Ectopic Foci Cause Premature Contractions Of The Heart

The SA node is the normal pacemaker for the heart. Occasionally cells outside the SA node can become irritated and elicit an action potential before the SA node fires. When this occurs, it is termed an ectopic focus and that action potential will conduct over the entire heart, causing a premature contraction often referred to as an extrasystole. Extrasystoles can originate in the atria, the ventricles, or the AV node region. Premature atrial contractions have an early P wave, usually with a modified shape because the pacemaker is outside the SA node. The QRS complex may be normal or may be altered if part of the ventricle is still partially a refractory. Because the SA node will be reset, the following beat will usually occur with a normal R-R interval.

Figure 10 shows an extrasystole that originated from an ectopic focus located in the ventricles, often referred to as a premature ventricular contraction (PVC). Because the SA node did not initiate this beat, there is no P wave preceding it. Furthermore, because the ectopic focus did not activate the ventricles by traveling down the Purkinje fibers, conduction is slow and the QRS complex is consequently very broad and unusually shaped. The AV node usually does not conduct well in the retrograde direction; hence, the SA node often is not reset by a PVC. This causes the ectopic beat to be followed by a compensatory pause while the ventricle waits for the next conducted impulse from the SA node. If the ectopic focus is in the AV node region, a near normal QRS complex will be seen since the normal conduction system is used to activate the ventricle. It will be early, however, and not preceded by a P wave. PVCs occur occasionally in all healthy hearts and are not a cause for alarm. When they suddenly become

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