on Death's Door and reduce their stress to 170. If they are already at 0 health when the heart attack occurs, the heart attack will kill the hero instead, disregarding Death Blow resistance. If your hero is currently Virtuous, they will not suffer a Heart attack, and instead lose the Virtue and reset to 0 Stress.
If a hero is brought back from Death's Door after suffering a heart attack, in addition to the Death's Door Recovery debuff, they will receive a Heart Attack debuff for the rest of the quest:
Heart attacks are similarly lethal, and are the end point of a Stress-focused team. Virtuous Heroes in this game mode will experience heart attacks normally like non-Virtuous ones, but will never lose their Virtue at 200 Stress.
Heroes suffering a Heart Attack emit a unique bark right before it occurs. There is one different bark for each Affliction, in addition to one general bark that any Afflicted hero may use.
IT IS UNBEARABLE!
~ An afflicted hero moments before suffering a heart attack.