In "The Tell-Tale Heart," what heart does the narrator hear at the end? What do you think this reveals about the narrator’s mental state? At what point in the narrative is the narrator pushed over the edge into both sounding an acting madly? What is Poe's "sublime"? How and when might this experience enter into the story? Some critics have suggested that Poe’s unnamed narrator is actually a woman. How might this identity alter our understanding of the story, particularly how we determine who is possibly the first victim of this story?

At the end of the story, the narrator hears his victim's heart beating underneath the floorboards. His heightened sensitivity to imagined sounds demonstrates his paranoia and mental instability. It's also possible he mistakes the sound of his own accelerating heartbeat for the dead man's. As to what point in the narrative the narrator begins sounding and acting mad, it can be argued that he reveals his acute psychosis from the beginning of the story. For example, he asserts that his feelings of enmity stem from his hatred of the old man's "pale blue eye." He then maintains that he will kill the old man in order to rid himself of the eye "forever." Essentially, the narrator demonstrates his mental deterioration by proclaiming that a human being can be separated into distinct and autonomous parts.
The word "sublime" means exalted or awe-inspiring. Poe viewed the sublime as the inclusion of light and dark. In his interpretation of the sublime, obscurity and power had equal emphasis. Poe was interested in how his verbal descriptions could make his readers perceive objects on a more visceral level than an academic one. His conception of beauty embraced all the elements of ideality (perfection): the sublime, the grotesque, the arabesque, and the picturesque. Additionally, Poe's idea of the sublime challenged the rational order of things (remember that light and darkness had equal emphasis in Poe's world). 
If Poe's sublime challenged rationality, it is conceivable that his stories would be imbued with elements of irrationality or mental deviance. In "The Tell-Tale Heart," Poe has created a world where the surreal mingles with the lucid. The narrator perceives the old man as both a physical being as well as a conglomerate of body parts. He does this from the beginning of the narrative, when he proclaims his desire to kill the old man in order to be rid of his "Evil Eye." Essentially, Poe's idea of the sublime incorporated the interaction between light and darkness in the human soul.
It is true that some critics have suggested that Poe's narrator is a woman. Our perception of the narrator largely depends upon our biases and preconceptions about gender. For example, are we more inclined to feel sympathy towards a female narrator? If so, are we then more predisposed to the idea of the narrator as the victim rather than the cold perpetrator of a horrendous crime? Is it conceivable for a woman to kill someone she loved? If so, how could she justify it? These are just a few of the questions we can ask ourselves as we ponder the ramifications of this unique short story.
Source:
Edgar Allan Poe: The Sublime, the Picturesque, the Grotesque, and the Arabesque, Frederick L. Burwick, Amerikastudien / American Studies, Vol. 43, No. 3, The American Sublime (1998), pp. 423-436.