What does the opening paragraph of "The Tell-Tale Heart" imply about the narrator's reliability?

When the narrator opens with the words that he is "very,very dreadfully nervous" and asks the reader why he thinks he is "mad", this obviously produces some doubt in the narrator's reliablity. The narrator continues by saying that he "heard all things in the heaven and in the earth." That comment alone casts doubt about his sanity because no one hears everthing, especially everything in heaven. Finally, the narrator has to call our attention to "how calmly I can tell you the whole story." By this time we are convinced that something is dreadfully wrong with the speaker because he is implying that he is usually not calm, but agitated.