[Herodotus38]

To be a little more clear pacemaker != defibrillator, although often they are combined into one device. A defibrillator gives a large shock to the heart to knock it out of a dangerous rhythm such as ventricular tachycardia or ventricular fibrillation. A pacemaker is designed to send a smaller shock in such a way as to stimulate the atria and/or ventricles to depolarize and contract (recreating the normal rhythm).

Pacemakers are designed to either constantly go off or to detect missed beats and give shocks. They do not knock people down and I've never seen anybody ever complain of a pacemaker shock being even detectable. A defibrillator can be part of a pacemaker (in which case it's called an ICD), and it WILL hurt if it goes off. I think that is what you mean.