Kaiba's a vampire. He doesn't need to watch the monitor to hear Yuugi's heart dancing a quicker tempo in his ears. Focusing on it is helping to drown out the rest, but for some reason, he's perturbed by it.

Vampires have the predatory ability to smell changes in the blood, to tell how their prey was feeling. Maybe, they even preferred certain profiles of emotion in the blood.

Kaiba, the one who had denied himself until this man came into his life, could read the book written in Yuugi's blood, and it discomforts him.

"If you don't calm down, they may change their minds about keeping you here," he says quietly. "Breathe."

He says it commandingly, despite his low tone.