
Determine the topic of the passage.
"A premature ventricular contraction (PVC), also known as a premature ventricular complex, ventricular premature contraction (or complex or complexes) (VPC), ventricular premature beat (VPB), or ventricular extrasystole (VES), is a relatively common event where the heartbeat is initiated by Purkinje fibres in the ventricles rather than by the sinoatrial node , the normal heartbeat initiator."
Topic: