[[Image:Rhythm_ventricular_premature.png|thumb|A [[Premature ventricular beats(PVB) / Venticular extrasystoles (VES)|ventricular extrasystole]] with a compensatory pause]]
[[Image:pacemaker_rates.png|thumb|Mycoardial cells with pacemaker activity]]
The pacemaker cells in the sinus node are not the only cells in the heart that can depolarize spontaneously. Actually all cardiomyoctyes have this capacity. The only reason why the sinus node 'rules' is that it is the fastest pacemaker of the heart.
From sinus node to ventricle all healthy cardiomyocytes can function as ectopic pacemakers. Ectopic pacemaker activity can originate from the atria (60-80 bpm), AV-node (40-60 bpm) and the ventricles (20-40 bpm). So, as the sinus rate drops (e.g. during atrial infarction), other cells can take over. The configuration of ectopic beats or extrasystoles, as seen on the ECG, reveals its origin, whether they are [[Atrial_Premature_Complexes|atrial]], nodal or [[ves|ventrical]].
The duration of the interval following a premature beat can help in the differentiation of a atrial or ventricular premature beat.
===Non-compensatory pause, following an atrial premature
beat===If a sinus complex is followed by an atrial premature complex, the electrical activity of the atrial premature complex can enter the sinus node and reset its timing. If the sinus rate were for example 60 /min, the reset pacemaker cells will restart firing after 1 full second has passed. The time from the extra beat to the next QRS complex in this example is thus 1 second. The sequence "normal sinus complex" - "atrial premature complex" - "normal sinus complex" will therefore be shorter than the 3 seconds it would have taken if the premature complex had not occurred. This is called a non (fully) compensatory pause.
Sometimes the electrical activity of the atrial premature complex cannot enter the sinus node due to refractoriness of the sinus node (if the interval between previous sinus complex and atrial premature complex is short) or due to electrical block to enter the sinus node (as in sinus nodal disease).