ECG Basics Part 1: Start here Lesson 2 of 22
The heart's wiring: the conduction system
Specialised tissue starts every heartbeat and times it. Each wave on the ECG is one stage of this path.
Six stops from top to bottom
- SA (sinoatrial) node — in the wall of the right atrium where the superior vena cava joins it. It is the natural pacemaker and normally fires 60–100 times a minute.
- Atria — the impulse spreads from cell to cell across the right atrium and, through Bachmann's bundle, to the left atrium. This makes the P wave.
- AV (atrioventricular) node — low in the wall between the atria, on the right atrial side, just above the tricuspid valve and near the opening of the coronary sinus (an area called the triangle of Koch). It is the slowest part of the path. It holds the impulse back for roughly a tenth of a second so the atria can finish emptying before the ventricles contract, and it limits how many very fast atrial impulses, for example in atrial fibrillation, reach the ventricles.
- His bundle — a ring of fibrous tissue insulates the atria from the ventricles, and normally the AV node and His bundle are the only electrical bridge across it. The His bundle passes through this ring to the top of the interventricular septum.
- Bundle branches — the right bundle branch runs down the right side of the septum; the left bundle branch fans out over the left side into an anterior and a posterior fascicle.
- Purkinje fibres — a fast network just under the inner lining of both ventricles. The septum is activated first, from left to right, then the apex and the free walls, and the base of the heart last. Each wall is activated from the inside (endocardium) outwards (epicardium).
How long each step takes
Approximate textbook timings for one beat in a healthy adult heart (they vary between people):
| Timing | Where the impulse is |
|---|---|
| By about 0.03 s | Reaches the AV node |
| The next 0.13 s or so | Held up in the AV node and His bundle |
| By about 0.16 s | Reaches the ventricular muscle (close to the PR interval) |
| About 0.06 s more | All ventricular muscle activated (close to the QRS duration) |
Rows that start with “By” count from the moment the SA node fires; the other two rows are how long that stage lasts (0.03 + 0.13 ≈ 0.16 s).
On a surface ECG the normal adult PR interval is 0.12–0.20 s and the QRS lasts less than 0.12 s.
Mapping the wiring onto one beat
| Stage of the path | What you see on the ECG |
|---|---|
| SA node fires | Nothing: too few cells to see from the skin |
| Atria depolarise | P wave |
| AV node, His bundle, bundle branches | Flat PR segment: too few cells to make a visible wave |
| Ventricles depolarise | QRS complex |
| Ventricles recover (repolarise) | T wave |
Backup pacemakers
If the SA node stops or its impulses are blocked, a lower pacemaker takes over with an escape rhythm. The AV junction usually fires at 40–60 /min. Its impulse still travels down the bundle branches, so the QRS stays narrow, as in this junctional escape example with no visible P waves:
A ventricular escape rhythm usually fires at 20–40 /min. The impulse starts in ventricular muscle and spreads slowly from cell to cell instead of through the fast wiring, so the QRS is wide. In this example of complete AV block the P waves carry on regularly but none of them reaches the ventricles, and wide escape beats come at about 30 /min:
More about pacemaker hierarchy is in Lesson 12: Why the heart beats by itself.
Where problems show up
- Slow or failing conduction through the AV node lengthens the PR interval or drops beats (Lesson 16: PR interval and AV block).
- A blocked bundle branch widens the QRS (Lesson 17: Narrow vs wide QRS: bundle branch block).
- Sympathetic drive speeds up the SA node and AV node conduction; vagal drive slows both.
Key points
- Path: SA node → atria → AV node → His bundle → bundle branches → Purkinje fibres.
- The AV node delays the impulse; this delay is most of the PR interval.
- P = atria, flat PR segment = AV node and His–Purkinje system, QRS = ventricles, T = ventricular recovery.
- Junctional escape: narrow QRS, 40–60 /min. Ventricular escape: wide QRS, 20–40 /min.
Check yourself
Which structure delays the impulse so the atria can finish emptying?
Answer: the AV node.
During the flat PR segment, where is the impulse?
Answer: in the AV node, His bundle and bundle branches; too few cells are active to make a visible wave.
If the ventricles pace themselves, is the QRS narrow or wide?
Answer: wide, because the impulse spreads slowly through muscle instead of the fast wiring.
Watch the conduction path light up beat by beat in the Heart view of the ECG Lab.
References
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021.
- 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.
- Surawicz B, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part III: intraventricular conduction disturbances. J Am Coll Cardiol 2009;53:976–981.
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