Why did Ann think Loomis died?

Ann thought Loomis had died because she could not hear his breathing.

Once I was sure he had died. I slept on the sofa in the living room, and all night he did not make a sound. I could not really sleep for any length of time, but dozed now and then, and in between I would go in and see how he was. About two in the morning I went in, listened for his breathing, and did not hear it. I felt so frightened I thought my own heart was going to stop.