The narrator of this story has just killed a man and stuffed him under the floorboards, and he has spent a good amount of time, as he tells the story, trying to justify his actions to a seemingly unsympathetic audience. Right from the beginning he proclaims he is "not mad", and that he had good reason for what he did; this indicates that he is defensive, and feeling guilty for his crime. As he recounts the tale, the beating heart starts only after the crime has been committed, and gets more and more intense the more he tries to confidently deny his crime. So, the beating heart symbolizes the narrator's conscience, or his sense of guilt and wrongdoing. After all, he HAD killed a guy. Outwardly, he seems cocky and unregretful. He asks, "what had I to fear" from the cops? And it was in "the enthusiasm of my confidence" that he seats them right on top of the body. So, to look at him, or hear him describe it, he had no guilt. However, Poe decided to symbolize this man's subconscious guilt through the heartbeat. Since the narrator himself refuses to admit any wrongdoing for killing the man, Poe had to show that he did feel bad at some level, and chose to do that through the heart. It symbolizes how what he did cannot be hidden from himself, or from the old man. He feels guilty; each beat of the heart reminds him of his crime, and it is that, his guilt, that leads him to confess.