We learn in the first sentence of the story that Mrs. Mallard has some sort of heart trouble and that it is the news of her husband's death that she is being protected from for fear this terrible news might cause her to have a heart attack or even kill her. Her sister, Josephine, breaks the news in "broken sentences" (para. 1) and "veiled hints" (para. 1). Richard is a close friend of Mr. Mallard's, and he has come to bear the news more gently and carefully than someone else might have done, since he, too, knows about Mrs. Mallard's delicate condition. Together, they manage to make Mrs. Mallard understand that her husband has been killed.
The irony in the story stems from how these elements, the "heart trouble" and the seeming loss of a spouse, take on new meaning as the story goes on. Yes, Mrs. Mallard does in fact have a physical kind of heart trouble, but the reader comes to understand that she has a very different kind of heart trouble, too, a marriage in which she is repressed and unhappy. The idea of Mr. Mallard's death cures the latter problem, for a brief time, but then, when Mrs. Mallard learns that her husband is alive, it is the new shock, of the loss of freedom, that stops her heart because of the physical heart trouble.