Second-degree AV block (Mobitz II)
A second-degree AV block in which conduction fails abruptly, without the progressive AV-nodal fatigue of Mobitz I.
Key ECG features
- ✓PR fixed on every conducted beat
- ✓Non-conducted P waves appear without warning
- •Nothing builds up to the dropped beat
- ±P-P regular straight through the drop
✓ required · • typical · ± may be present
ECG characteristics
The conducted beats keep a fixed, constant PR interval and non-conducted P waves appear without warning - nothing builds up to the dropped beat. The QRS may be wide if the block sits below the bundle of His.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Usually disease below the AV node, in the His-Purkinje system, rather than vagal tone.
Why it matters
More likely than Mobitz I to progress to complete block, so it warrants closer evaluation, and a symptomatic patient becomes a pacing candidate. Unlike vagally mediated block it typically does not improve with atropine, which is why the atropine response is used to tell the two apart.
Differentiate from
- Second-degree AV block (Mobitz I)Mobitz I lengthens the PR interval progressively before dropping a beat; here it stays constant and the drop is abrupt.
- Third-degree (complete) AV blockConducted beats still exist here, with a fixed PR interval. In complete block nothing conducts and the PR varies constantly.