ECG Basics · Lesson 2 · From cell to trace
Why the heart beats by itself
Nodal cells have no stable resting potential. They slowly depolarise after every beat until they fire again, which is why the heart keeps a rhythm without any outside signal.
Three phases, not five
Sinoatrial (SA) and atrioventricular (AV) nodal cells start each cycle from a maximum diastolic potential of about −60 mV, much less negative than working muscle. From there:
- Phase 4, diastolic depolarisation: a slow upward drift carried by the "funny" current (If, through HCN channels), T-type calcium current and calcium released inside the cell.
- Phase 0, upstroke: carried by L-type calcium current, so it is slower and smaller than the sodium upstroke of muscle cells. This is one reason the AV node conducts slowly.
- Phase 3, repolarisation: delayed-rectifier potassium currents bring the cell back down, and the drift starts again.
What sets the rate
Rate depends on how long phase 4 takes. A steeper slope, a lower threshold or a less negative starting point reaches threshold sooner.
- Sympathetic drive (noradrenaline, β1 receptors) increases If and calcium currents, so the slope steepens and rate rises.
- Vagal drive (acetylcholine) opens IK,ACh, which makes the starting point more negative, and reduces If, so the slope flattens and rate falls.
A hierarchy of backups
Several tissues can pace the heart. The fastest one wins: each of its beats resets the slower pacemakers before they reach threshold, and being driven faster than their own rate further depresses their automaticity (overdrive suppression).
| Pacemaker | Usual rate when in charge | QRS when it takes over |
|---|---|---|
| SA node | 60–100 /min | Narrow, preceded by upright P in II |
| AV junction | 40–60 /min | Usually narrow, P absent, hidden or retrograde |
| Ventricle | 20–40 /min | Wide |
When the SA node fails or impulses are blocked, the next pacemaker down produces an escape rhythm. When a lower pacemaker fires faster than its own escape range but slower than a tachycardia, it is an accelerated rhythm: accelerated junctional rhythm (about 60–100 /min) or AIVR (about 40–120 /min). It shows on the ECG when that rate is above the sinus rate, or when sinus slows below it.
References
- DiFrancesco D. The role of the funny current in pacemaker activity. Circ Res 2010;106:434–446.
- Kusumoto FM, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation 2019;140:e382–e482.
- Inada S, Hancox JC, Zhang H, Boyett MR. One-dimensional mathematical model of the atrioventricular node including atrio-nodal, nodal, and nodal-His cells. Biophys J 2009;97:2117–2127 (model used for the pacemaker figure).