What is the nature of Mrs. Mallard's "heart trouble," and why would the author mention it in the first paragraph? How is this symbolic or ironic?

Louise Mallard's heart ailment is both physical and mental. During the introduction, her frailty is addressed as a medical condition. Over the course of the story, however, we come to the conclusion that her heart trouble might be more of an emotional problem than anything else. In the end, Louise is said to have died from "the joy that kills", a phrase that is filled with irony as we can infer from the text that her last moments were filled with both joy and disappointment.