Ectopic foci are abnormal pacemaker sites within the heart (outside of the SA node) that display automaticity. Their pacemaker activity, however, is normally suppressed (overdrive suppression) by the higher rate of the SA node. They can occur within the atria or ventricles.
Ectopic foci can cause additional beats (observed as premature beats) or take over the normal pacemaker activity of the SA node. These ectopic pacemakers can lead to either tachycardia or bradycardia depending upon their location and surrounding electrical conditions. For example, an ectopic foci in the ventricle when coupled with a reentry pathway can precipitate ventricular tachycardia. In third degree AV nodal block, a ventricular rhythm still occurs because of the expression of normally latent ectopic pacemaker sites within the ventricle. These ectopic ventricular pacemakers generally produce a rhythm (30-40 beats/minutes); this resting rate is much slower than that generally produced by the SA node (60-100 beats/min).
When an ectopic foci drives the rhythm of the heart, the spread of depolarization generally does not follow the normal, fast conducting pathways within the heart. Because of this, the depolarization wave takes longer to spread throughout the myocardium. This typically causes a shape change in QRS complex and prolongs its duration (>0.10 sec) because more time is required for the entire ventricle to become depolarized.