AV delay

The AV delay is a deliberate, well-timed pause built into the heartbeat. As the impulse from the atria reaches the atrioventricular node on its way to the ventricles, the node slows it down for a fraction of a second before letting it through. This brief hesitation is not a flaw — it is exactly what makes the pump work in the right order.

The reason is mechanical timing. The atria need a head start: they contract first to top off the ventricles with a final push of blood, and only then should the ventricles contract. The pause at the node — typically around a tenth of a second — gives the atria time to finish emptying before the ventricles squeeze. Without it, atria and ventricles would contract almost together, and the ventricles would be pumping before they are properly filled.

On the ECG this delay is the major component of the PR interval, the gap between the start of the P wave and the start of the QRS complex. When conduction through the node slows abnormally, the PR interval lengthens (first-degree heart block); when it fails intermittently or completely, beats are dropped or the atria and ventricles march independently. Implanted pacemakers even let clinicians program an artificial AV delay to recreate this natural timing.