ECG rhythms AV blocks
Third-degree AV block with a junctional escape ECG
Third-degree AV block with a narrow escape rhythm. Regular P waves march through at a faster rate than the regular, narrow, slow QRS complexes, with no fixed PR.
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 | — |
| QRS | 84 ms |
| QT | 426 ms |
| QTc (Bazett) | 356 ms |
| QTc (Fridericia) | 378 ms |
What it is
Complete (third-degree) AV block: no atrial impulse reaches the ventricles. Atria and ventricles beat independently; the P waves march through at their own regular rate, and a narrow junctional escape rhythm (usually 40–60/min; about 42/min on this recording) drives the ventricles.
How to read this strip
| Rate | Atrial faster than ventricular; ventricular slow (below 50/min). |
|---|---|
| Rhythm | P–P regular; R–R regular; the two are independent. |
| P waves | Regular, none conducted. |
| PR | Varies on every beat (no true PR). |
| QRS | Narrow. |
| QT | Normal for the slow rate. |
Causes
- Inferior myocardial infarction (often temporary, at the AV node).
- Drugs: beta-blockers, calcium channel blockers, digoxin.
- Congenital complete heart block.
- Lyme carditis.
- Degenerative conduction disease.
- Hyperkalemia; after cardiac surgery or valve replacement.
What to consider
- Assess perfusion: a narrow junctional escape is steadier than a wide ventricular one, but the patient can still be unstable.
- Have transcutaneous pacing ready; unstable patients follow the bradycardia algorithm.
- Atropine is more likely to help when the block is at the AV node (as with a narrow escape or inferior infarction), but it should not delay pacing in an unstable patient.
- Treat the reversible causes listed above before deciding on a permanent pacemaker.
- Persistent complete block without a reversible cause generally needs a permanent pacemaker.
Key point
Complete block: regular P waves, regular QRS complexes, no relationship; a narrow escape is junctional and usually more stable than a wide one.
Not to be confused with
- Sinus bradycardia — There are more P waves than QRS complexes, and they are not linked.
- Second-degree AV block, Mobitz II — In Mobitz II most P waves conduct with a fixed PR. Here the PR changes on every beat because none conduct.
- Junctional escape rhythm without AV block (simulator) — In a junctional rhythm without block, any P waves are retrograde (inverted in II) or sit close to the QRS; they do not march through at a faster, independent rate. Here regular sinus P waves march through and none conduct.