Hearts are frequently used to symbolize love in Western art and culture. We bet you've seen this emoticon before: <3. Right? Frequent texters probably know that "<3" means "heart," and people end up using "heart" as a shorthand for "love." You know, "I <3 New York," or "I heart Chuck Bass."
That said, the fact that Mrs. Mallard has "heart trouble" (1) should be taken as more meaningful than just the idea that she's unhealthy. If that were true, she could have ulcers or be allergic to pineapples. But no. Of all the potential illnesses she could have, she has problems with her heart.
The late nineteenth-century way this condition is described, as "heart trouble," only reinforces its symbolic meaning. Sure, it's a polite way of describing her condition, but it also reinforces the "trouble" Mrs. Mallard is having with her "heart" within her marriage.
Having "heart trouble" makes it easier for her to dismiss the concept of love with the grand statement, "What did it matter!" (15). Even if Mrs. Mallard wasn't sick, she'd still have "heart trouble" of the emotional kind.