How to proceed with metoprolol (beta-blocker) in a patient with a history of Coronary Artery Bypass Grafting (CABG) and reduced Left Ventricular Ejection Fraction (LVEF) who experiences episodes of Mobitz type II heart block?

Management of Beta-Blocker Therapy in Post-CABG Patient with Reduced LVEF and Mobitz Type II Heart Block

Beta-blocker therapy should be temporarily discontinued in this post-CABG patient with severely reduced LVEF who is experiencing Mobitz type II heart block, and permanent pacemaker implantation should be considered before resuming metoprolol.

Assessment of the Clinical Situation

This case presents a challenging scenario with competing priorities:

1. The patient has severely reduced LVEF (25-30%), requiring guideline-directed medical therapy (GDMT) including beta-blockers
2. The patient has developed Mobitz type II heart block, which is:
   • Occurring at night during sleep
   • A potentially dangerous form of heart block that can progress to complete heart block
   • Often located below the AV node (infranodal)

Management Algorithm

Step 1: Immediate Management

• Temporarily discontinue metoprolol since beta-blockers can worsen conduction abnormalities, especially with pre-existing heart block 1
• Monitor the patient with continuous ECG for at least 48 hours to assess for progression of heart block 2

Step 2: Evaluate for Permanent Pacemaker

• Arrange urgent cardiology consultation for permanent pacemaker evaluation
• Mobitz type II block is concerning because:
  • It has a higher risk of progression to complete heart block compared to Mobitz type I 1
  • The site is almost always below the AV node (infranodal) 1
  • It can lead to Stokes-Adams attacks (syncope) 1
  • Beta-blockers will likely worsen the conduction abnormality

Step 3: Resume GDMT After Pacemaker Implantation

• After pacemaker implantation, restart metoprolol at a low dose (12.5-25 mg daily) 3
• Titrate gradually every 2 weeks as tolerated toward target dose 3
• Beta-blockers (specifically bisoprolol, carvedilol, or sustained-release metoprolol succinate) are Class I recommended therapy for patients with:
  • LVEF ≤40% 2
  • Prior or current symptoms of heart failure 2
  • History of myocardial infarction 2

Evidence-Based Rationale

Beta-Blocker Importance in Reduced LVEF

Beta-blockers are a cornerstone of therapy for patients with reduced LVEF, with multiple guidelines supporting their use:

• The ACC/AHA guidelines recommend beta-blockers for all patients with current or prior symptoms of HF and reduced LVEF (Class I recommendation) 2
• Beta-blockers have been shown to reduce mortality by 34-35% in patients with systolic heart failure 3
• Specifically, metoprolol succinate is one of the three beta-blockers (along with bisoprolol and carvedilol) proven to reduce mortality in heart failure 2

Mobitz Type II Heart Block Concerns

Mobitz type II heart block represents a significant contraindication to beta-blocker therapy without pacemaker protection:

• It typically occurs below the AV node and has a higher risk of progression to complete heart block 1
• Beta-blockers can further depress AV conduction, potentially precipitating complete heart block
• Nocturnal heart block may be exacerbated by increased vagal tone during sleep, and beta-blockers could worsen this effect

Special Considerations

Post-CABG LVEF Improvement

• More than 50% of patients with pre-operative EF ≤35% improve to >35% after CABG 4
• Improvement is more likely in patients with preoperative EF between 26-35% compared to those with EF ≤25% 4
• Consider reassessing LVEF after 3 months to determine if there has been improvement

Beta-Blocker Titration After Pacemaker

• Start with low dose (metoprolol succinate 12.5-25 mg daily) 3
• Titrate gradually every 2 weeks if well-tolerated 3
• Monitor heart rate, blood pressure, and clinical status after each dose titration 3
• Target dose for metoprolol succinate is 200 mg daily 3

Pitfalls to Avoid

1. Do not continue beta-blocker therapy in the presence of untreated Mobitz type II heart block, as this could precipitate complete heart block and hemodynamic collapse

2. Do not permanently discontinue beta-blocker therapy without addressing the underlying conduction issue, as beta-blockers are crucial for improving survival in patients with reduced LVEF

3. Do not delay pacemaker evaluation, as Mobitz type II block has a high risk of progression to complete heart block

4. Do not miss the opportunity to optimize other GDMT including ACE inhibitors/ARBs, mineralocorticoid receptor antagonists, and potentially SGLT2 inhibitors once the conduction issue is addressed