Poe's use of rhetoric and syntax clearly conveys the narrator's "madness". His short, choppy sentences laden with repetition coupled with flowery, eloquent, descriptive narration detailing the plans to murder the old man demonstrate an unstable mind whose ideas for a brilliant plot are constantly interrupted by his paranoid and guilty thoughts and his need to justify his sanity. His quasi-asides about his stability and perceived state of his environment become more and more desperate, culminating with his recounting of the deafening beating of the dead old man's "hideous heart".
From the outset of the story, Poe shows us that his narrator is definitely not sane:
"He speaks with trepidation from the famous first line of the story: 'True—nervous—very, very dreadfully nervous I had been and am; but why will you say that I am mad?'" (eNotes)
The narrator goes onto explain to us that he "loved" the old man, except for his cloudy eye, which must have been evil, and so led him to murder the old man. This is definitely not a person grounded in reality.
Check out the eNotes study guide (link below) for more information on this great story!