ECG rhythms AV blocks
Second-degree AV block, Mobitz I ECG
Second-degree AV block, Mobitz I (Wenckebach). The PR lengthens beat by beat until one P wave is not conducted, then the cycle restarts with a shorter 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 | 56/min |
|---|---|
| PR | 158–278 ms, changing beat to beat |
| QRS | 84 ms |
| QT | 374 ms |
| QTc (Bazett) | 398 ms |
| QTc (Fridericia) | 390 ms |
What it is
A second-degree AV block that is usually in the AV node: each impulse finds the node a little more tired, so conduction slows beat by beat until one P wave cannot pass. The pause lets the node recover, and the cycle repeats as group beating.
How to read this strip
| Rate | Ventricular rate slower than the atrial rate. |
|---|---|
| Rhythm | Group beating; irregular. |
| P waves | Regular sinus P waves; some not conducted. |
| PR | Lengthens progressively before each dropped beat. |
| QRS | Narrow. |
| QT | Normal. |
Causes
- High vagal tone: sleep, athletes.
- AV-nodal blocking drugs.
- Inferior myocardial infarction.
- Myocarditis, Lyme disease, after cardiac surgery.
- Degenerative conduction disease.
What to consider
- Symptoms or hypotension make it significant; unstable patients follow the bradycardia algorithm.
- Review AV-nodal blocking drugs: they are a common reversible cause.
- Persistent symptomatic Mobitz I without a reversible cause may need a permanent pacemaker.
- Wenckebach with a wide QRS can arise below the AV node and behave more like Mobitz II.
Key point
Mobitz I usually sits in the AV node, is common during sleep and in athletes, and rarely needs treatment when the patient is well.
Not to be confused with
- Second-degree AV block, Mobitz II — In Mobitz II the PR is constant before the dropped beat. Here it lengthens.
- Sinus rhythm with blocked PACs — The non-conducted P waves arrive on time at the sinus rate, not early.
- Third-degree AV block — Most P waves conduct with a PR that changes in a pattern. In complete block none conduct.