the speaker (prufrock) compares the sunset to a ¨patient etherised upon a table¨.
The allusion to anesthesia has nothing to do with surgery itself, but rather as a form of painless exploration. Prufrock wishes to explore the city, and the exploration is compared to the intricate makings of the human body.
He is living in a world inwhich he can barely feel his existence, and the evening ends when the sun goes down and slowly gives us the orange color or yellow. these two colors usually mean palness and sickness, and when Prufrock compares the evening with a patient, we get the feeling that the sun is almost gone, and the yellow and orange are the ones left, it is sickness that Profrock has inside of him,we get the feeling that he is not as functional as a normal human being, in fact, he has been in this state for so long that he has long abandoned the feeling of living, just like a patient uder surgery, who is not dead, but not alive at the same time, he's not concious and the only thing that exists is his body, other than that, his soul is hanged inbetween life and death where he can not feel his existence. similarly, when the evening comes, it is Profrock who feels like a patient unable to live fully and instead is trapped in a state, under the effect of a drug given to patients under surgery.
