It may be caused by an irritable area in the ventricle acting as a rapid pacemaker, or may originate via a circular motion of electricity in an area of damaged myocardium. If atrial activity continues during the episode of V-tach, P waves may be seen deforming the ventricular complexes. Occasionally, an atrial depolarization may be conducted, resulting in a narrower QRS complex. This narrow QRS may resemble the patient's standard QRS complex, or may be a hybrid between the V-tach QRS and the normal QRS, called a fusion beat. Either of these findings confirms that the tachycardia is V-tach rather than an aberrantly conducted supraventricular tachycardia.
Ventricular fibrillation is chaotic irregular electrical activity within the ventricles.
No QRS complexes are seen. Lead II and lead V1 are most useful for differentiating premature beats, because the P waves are most obvious in II and the QRS shape is most helpful in V1. The P wave often has a different shape or size compared to the patient's normal P's. Premature beats that cause an inverted P wave to appear during the ST or T wave are originating in the AV node. The QRS is usually narrow.