Mr. Casaubon has never asked Lydgate anything about his illness, and Dorothea has never hinted to her husband what Lydgate had told her – that Casaubon's illness could come back at any time, and that the next time it might kill him.
But Casaubon decides that he needs to know what his prognosis is, partly because he wants to know whether he'll ever be able to finish his book, the "Key to all Mythologies," and partly (though he doesn't like to admit this even to himself) because he's worried about the idea of Dorothea remarrying.
Casaubon is unhappy – even though Dorothea is good to him, he suspects her of secretly disrespecting him.
His unhappiness about Dorothea and Will only makes his anxiety about his health worse.
Casaubon is toying with the idea of rewriting his will to make it difficult or impossible for Will to marry Dorothea if he should die suddenly.
So he calls Lydgate in for a consultation.
Lydgate is upfront with him, and tells him that heart disease is difficult to predict. He might have another heart attack tomorrow, or he might live many years.
Lydgate also tells Casaubon that he's already warned Dorothea about the uncertainty of heart disease.
Dorothea goes out to meet Casaubon after Lydgate's left, and is affectionate towards him, but he doesn't respond.
They go in, and he shuts himself in his library to deal with his sadness alone.
Dorothea goes to her own room, miserable because he's so distant with her.
She sits up without supper thinking things over, and finally decides that he's still worthy of her pity.
So she waits for him to come up from the library, and he takes her by the hand gently.