Has anyone used a window-less den as a baby's sleeping room?

We saw a great apartment that we really liked. Only thing is, one of the rooms is a den with a sliding door that doesn't have a window.

It does have a heating vent, but I'm not sure how well ventilated it is. Has anyone used a room like this as a baby's room? I'm so worried about the ventilation aspect.

The den is approx 9' x 8'.