In "Macbeth", what does the phrase "heat oppressed brain" mean?

The phrase comes from the end of Act II scene 1, where Macbeth ponders killing Duncan.  He clutches a dagger, and proceeds to consider and examine his thoughts about murder, the dagger itself becoming the physical embodiment of the act.  He then questions himself, "Is this just a dagger, or is this murder?" Or in other words, "am I really going through with this act?" and these heavy thoughts cause and continue to cause more heavy thoughts, originating from a "heat oppressed" (headache?) brain.

Later in the soliloquy, he mentions the phrase "heat of deeds," heat in the passage suggesting passion; he's therefore not thinking logically, but reacting emotionally to the murder he's about to commit.