• A premature ventricular contraction (PVC) is generated from an irritable impulse from a ventricular automaticity focus that is a bit earlier than than the expected sinus impulse. As a result, the expected sinus impulse cannot be conducted because the ventricular cells will enter refractory period once it reaches them.
• Because PVC is not conducted down the AV-His-Purkinje pathway, but rather spreads directly to nearby ventricular cells, the resulting ventricular depolarization is uncoordinated and QRS complex is widened, tall, and deep.
• Meanwhile, the expected sinus beat reaches the atrial cells (which have been left untouched by the PVC), causing atrial depolarization and sinus P wave. Depending on the chronological relation between the PVC and the expected sinus beat, P wave might end up buried in QRS complex of PVC which might be invisible or seen as a small P notch.
• Unifocal PVCs are identically-appearing PVCs coming from the same ventricular automaticity focus.
• Multifocal PVCs are distinctively-appearing PVCs coming from multiple ventricular automaticity foci.
• When there is a run of 3 or more consecutive unifocal PVC’s at fast rate, it is called ventricular tachycardia (because sinus beats are discharged concurrently at their own pace, AV dissociation is often seen).
If a run lasts > 30 secs = “sustained”
If a run lasts < 30 secs = “non-sustained”
• When there is 6 or more unifocal PVCs per minute, hypoxia is suspected and there is an increased risk of ventricular fibrillation.
• When PVC occurs during the vulnerable period (repolarization of Purkinje fibers) or falls at the peak or the first part of downslope of T wave, ventricular tachycardia or ventricular fibrillation can develop as a result.
• Coupling of a premature ventricular beat to one, two, or three normal sinus beats are called ventricular bigeminy, ventricular trigeminy, or ventricular trigeminy, respectively.
• Each coupling is called “couplet”.
• When there is a run of 3 or more consecutive unifocal PVC’s at normal rate = accelerated idioventricular rhythm (AIVR).
• AV dissociation could also be seen.
• AIVR is also referred to as "slow ventricular tachycardia" because this rhythm is essentially ventricular tachycardia that runs at a slower (normal) rate.
• This is often seen in reperfusion injury post-STEMI.
