What is the irony in the use of the phrases"heart trouble" at the beginning of the story, and "heart disease" as the cause of her death at the end?

To unify the story under a central theme, Chopin both begins and ends with a statement about Louise Mallard's heart trouble, which turns out to have both a physical and a mental component. In the first paragraph of "The Story of an Hour," Chopin uses the term "heart trouble" primarily in a medical sense, but over the course of the story, Mrs. Mallard's presumed frailty seems to be largely a result of psychological repression rather than truly physiological factors. The story concludes by attributing Mrs. Mallard's death 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.