Are people always put on a ventilator during cardiac bypass?

Yes.: Actually, by definition, people are put on a ventilator during all operations with general anesthesia. Due to the complexity of the surgery, many people are kept on the ventilator after open-heart surgery, however this is usually only for a short time. This is actually a quality metric reported by many cardiac surgery programs: see www.Sts.Org (risk adjusted prolonged intubation).

Interesting: Patients are mechanically ventilated immediately upon start of anesthesia. When on cardiopulmonary bypass, circulation to lungs and heart stops. Ventilator turned off and oxygenated through bypass pump. Ventilation resumes when done with bypass until extubated. Off-pump cabg, you stay on ventilator entire time as no bypass pump used.