The story begins with a description of Louise Mallard as someone with "a" heart trouble. Therefore, the news of her husband's death would have to be given to her gently. Initially, the reader is given the impression that either Mrs. Mallard is emotionally and/or physically frail or that those around her simply perceive her to be so frail.
Upon hearing the news, Louise wept at once and left to be alone in her room. While alone, she moves from grief to abundant joy in the new freedom she foresaw in her future. The sequence of events from shock to grief then goes to a subtle realization of independence which increasingly rises to a climax of exuberance of self-discovery and hope for Louise.
There was a feverish triumph in her eyes, and she carried herself unwittingly like a goddess of Victory. She clasped her sister's waist, and together they descended the stairs.
As Miner points out in the article (one of the links below), the indefinite article "a" which modifies heart trouble is, by definition: indefinite. We don't know exactly what that means. It could be quite literal. It could also be something Mrs. Mallard's husband ascribed to her as he may have believed women as a more frail, weaker sex.
Just as the news is gently broken to Louise that Mr. Mallard had been "killed," the narrator is equally elusive or subtle in revealing Mr. Mallard's return. "Some one was opening the front door with a latchkey." This sentence is unnecessary. Chopin used this to increase the suspense and tension to augment the rising climax but also transfer Louise's emotion from joy to grief as instantaneously as possible.
This story is generally understood as a woman's discovery of her self, her independence and her freedom in the wake of her husband's death. Some of the wording is ambiguous. In other words, phrases such as "possession of self-assertion" suggests a passive as well as an active self-discovery. However, this passive (possession) trace en route to her self-discovery is simply that: a trace of her former dependence upon her husband. What we learn about Louise is that she has felt complacent but stifled in her life with her husband. Hearing news of his death, she grieves but is empowered. And during that hour, she becomes more and more free. The climax reaches such a height that the shock of seeing Mr. Mallard (the shock of losing that freedom) is too much to bear: "heart trouble" or not.