When performing cardiac compression, you may feel some cracking (of ribs).
Just keep going.
It seems the old technique of pounding on the chest to shock the heart out of fibrilation is noW discouraged because so few people were aware that you are only supposed to raise your fist one fist-width above the chest before slamming down to limit your force so people where lifting their fist up about a foot or more and slamming down and rupturing the heart (killing the patient). (talk about kicking someone when their down.. XD) It also helps to ask a nearby female for their cosmetic mirror to hold over the nose to look for exhalation to confirm the patient is indeed not breathing. Some people don't know that so they stand there next to a woman who has half a dozen cosmetic mirrors in her purse and wait until the patient turns blue in the face before deciding they aren't breathing. (by then it's almost too late) It's also a good idea to use a small pen flashlight to check the pupils for pupil response. The left pupil indicates right side brain function and right pupil indicates left side brain function (I think that' how it works) . As long as you get a pupil response when you first arrive, the brain is still alive and there is still hope. If you arrive and no response, well , that's dead issue if you know what I mean. If you're doing the resusitation, get someone to check the pulse with one hand and touch their thumb and forefinger together with their other hand every time they feel a pulse (human pulse monitor). If allows the resuscitator to know if they are applying enough pressure to force the blood flow. If they get one thumb/forefinger touch every time they pump they know there is blood flow and it's working. (I'm not a paramedic but I worked in an ICU in a hospital in Japan when I was in the AF. Don't worry, most people in the ICU are unconscious so there's not much conversation. When they are conscious, the nurse usually doesn't want them to talk much anyway.) and yes I was good at my job. I made sergeant in less than two years)