I know this will sound awful, but if the ribs broke, it means you did it correctly. By compressing the chest deeply, about 5 to 6 cm, and at a rate of 100 to 120 beats per minute, you are within the limits of the bone structure, and it can potentially lead to fractures. The duration of CPR also plays a role. It is a freaking scary sound, but we saved lives because of it…I think maybe it's not a bad idea for all of us to have first-aid training once in a while…