Difference between bundle branch block and Mobitz Type 2 block
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"bundle branch block" AND "atrioventricular block"
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Merck Manual bundle branch block Mobitz type II atrioventricular block ECG
Both involve the ventricular conduction system, but they interrupt conduction at different levels and produce different ECG findings.
| Feature | Bundle branch block (BBB) | Mobitz type II AV block |
|---|---|---|
| Where conduction fails | In the right or left bundle branch, after the His bundle | Usually in the His-Purkinje system, below the AV node |
| What happens to atrial-to-ventricular conduction | Every P wave is usually conducted to a QRS | Some P waves are suddenly not conducted, so a QRS is dropped |
| PR interval | Usually normal or unchanged | Constant in conducted beats, then an unexpected nonconducted P wave |
| QRS width | Typically wide, at least 120 ms in complete BBB | Often wide because the underlying disease is in the His-Purkinje system, but it can be narrow if the block is intra-Hisian |
| Key ECG pattern | Wide, altered QRS morphology: RBBB or LBBB pattern | Regular P waves with intermittent missing QRS complexes, without preceding PR prolongation |
| Clinical implication | May be incidental or reflect structural/ischemic heart disease, depending on type and context | Pathologic and higher risk: can abruptly progress to complete heart block, often requiring a permanent pacemaker |
The simplest distinction
- BBB: the electrical impulse still reaches the ventricles, but travels slowly through an alternative route. Thus, there is a wide QRS.
- Mobitz II: some electrical impulses from the atria do not reach the ventricles at all. Thus, there are dropped QRS complexes.
Example
Right bundle branch block:
Every P wave produces a QRS, but each QRS is broad.
P → wide QRS, P → wide QRS, P → wide QRSEvery P wave produces a QRS, but each QRS is broad.
Mobitz II:
PR intervals before conducted QRS complexes stay the same, then one P wave is not followed by a QRS.
P → QRS, P → QRS, P → no QRS, P → QRSPR intervals before conducted QRS complexes stay the same, then one P wave is not followed by a QRS.
They can coexist. For example, Mobitz II with a pre-existing bundle branch block suggests more extensive His-Purkinje disease and is concerning. Merck's AV-block reference notes that Mobitz II commonly progresses to complete heart block. Harrison's describes bundle branch block as a conduction delay/block in a bundle branch, producing characteristic ventricular conduction patterns. Harrison’s Principles of Internal Medicine, 22e, section “Bundle Branch Blocks and Related Patterns.”
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