Second-degree AV block (Mobitz I)
The Wenckebach form of second-degree AV block: the AV node fatigues progressively with each beat until one impulse fails to get through, then recovers and the cycle repeats.
Key ECG features
- ✓PR interval lengthens beat by beat
- ✓A QRS is eventually dropped
- •PR before and after the pause visibly differ
- •Cycle restarts after the pause
- •P-P interval stays regular
✓ required · • typical
ECG characteristics
The PR interval lengthens over successive beats until one P wave fails to conduct and a QRS is dropped. Comparing the PR immediately before the pause with the one immediately after it is the reliable test - they are visibly different.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Typically delay within the AV node itself, often vagally mediated. Also seen with drugs that slow AV conduction.
Why it matters
Generally lower-risk than Mobitz II. It is often benign and frequently improves with exercise or atropine, which is itself a useful diagnostic maneuver.
Differentiate from
- Second-degree AV block (Mobitz II)The PR interval lengthens beat by beat before the drop here and stays fixed in Mobitz II. The distinction changes the prognosis and the pacing decision.
- Sinus arrhythmiaBoth give a regularly irregular rhythm, but Wenckebach drops a QRS and sinus arrhythmia never does.