Actually, 60% isn’t that uncommon, having to do with fluid dynamics.
It’s the old “garden hose rule”, which is to take a garden hose with a kink in it, and watch the flow of water as you tighten the kink. It will continue with very little obvious decrease until it is almost completely cut off. Then suddenly it stops.
The same rule applies with plaque build up in the arteries.
It is also a bugger to diagnose without using medical imagining, which they do a lot of.
http://en.wikipedia.org/wiki/Cardiac_imaging
The most recent trendy diagnosis for people over 60 is to check for such buildup in their neck, in the carotid artery and jugular vein. These are the bodies high pressure fire hoses, so the assumption was that they were less prone to narrowing. This was a wrong assumption, and this common condition can be deadly. So elderly people are getting systematically checked for this.
My Mom, before she passed two weeks ago at 89, had both carotid arteries bypassed for major blockages, several years ago. She’d had 6 minor TIAs as a result of the blockages.