Ectopic Complexes
The pacemakercells in the sinusnode are not the only cells in the heart that can depolarize spontaneously. Actually all cardiomyoctyes have this capacity. The only reason why the sinusnode 'rules' is that it is the fastest pacemaker of the heart. From sinusnode to ventricle all healthy cardiomyocytes can function as a ectopic pacemaker. 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, nodal or ventrical.
Ectopic pacemakers
| Celltype | Frequency | QRS width (*) |
|---|---|---|
| SA node (not ectopic) | 60-100 bpm | narrow |
| Atrial | 55-60 bpm | narrow |
| AV Nodal ectopic pacemaker | 45-50 bpm | narrow |
| His bundle | 40-45 bpm | narrow |
| Bundle branch | 40-45 bpm | narrow or wide |
| Purkinje cells | 35-40 bpm | wide |
| Myocardial cells | 30-35 bpm | wide |
(*) QRS width can only be narrow if the conduction system downstream is normal (i.e. no bundle branch block)