the story of an hour

The shocking conclusion of the story finds Mrs. Mallard's heart failing, as she comes to see that her husband (presumed dead) is still alive. Mrs. Mallard's death is attributed to heart disease, where heart disease is "the joy that kills." This last phrase is purposefully ironic, as Louise must have felt both joy and extreme disappointment at Brently's return, regaining her husband and all of the loss of freedom her marriage entails. The line establishes that Louise's heart condition is more of a metaphor for her emotional state than a medical reality.

Everyone thought she would have a broken heart because of the death of her husband.  The reader knows that she actually begins to feel free.  When it is revealed that he is alive, she actually has a "broken heart" and dies.  This is ironic.