Each heartbeat starts as an electrical signal that the heart makes on its own. Specialized conduction tissue carries the signal in a fixed order so the atria contract before the ventricles.
Sinus node
The sinus (SA) node sits in the wall of the right atrium and fires about 60 to 100 times per minute at rest. It is the natural pacemaker, and the nervous system speeds it up or slows it down.
AV node
The signal crosses the atria to the atrioventricular (AV) node, which holds it for a fraction of a second. This delay lets the atria finish filling the ventricles and protects the ventricles from very fast atrial rhythms.
Bundle of His, bundle branches and Purkinje fibers
From the AV node the signal runs down the bundle of His, splits into the right and left bundle branches and spreads through the Purkinje fibers. This fast network makes both ventricles contract almost at once, from the apex upward.
Backup pacemakers
If the sinus node fails, lower tissue takes over at a slower rate. The AV junction fires about 40 to 60 times per minute and the ventricles about 20 to 40.
| Pacemaker | Usual rate (per minute) | ECG clue |
|---|---|---|
| Sinus node | 60 to 100 | Normal P wave before each QRS |
| AV junction | 40 to 60 | Absent or inverted P waves, narrow QRS |
| Ventricles | 20 to 40 | Wide QRS, no related P waves |
At the bedside
A slow pulse with dizziness or low blood pressure points to a failing pacemaker or a heart block. Put the patient on a monitor, get a 12-lead ECG and have atropine and pacing ready.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).