Cardiologists often are called to explain electrocardiograms of pacemaker/ implantable cardioverter (PMK/ICD) patients during arrhythmic events. The most frequent arrhythmia is atrial fibrillation (AF) whether in PMK or in ICD patients. Generally, it is not difficult to diagnose, it can affect the quality of life of this subgroup because it can generate inappropriate and painful therapies. Arrhythmias as atrioventricular block or other bradyarrhythmias can show a particular way of presentation specially for the device’s intervention using specific algorithms that cardiologists should know to adequately interpret the phenomenon. For example, Rate Drop Response algorithm (to prevent syncope) or other antiarrhythmic functions for atrial tachyarrhythmias (Post Mode Switching Overdrive Pacing (PMOP) or Atrial Rate Stabilization) can alter surface electrocardiogram after AF or atrial ectopic beats. Ventricular arrhythmias in ICD patients are frequent. Burst, ramp and shock are therapies with a high percentage of efficacy. However, sometimes, supraventricular arrhythmias (SVT) can induce inappropriate interventions. In other cases appropriate burst, ramp or shock (during a ventricular tachycardia (VT)) can degenerate it in a fast ventricular tachycardia or in ventricular fibrillation with consequent shocks. Wavelet, onset, stability, and other algorithms, specifically in dual chamber ICDs, are used to discriminate SVT from VT assuring more specific interventions.