Foreshadowing in Crime and Punishment
Raskolnikov thinks about this impression later as a presentiment, a sign or feeling, that something is going to happen. Dostoevsky uses this line to indicate to the readers the importance of this meeting between Raskolnikov and not-yet-named clerk.
Nastasya is saying that the blood circulating in Raskolnikov's body is pounding in his brain and is giving him hallucinations. However, the causes for this dream are more likely associated with his guilt, and Dostoevsky uses sleep and dreams to foreshadow the beginning of Raskolnikov's internalization of his guilt.
If not for the sudden confession from Nikolay, Raskolnikov was likely very close to being arrested himself. This turn of events gives Raskolnikov a renewed sense of life and freedom similar to how he felt when he gave all his money to Katerina Ivanovna. However, if this moment is like the other, we know that his guilt will return and that this feeling of joy will be short lived.
Readers should view this line with suspicion. Having been rebuked by Dounia, Svidrigailov foreshadows that he has some other plan or plans in mind. His concern for his own health here represents a somewhat ironic statement on his part, especially considering what he plans to do. However, it does reinforce the image of Svidrigailov as a character who holds to certain ideas of decorum despite doing whatever he pleases with those around him.