In "The Tell-Tale Heart,"how does imagery (the beating of the heart) describe the narrator's unbalanced nature?

This excellent story, like "The Black Cat," shows an
unreliable narrator who gives himself away because of his own unbalanced nature. For me,
a key quote to consider in how Poe uses the heart beat of the old man to show the mental
instability of the narrator is the first mention of the
heart-beat:

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And have I not told you that what you mistake
for madness is but over-acuteness of the sense? - now, I say, there came to my ears a
low, dull, quick sound, such as a watch makes when enveloped in cotton. I knew that
sound well, too. It was the beating of the old man's heart. It increased my fury, as the
beating of a drum stimulates the solder into
courage.

Of course, because
of the character of the narrator, we disagree with his first sentence. His "hearing" of
the heart beat reflects his own inner mental instability. It is interesting how, once
the man is dead and disposed with, that the beating begins again, just at the time when
it looks as if the narrator is going to be successful in getting away with the crime.
The beating of the heart then could be said to describe both the madness of the narrator
but also his guilty conscience at some kind of level, representing the guilt he feels
about killing the man.