What is the nature of Louise Mallard's heart trouble?

In the beginning of the story, we are told that Mrs. Mallard suffers from a heart condition with no explanation afforded.

Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death.

The story end with the same information and a bit more, as it is alluded that "joy" was the cause of her heart giving out.

When the doctors came they said she had died of heart disease - of joy that kills.