The last line of the story is : " When the doctors came they said she had died of heart disease-of joy that kills." In what ways is this an ironic statement?

Mrs. Mallard immediately comes to terms with her husband's death, and soon is transported by thoughts she will be free in a way she's never known. Ironically, in the end, her husband lives, and she is the one who will die..... free of earthly bondage at last.