Atrioventricular Node

The atrioventricular node (AV node) is a small cluster of specialised cardiac cells located at the junction between the atria (upper chambers) and the ventricles (lower chambers) of the heart. It plays a critical role in the normal cardiac conduction system, acting as both a relay station and a gatekeeper for electrical signals passing from the atria to the ventricles.

Function of the AV node

In a normal heartbeat, an electrical impulse begins in the sinus node (the heart's primary pacemaker) and spreads across the atria, causing them to contract. The impulse then reaches the AV node, which deliberately slows it down before allowing it to pass to the ventricles via the bundle of His and the bundle branches. This brief delay (the PR interval on an ECG, normally 0.12 to 0.20 seconds) is essential: it ensures that the atria have finished contracting and pushing blood into the ventricles before the ventricles contract to pump blood out.

What happens when the AV node is blocked

Heart block occurs when conduction through the AV node is impaired:

• First-degree heart block: the impulse is delayed but all beats get through (PR interval prolonged)
• Second-degree heart block: some impulses are blocked, causing occasional missed beats
• Third-degree (complete) heart block: no impulses pass through; the atria and ventricles beat independently

Complete heart block is a cause of cardiac arrest and requires emergency pacing.

AV node ablation

In patients with uncontrolled rapid heart rates (particularly from atrial fibrillation) that cannot be controlled by medication or other means, the AV node can be intentionally destroyed by radiofrequency ablation. This stops all conduction from the atria to the ventricles and requires a permanent pacemaker to take over ventricular pacing.