Guidelines in the US and Europe recommend compressions which are four or five cm deep, and that's a pretty significant percentage of the distance from your sternum to your back. Remember of course that one of the things that your rib cage is supposed to protect against is blows to your heart and lungs, so it shouldn't be too much of a surprise that a procedure that is supposed to compress your heart is also going to cause damage to your ribcage.
Yes this happened to my father. I remember the crunching sound of his ribs to this day. This was at waterloo station in London in 1993. He just keeled over. Fortunately for him, someone had just got stabbed 100 yards away so people were already on hand. The paramedic didn't even blink. Everyone else shit a brick.
He survived though and after a bypass op is fine to this day.
It probably helped as well that St Thomas's is literally within a few hundred yards of Waterloo Station.
The speed with which you can be in specialist care is absolutely critical when it comes to cardiac problems - which is a large part of the reason why survival rates are far higher in places like Central London where you've got both fast response time for paramedics (for London as a whole, ~75% of 'immediately life-threatening' calls have a response time of under eight minutes) and multiple world class Hospitals with specialist cardiac centres.
Even so, that's one hell of a lucky break your father had and fantastic that he both survived and is fine still!
There's new technology that automates this and the incident of rib-breakage is, as I understand, drastically minimized. (http://en.wikipedia.org/wiki/AutoPulse)
As an anecdote, my 70+ year old dentist (i.e. not the demographic with the strongest bones) had a heart attack (clinically dead). The responding EMTs used the AutoPulse, which saved his life. It pumped his heart for 10+ minutes while 1 EMT gave him air and the other drove to the hospital. No brain damage, no broken ribs.
Is that always necessarily true...?