I think they've kinda got the wrong idea there - it's actually opening the bowels which can precipitate a cardiac arrest, as the nerve which slows the heart down is the same as that which makes you want to open your bowels (although there is some debate as to whether it's actually the slowing down of the heart that makes you want to go to the loo...). I've come across it happening only 2-3 times even though I've worked in cardiology/A&E/critical care for quite a while, so I'm not convinced it's that common. Generally I don't think you can stop someone going to the loo, but it's usually a good idea (as with any reasonably ill patient) to either go with them or make sure they keep the door unlocked.