If it is an arrhythmia like ventricular fibrillation, as in sudden cardiac death, and cpr is done right away you can save a life.
If a person is dying of something else it might not work.
And if it isn't started soon enough you might end up with someone brain damaged from lack of oxygen.
A lot of doctors don't want cpr on themself for that reason.
Thank you for your intelligent response to the article posted.