Electrical conduction of the heart
A wiring system of specialised muscle cells spreads each heartbeat from the sinoatrial node through the atria, the atrioventricular node and the bundle branches to the ventricles.
The impulse starts in the sinoatrial node, sweeps across both atria (the P wave of the ECG) and reaches the atrioventricular node, which delays it for about a tenth of a second so the atria finish emptying. It then races down the bundle of His and the right and left bundle branches to the Purkinje fibres, depolarising the ventricles from the apex upward (the QRS complex). Recovery of the ventricles produces the T wave.
Because the wave follows a fixed path, its timing on an ECG reveals blocks (a long PR interval or a bundle branch block), extra pathways and the chaotic atrial activity of atrial fibrillation. Damaged muscle after a heart attack changes the shape of the QRS and ST segments.
Pacemakers replace a failing sinoatrial node or a blocked atrioventricular node; ablation destroys tissue that generates abnormal rhythms.
Key facts
| Fact | Value |
|---|---|
| AV node delay | ≈0.1 s |
| Normal PR interval | 0.12–0.20 s |
| Normal QRS duration | <0.12 s |