How to diagnose myocardial infarction (MI) in patients with left bundle branch block (LBBB)?

Diagnosing Myocardial Infarction in Left Bundle Branch Block

The diagnosis of myocardial infarction in patients with left bundle branch block (LBBB) should utilize the Sgarbossa criteria combined with high-sensitivity troponin testing at 0/1 or 0/2 hours for accurate and early diagnosis. 1, 2

ECG Criteria for MI in LBBB

Sgarbossa Criteria

• Concordant ST elevation ≥1 mm in leads with positive QRS deflections - highly specific (95-100%) but low sensitivity (1-12%) 2, 3
• Concordant ST depression in leads V1-V3 - highly specific but low sensitivity 3
• Excessive discordant ST elevation (≥5 mm) opposite to the direction of the QRS complex - more frequent (19%) but less specific 3

The presence of concordant ST elevation appears to be one of the best indicators of ongoing myocardial infarction with an occluded infarct artery 1.

Diagnostic Algorithm

1. Initial Assessment:

   • Obtain 12-lead ECG immediately
   • Compare with previous ECGs when available 4
   • Look specifically for Sgarbossa criteria

2. Laboratory Testing:

   • Obtain high-sensitivity cardiac troponin (hs-cTn) immediately
   • Repeat hs-cTn at 1-2 hours after presentation 2
   • Note: hs-cTn has very high diagnostic accuracy (AUC 0.89-0.91) in LBBB patients 2

3. Decision Pathway:

   • High-risk features (ongoing symptoms + positive Sgarbossa criteria): Proceed to immediate coronary angiography without waiting for troponin results 1, 4
   • Intermediate risk (clinical suspicion without definitive ECG changes): Use point-of-care troponin testing 1-2 hours after symptom onset to help decide whether to perform emergency angiography 1, 4
   • Low risk: Serial troponin measurements and observation

Important Considerations

• Only about 30% of patients with LBBB presenting with chest pain will ultimately be diagnosed with AMI 2
• There is no significant difference in AMI incidence between new LBBB (35%) and known LBBB (29%) 2
• The European Society of Cardiology no longer considers new or presumably new LBBB alone as an ST-elevation equivalent requiring immediate reperfusion 1, 5
• In hemodynamically stable patients with LBBB, the result of hs-cTn measurement should be integrated into the decision regarding immediate coronary angiography 1

Additional Diagnostic Tools

• Echocardiography: Should be performed immediately in patients with cardiac arrest or hemodynamic instability to assess for regional wall motion abnormalities 1, 4
• Additional ECG leads: Consider recording right precordial leads (V3R, V4R) to identify right ventricular involvement in inferior MI with LBBB 4
• Serial ECGs: Frequent serial ECGs can help detect dynamic changes suggestive of ongoing ischemia 6

Pitfalls to Avoid

• Overreliance on LBBB chronicity: Both new and known LBBB have similar rates of AMI 2
• Waiting for biomarker results in high-risk patients: When clinical suspicion is high with positive Sgarbossa criteria, reperfusion therapy should not be delayed 4
• Ignoring other diagnoses: More than 50% of patients presenting with chest pain and LBBB will have diagnoses other than MI 1

By combining ECG criteria with high-sensitivity troponin testing, clinicians can achieve early and accurate diagnosis of MI in patients with LBBB, avoiding unnecessary interventions while ensuring timely treatment for those with true MI.