What makes Fortunato sober up quickly?

When Montresor, the narrator, encounters Fortunato, Fortunato is already somewhat drunk. Montresor takes the opportunity to entice Fortunato into the catacombs by appealing to his ego and inviting him to taste a cask of Amontillado to verify its quality. Fortunato has a cough, and he coughs frequently as the pair descend underground. The air is damp and stuffy. Montresor leads Fortunato deeper and deeper into the family catacombs, past skeletons that foreshadow Fortunato's end and mineral deposits of nitre. Finally, he takes advantage of Fortunato's drunkenness and locks him to a chain inside a small crypt, the same crypt in which the Amontillado is supposedly housed. As Fortunato comes to his senses, Montresor begins to build a wall with mortar and brick. Montresor had "scarcely laid the first tier of the masonry when [he] discovered that the intoxication of Fortunato had in a great measure worn off."
The reader can surmise that Fortunato begins to sober up as soon as he realizes that his chains are secure. He gets more and more sober as he listens to Montresor scraping mortar and stacking bricks. Fortunato screams in fear and desperation, to no avail.