How to diagnose ST-Elevation Myocardial Infarction (STEMI) in patients with Left Bundle Branch Block (LBBB) or paced rhythm?

Diagnosing STEMI in LBBB or Paced Rhythm

In patients with suspected STEMI who have LBBB or paced rhythm, the Sgarbossa criteria should be used for diagnosis, with concordant ST-segment elevation ≥1mm in any lead being the most specific finding warranting immediate reperfusion therapy. 1

Diagnostic Challenges

Diagnosing STEMI in the presence of LBBB or paced rhythm is challenging because:

• The baseline ECG already shows ST-segment and T-wave abnormalities
• Normal discordance (ST-segment deviation opposite to the main QRS deflection) can mimic ischemic changes
• Traditional STEMI criteria cannot be applied directly

Sgarbossa Criteria for LBBB and Paced Rhythm

The Sgarbossa criteria are the most validated tool for diagnosing STEMI in LBBB and can be applied to paced rhythms as well 1, 2:

1. Concordant ST-segment elevation ≥1mm in leads with positive QRS complexes (5 points)

   • Highest specificity (92-98%) 1, 3
   • Most reliable indicator of acute coronary occlusion

2. Concordant ST-segment depression ≥1mm in leads V1-V3 (3 points)

   • Specificity 96% 1

3. Excessive discordant ST-segment elevation ≥5mm in leads with negative QRS complexes (2 points)

   • Specificity 82% 1

A score ≥3 is highly specific for STEMI and warrants immediate reperfusion therapy 3.

Modified Sgarbossa Criteria (Smith's Modification)

The modified criteria replace the third Sgarbossa criterion with:

• Excessive discordant ST elevation defined as ST/S ratio ≥0.25 (ST elevation to S wave depth ratio)
• Improves sensitivity while maintaining specificity

Diagnostic Algorithm

1. Initial Assessment:

   • Obtain 12-lead ECG within 10 minutes of first medical contact 1
   • Assess for clinical symptoms consistent with ischemia

2. Apply Sgarbossa Criteria:

   • If score ≥3 (especially with concordant ST elevation): Diagnose STEMI and proceed with reperfusion
   • If score <3 but high clinical suspicion: Proceed to next steps

3. Additional Diagnostic Steps:

   • Obtain serial ECGs to detect evolving changes 1
   • Measure cardiac troponin (elevated troponin significantly increases predictive value of LBBB for STEMI) 4
   • Consider echocardiography to detect wall motion abnormalities 1
   • If still uncertain with high clinical suspicion: Consider emergency angiography 1

Important Caveats

• New or presumably new LBBB alone should not be considered diagnostic of STEMI in isolation 1
• The 2013 ACC/AHA guidelines removed the recommendation that new LBBB alone is a STEMI equivalent 1, 3
• Patients with LBBB and suspected ischemia have higher baseline mortality and receive greater benefit from reperfusion when STEMI is confirmed 5
• For paced rhythms, temporarily reprogramming the pacemaker (if patient is not pacemaker-dependent) may allow better ECG interpretation 1

Special Considerations

• In patients with RBBB, standard STEMI criteria can still be applied 1
• Consider obtaining posterior leads (V7-V9) in patients with isolated ST depression in V1-V3 to detect posterior MI 1
• Patients with LBBB and chest pain represent a high-risk population - when in doubt with strong clinical suspicion, emergency angiography should be considered 1

Pitfalls to Avoid

1. Don't assume all LBBB with chest pain is STEMI - this leads to false catheterization lab activations
2. Don't dismiss STEMI possibility in LBBB - some patients do have acute coronary occlusion requiring reperfusion
3. Don't rely solely on ECG - integrate clinical presentation, biomarkers, and imaging when available
4. Don't forget to obtain serial ECGs - changes over time may reveal evolving STEMI

The Sgarbossa criteria remain the most validated tool for diagnosing STEMI in LBBB and paced rhythm, with concordant ST elevation being the most specific finding that should trigger immediate reperfusion therapy.