When do Premature Ventricular Complexes (PVCs) occur?

Premature Ventricular Complexes (PVCs) occur when a site in the ventricle fires prematurely. PVCs are a type of arrhythmia characterized by an ectopic heartbeat originating from the ventricles rather than the sinoatrial (SA) node, which is the natural pacemaker of the heart. This premature electrical impulse causes the ventricles to contract earlier than they would during a normal heartbeat, leading to an irregular rhythm.

The characteristics of PVCs include the presence of a wide and abnormally shaped QRS complex on an electrocardiogram (ECG), as the ventricular contraction happens without the normal preceding atrial contraction and possibly not following the expected conduction pathway. Understanding the mechanism of PVCs is important for evaluating cardiovascular health and guiding treatment options if necessary.

The other options pertain to different cardiac conditions or mechanisms: atrial contractions that occur prematurely are related to atrial arrhythmias, a heart rate exceeding 100 bpm signifies tachycardia but does not specifically indicate PVCs, and electrical signals being blocked at the AV node leads to different arrhythmias, such as heart block, rather than PVCs.