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.
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.
RK Revised 04/06/07