ECG rhythms AV blocks
Second-degree AV block, Mobitz II ECG
Second-degree AV block, Mobitz II. The PR is constant on every conducted beat, then a P wave suddenly fails to conduct.
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 | 54/min |
|---|---|
| PR | 158 ms |
| QRS | 84 ms |
| QT | 375 ms |
| QTc (Bazett) | 408 ms |
| QTc (Fridericia) | 397 ms |
What it is
Second-degree AV block in which the PR is constant on the conducted beats and a P wave suddenly fails to conduct. The block is usually below the AV node, in the His–Purkinje system, and the QRS is often wide.
How to read this strip
| Rate | Ventricular rate slower than the atrial rate. |
|---|---|
| Rhythm | Irregular because of dropped beats. |
| P waves | Regular sinus P waves; some not conducted. |
| PR | Normal and constant on conducted beats. |
| QRS | Narrow in this tracing; Mobitz II is usually below the AV node and often comes with a bundle branch block. |
| QT | Normal. |
Causes
- Degenerative conduction system disease.
- Anterior myocardial infarction.
- After cardiac surgery or transcatheter aortic valve replacement.
- Infiltrative disease (sarcoidosis, amyloidosis), myocarditis.
- Hyperkalemia.
What to consider
- It can progress suddenly to complete heart block: monitor the patient with transcutaneous pacing available.
- Without a reversible cause, a permanent pacemaker is generally recommended (2018 ACC/AHA/HRS).
- Atropine acts on the AV node and is unlikely to help a block below it; it can even worsen the block by speeding the sinus rate. In an unstable patient it may be tried per the algorithm but should not delay transcutaneous pacing.
- The PR after the dropped beat is the same as before it: that is what separates it from Mobitz I.
- 2:1 AV block cannot be called Mobitz I or Mobitz II from the strip alone.
Key point
A constant PR with sudden dropped beats means Mobitz II: monitor closely, keep pads at hand and plan for pacing.
Not to be confused with
- Second-degree AV block, Mobitz I (Wenckebach) — The PR does not lengthen before the dropped beat.
- Sinus rhythm with blocked PACs — The non-conducted P waves come on time, at the same rate as the others.
- Third-degree AV block — Most P waves conduct with a fixed PR. In complete block none conduct.