Are beta blockers (beta-adrenergic blocking agents) contraindicated in patients with Mobitz II second-degree atrioventricular (AV) block?

Beta Blockers are Contraindicated in Mobitz II Second-Degree AV Block

Beta blockers are absolutely contraindicated in patients with Mobitz II second-degree atrioventricular block due to their potential to worsen conduction abnormalities and precipitate complete heart block. 1

Pathophysiology and Risk Assessment

Mobitz II second-degree AV block is characterized by:

• Intermittent failure of atrial impulse conduction to the ventricles
• No progressive PR interval prolongation before blocked beats
• Block typically located at or below the His bundle (infranodal) 2
• Higher risk of progression to complete heart block compared to Mobitz I 3

This conduction disorder is fundamentally different from Mobitz I (Wenckebach) block, which:

• Occurs at the AV node level
• Shows progressive PR interval prolongation before blocked beats
• Usually has a more benign prognosis
• Often responds to atropine

Medication Contraindications in Mobitz II

Agents that slow AV conduction are specifically contraindicated in Mobitz II block:

1. Beta blockers - directly contraindicated 1
2. Calcium channel blockers (non-dihydropyridine)
3. Digitalis
4. Amiodarone

The 2018 ACC/AHA/HRS guideline on bradycardia and cardiac conduction delay clearly identifies beta blockers as medications that can cause or exacerbate bradyarrhythmias 1.

Clinical Implications and Management

When managing patients with Mobitz II block:

• Avoid all medications that slow AV conduction, especially beta blockers 1
• Consider temporary pacing for symptomatic patients 1
• Evaluate for permanent pacemaker implantation, as Mobitz II is typically an indication for permanent pacing
• If beta blockers are absolutely necessary for another condition, they should only be used after pacemaker implantation

The European Society of Cardiology guidelines specifically warn that agents that slow AV conduction (including beta-blockers) should be used with caution in patients with AV block 1.

Special Considerations

In rare circumstances, some cases initially appearing as Mobitz II may actually be:

• Hisian extrasystoles mimicking Mobitz II block 4
• Tachycardia-dependent AV block 5, 6

However, these are specialized diagnoses requiring electrophysiological studies and should not change the general approach of avoiding beta blockers in apparent Mobitz II block.

Clinical Pitfalls to Avoid

1. Do not confuse Mobitz I and Mobitz II blocks - they have different anatomical locations and prognoses
2. Never administer beta blockers to patients with known or suspected Mobitz II block
3. Avoid assuming a 2:1 AV block is either Mobitz I or II without additional diagnostic information 2
4. Remember that beta blockers can convert partial AV block to complete heart block in susceptible patients

In summary, beta blockers are firmly contraindicated in Mobitz II second-degree AV block due to their potential to worsen conduction and precipitate complete heart block, which could lead to significant morbidity and mortality.