However, in the instance of either a malfunctioning SA node or an ectopic focus bearing an intrinsic rate superior to SA node rate, ectopic pacemaker activity may take over the natural heart rhythm.

Ectopic pacemakers can occur within healthy hearts in response to various stimulating events, they can be caused by automaticity or triggered activity, such as: They can also occur within unhealthy hearts, caused by: An ectopic pacemaker can reside within a part of the electrical conduction system of the heart, or within the muscle cells of the atria or ventricles.

[13][14] On an ECG, the QRS complex will be abnormally shaped when looking at ventricular ectopic activity, often it occurs earlier with an absent P wave.

[12] During atrial ectopic activity where the P wave is normally rounded can be inverted or peaked.

[10] Conversely, during junctional ectopic activity the P wave is frequently absent or can be hidden in the QRS complex.