What Causes Unresponsive And Breathlessness After Undergoing Heart Valve Replacement?

My stepfather has a heart valve replacement the 21st of February, was alert after surgery confused but knew where he was and what day it was. Then 2 days later they had to do cpr because he stopped breathing has been unresponsive since. They did a cat scan and said couldn t see any signs of a stroke has some brain activity, breathing somewhat on his own and opening eyes some. Put a trac in about a week ago why is he not waking up. Has a pacemaker so cant do a MRI.