Sinoatrial node 60 to 100, atrioventricular junction 40 to 60, ventricle 20 to 40
Mnemonic
“The faster tissue wins”, with intrinsic rates falling as you descend:
- Sinoatrial node - 60 to 100 per minute
- Atrioventricular node - 40 to 60
- Bundle of His and bundle branches - 40 to 45
- Purkinje fibres and ventricular myocardium - 20 to 40
Overdrive suppression means the fastest pacemaker sets the rhythm and suppresses the rest; a lower pacemaker only emerges as an escape rhythm when the one above it fails or is blocked.
The practical corollary is that the lower the escape focus, the slower and broader the rhythm, and the less reliable it is: a junctional escape is narrow and reasonably stable, whereas a ventricular escape in complete heart block is broad, slow and liable to fail, which is why it demands pacing.
Automaticity arises from the funny current (If), a slow sodium leak in phase 4, which is accelerated by sympathetic stimulation and blocked by ivabradine.
Expansion
| Pacemaker | Intrinsic rate |
|---|---|
| Sinoatrial node | 60 to 100 per minute |
| Atrial foci | 60 to 80 |
| Atrioventricular junction | 40 to 60 |
| Ventricular (Purkinje) | 20 to 40 |
The site with the steepest phase 4 depolarisation sets the rate and suppresses the others by depolarising them before they reach threshold; this is overdrive suppression.
Two clinical patterns follow. If the sinoatrial node fails, a junctional escape rhythm at 40 to 60 appears, with a narrow QRS and absent or inverted P waves. In complete heart block, a ventricular escape at 20 to 40 takes over, with a broad QRS, and is unreliable enough to cause syncope, which is the basis of Stokes-Adams attacks.
The lower the escape focus, the slower and less dependable the rhythm.