Ectopic Complexes

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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

Cells that have pacemaker activity
cell 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 broad
Purkinje cells 35-40 bpm broad
Myocardial cells 30-35 bpm broad

Examples