It's very odd, exactly the same thing happened to a friend earlier in the week. He was getting up out of his seat, next to me, and just keeled over. Not unconscious, but obviously couldn't move unaided - boy was he heavy to move. Luckily he didn't hit anything, but went down on one knee quite hard, which caused him some grief later.
The ambulance took an hour and a half to come, which I was very unimpressed with and could only feel glad that it wasn't a heart attack or stroke. They checked him over and took him home, close by. It comes as a bit of a shock when it happens though, as Peter can attest.
It seems that if BP related, even if it is regularly checked, it can drop very suddenly. Is there nothing that can be done?
