No beds are kept open, although if someone is well enough to be transferred out to a step-down unit, they'll push for that to happen sooner so that they have a bed available for emergencies.
One nurse in the ICU is assigned to be the Code Nurse for that shift, and attends all code blues along with the doctor. That nurse either has no assigned pt in the ICU or has a very stable pt. If there is a code blue, and there are no other beds, the most stable pt gets turfed out. Hey, publicly funded means that all hospitals in the city are under the same administration, so if in dire straits, they can transfer a pt to an ICU in another hospital.
We had a BAD winter this past year with every ICU bed in the entire city being full... someone on the ward codes, they have to send us someone in exchange. Usually someone who is not quite stable enough to be out of the ICU yet. That person will then crash a day or so later, and the cycle continues. We had quite a few pt's bouncing back and forth between the ICU and the wards because the system couldbn't handle the volume of sick pt's. It was a bad winter for staff burnout, as well as I'm sure pt's died as a partial result of this.