ECG rhythms AV blocks
Atrial fibrillation with third-degree AV block ECG
Atrial fibrillation with third-degree AV block and a regular escape rhythm. Fibrillatory baseline with no P waves, but a slow, regular, narrow QRS: no atrial impulse is conducting.
A 30-second recording of the heart model, played in real time. Run live starts the engine itself with this rhythm.
Computed by the TracingsMD heart model (Junkers engine): simulated for education, not a patient recording and not for diagnosis.
Measured on this recording
| Ventricular rate | 42/min |
|---|---|
| PR | Not assessable in this rhythm |
| QRS | 84 ms |
| QT | 426 ms |
| QTc (Bazett) | 356 ms |
| QTc (Fridericia) | 378 ms |
What it is
Atrial fibrillation in which no fibrillatory impulse reaches the ventricles: a slow, regular escape rhythm (narrow if junctional, wide if ventricular) drives them instead. The clue is a fibrillatory baseline with a regular, slow ventricular rate.
How to read this strip
| Rate | Ventricular rate slow. |
|---|---|
| Rhythm | Regular. |
| P waves | None; fibrillatory baseline. |
| PR | None. |
| QRS | Narrow: junctional escape. |
| QT | Normal for the slow rate. |
Causes
- Digoxin toxicity (the classic cause).
- AV-nodal blocking drugs in combination.
- Conduction system disease.
- Inferior ischemia.
- After AV node ablation (intended; a pacemaker is normally already in place).
What to consider
- AF with a regular, slow rate is not well-controlled AF: suspect complete heart block.
- In a patient taking digoxin, suspect toxicity: check the level and potassium, stop AV-nodal blocking drugs, and consider digoxin antibody (Fab) fragments for life-threatening toxicity.
- Manage as complete heart block: monitor, have pacing ready, and plan a pacemaker if no reversible cause is found.
Key point
AF that suddenly becomes slow and regular means the ventricles are no longer driven by the atria: think complete heart block, and in a patient on digoxin think toxicity.
Not to be confused with
- Atrial fibrillation with a controlled ventricular rate — Conducted AF gives irregular RR intervals. Here the RR intervals are regular, varying by only a few tens of milliseconds.
- Junctional escape rhythm with a flat baseline (simulator) — The baseline is not flat; it shows fine fibrillatory activity.
- Atrial flutter with 4:1 conduction — Flutter shows regular sawtooth waves. Here the baseline activity is disorganized.