I suspect you may have a battle for compensation if the effect wasn't instant. Proving it was the security scanner might be difficult unless the pacemaker has some kind of data logging built in.
You may very well be correct. But surely the point here is that the training and instructions given to security staff should be such that the risk isn't taken. Whether or not the pacemaker was actually affected, the elderly person had a heart condition and the stress of being forced to go through the arch which he knew was the wrong thing to do could in itself have made him ill.
I believe the complainant has a duty to press a complaint in order to prevent this sort of bullying being repeated on other, perhaps less fortunate, victims.