Premature ventricular contractions (PVCs), also known as extrasystoles, are a fairly common type of irregular heart rhythm (arrhythmia). In a normal heartbeat, an area of specialized cells called the sinoatrial (SA) node, located in the upper right atrium, generates an electrical impulse. This impulse spreads through the heart along a specialized conduction system, causing it to beat in a predictable, rhythmic fashion. However, there are other cells throughout the heart that can also generate electrical impulses. Some of them are located in the ventricles. When these fire early, they may cause the ventricles to contract, overriding the impulse from the SA node. In this case, the ventricles may still be recovering when the impulse from the SA node arrives and be unable to contract. This will result in a skipped, or missed, heartbeat.

PVCs are generally harmless and often have no symptoms. However, in patients with underlying heart disease, frequent PVCs may signify an increased risk of ventricular tachycardia (VT) – a serious arrhythmia in which the ventricles beat too rapidly and are unable to pump enough blood to the body. Ventricular tachycardia can lead to ventricular fibrillation, a potentially fatal arrhythmia in which the ventricles quiver and are unable to pump blood. Ventricular fibrillation requires immediate emergency medical attention to reestablish a normal heart rhythm. Heart patients who may be at increased risk due to PVCs include:

• Patients with reduced left ventricular ejection fraction. This is a measure of how much blood is pumped from the left ventricle with each contraction. Patients with heart failure frequently have a reduced left ventricular ejection fraction.
  
  
• Patients who suffer from an abnormally slow heart rate (e.g., bradycardia).
  
  
• Among patients with a previous heart attack, PVCs are associated with worse outcomes. Among these patients, beta blocker therapy is preferred. If the patient doesn't respond, the next treatment of choice is amiodarone, a potassium-channel blocker that is often used to treat arrhythmias.

Among people without clinically apparent heart disease, researchers have conducted multiple studies to uncover the relationship between PVCs and sudden cardiac death. Data from the Framingham Heart Study suggests that men who have PVCs with no symptoms are at greater risk of cardiac events than men without PVCs, even after other cardiac risk factors are taken into account.

In some cases, the cause of PVCs is unknown. In other cases, they may be due to known causes, such as: