Final answer:
The narrator mentions Mrs. Mallard's heart trouble to highlight her physical frailty and foreshadow her emotional state.
Step-by-step explanation:
The narrator mentions Mrs. Mallard's heart trouble in the first sentence to highlight her physical frailty. This detail helps set the stage for the emotional journey that Mrs. Mallard will undergo throughout the story. By mentioning her heart condition, the reader is prepared for the possibility of a physical or emotional shock that could impact Mrs. Mallard's well-being.