The heart is designed in such a way that it has backup pacemakers so that if the SA node fails to pace a normal heart, other cells will take over. There is a chain of command in place with a specific order along the cardiac conducting system of who will take over command of the heart if a superior pacemaker fails to show up to work. The rate of spontaneous depolarization of the cardiac pacemaker cells decreases down the conducting system (or chain of command):
- SA node (60-100 bpm)
- Atria (< 60 bpm)
- AV node (40-60 bpm)
- Ventricles (20-40 bpm)
When things are working well, the atria, AV node, and ventricular pacemakers are suppressed by the more rapid impulses that are coming from the SA node above. However, when the rate of the impulses coming from above become lower than the native or natural rate of a subordinate pacemaker (like the AV node or ventricles), then they will step up and take over pacing until their higher in command comes back and takes over. This is an amazingly effective chain of command that ensures that we remain alive.