When Montresor offers the medic wine to Fortunato, what is the reason he gives for wanting the other man to drink?

Shortly after Montresor convinces Fortunato to follow him into the depths of his family's extensive catacombs, Fortunato comments on the nitre, which is the naturally occurring white crystalline salt hanging from the ceiling of the vaults. Fortunato then begins to cough for several minutes, and Montresor responds by saying that he must leave the catacombs to avoid getting sick. Montresor pretends to be a concerned friend before mentioning Luchresi's name again, which he knows will persuade Fortunato to continue traveling down the catacombs. Montresor then offers a solution to Fortunato's cough by saying,

A draught of this Medoc will defend us from the damps. (Poe, 2)

Montresor has no intention of giving Fortunato a draught of Medoc to end his coughing fit; he simply wishes for Fortunato to remain intoxicated so that he can successfully carry out his plan. Montresor continues to offer Fortunato a couple of different wines before coming to the end of the vaults, where he proceeds to shackle Fortunato and build a wall around him.