What is the management plan for a patient with myocardial infarction (MI) and left bundle branch block (LBBB)?

Management of Myocardial Infarction with Left Bundle Branch Block

Patients with clinical suspicion of myocardial infarction and new or presumed new LBBB should receive immediate reperfusion therapy, preferably emergency coronary angiography with a view to primary PCI or, if unavailable, intravenous thrombolysis. 1

Diagnostic Challenges

The diagnosis of MI in the presence of LBBB is challenging due to the altered ventricular depolarization sequence, which affects ECG interpretation:

• LBBB masks the typical ST-segment elevation seen in STEMI
• Most patients with LBBB evaluated in emergency departments do not have acute coronary occlusion 1, 2
• The presence of concordant ST elevation (in leads with positive QRS deflections) is one of the best indicators of ongoing MI with an occluded infarct artery 1

Diagnostic Approach

1. Compare with previous ECGs if available to determine if LBBB is new 1

2. Look for specific ECG findings that suggest MI despite LBBB:

   • Concordant ST elevation ≥1 mm in leads with positive QRS
   • Concordant ST depression ≥1 mm in leads V1-V3
   • Excessively discordant ST elevation ≥5 mm in leads with negative QRS

3. Point-of-care troponin testing 1-2 hours after symptom onset may help decide whether to perform emergency angiography 1

Management Algorithm

Immediate Actions

1. ECG monitoring should be initiated as soon as possible to detect life-threatening arrhythmias 1
2. Oxygen therapy for patients with hypoxemia (SaO₂ < 90% or PaO₂ < 60 mmHg) 1
3. Pain relief with titrated IV opioids 1
4. Aspirin (162-325 mg) should be administered immediately 1

Reperfusion Strategy

For patients with clinical suspicion of MI and new or presumed new LBBB:

• Primary PCI is the preferred reperfusion strategy when available within 90 minutes of first medical contact 1
• Thrombolysis should be administered if PCI cannot be performed within 90-120 minutes 1
• Time targets:
  • Door-to-balloon time: ≤60 minutes in primary PCI hospitals 1
  • First medical contact to PCI: ≤90 minutes (≤60 minutes if early presenter with large area at risk) 1
  • First medical contact to fibrinolysis: ≤30 minutes 1

Monitoring for Complications

LBBB in the setting of MI indicates extensive myocardial damage and higher risk for complications:

• Continuous cardiac monitoring for at least 24 hours 1
• Monitor for progression to complete heart block - patients with LBBB developing in acute MI are Class I indications for temporary pacing 1
• Watch for signs of heart failure which is more common in patients with LBBB and MI

Pacemaker Indications

Temporary pacing is indicated for:

• LBBB developing in acute MI (Class I) 1
• LBBB with first-degree heart block of unknown duration (Class IIb) 1
• Complete heart block 1

Prognosis and Special Considerations

• Patients with MI and LBBB have higher mortality rates compared to those without bundle branch block 3
• The presence of LBBB in MI often indicates more extensive myocardial damage
• Consider early invasive management in patients with LBBB and MI who develop heart failure or cardiogenic shock 1

Common Pitfalls to Avoid

1. Delaying reperfusion therapy while waiting for definitive diagnosis - when in doubt with a clinically suspicious presentation and new LBBB, proceed with reperfusion 1

2. Overdiagnosis of MI in patients with chronic LBBB - most patients with LBBB and chest pain do not have acute coronary occlusion 2, 4

3. Failure to recognize high-risk features - LBBB with MI carries higher mortality and requires aggressive management 3

4. Missing progression to complete heart block - patients with new LBBB in MI should be monitored closely for progression to higher-degree blocks 5

The management of MI with LBBB requires a high index of suspicion, prompt recognition of high-risk features, and timely reperfusion therapy to improve outcomes in this high-risk population.