How to diagnose acute myocardial infarction (MI) in patients with ST elevation in paced precordial leads using the Scarbossa criteria?

Diagnosing Acute Myocardial Infarction in Patients with Paced Rhythms Using Sgarbossa Criteria

In patients with ventricular paced rhythms, the modified Sgarbossa criteria should be used to diagnose acute myocardial infarction, with any positive criterion warranting urgent cardiac catheterization. 1, 2, 3

Understanding Sgarbossa Criteria for Paced Rhythms

The standard Sgarbossa criteria were originally developed for LBBB but have been validated for ventricular paced rhythms. In paced rhythms, the following three ECG findings are independently predictive of AMI 1:

1. ST-segment elevation ≥1 mm concordant with the QRS complex (sensitivity 18%, specificity 94%)
2. ST-segment depression ≥1 mm in leads V1-V3 (sensitivity 29%, specificity 82%)
3. ST-segment elevation ≥5 mm discordant with the QRS complex (sensitivity 53%, specificity 88%)

Modified Sgarbossa Criteria: Improved Diagnostic Accuracy

The modified Sgarbossa criteria replace the third criterion with a proportional measurement that significantly improves sensitivity while maintaining high specificity 4:

1. ST-segment elevation ≥1 mm concordant with the QRS complex
2. ST-segment depression ≥1 mm in leads V1-V3
3. ST/S ratio ≤-0.25 in leads with discordant ST-segment elevation (replaces the ≥5 mm absolute criterion)

This modification has shown superior diagnostic performance with:

• Sensitivity of 91% (vs. 52% for weighted original criteria)
• Specificity of 90%
• Positive likelihood ratio of 9.0

Diagnostic Algorithm for Suspected MI in Paced Rhythms

1. Obtain 12-lead ECG immediately in patients with symptoms suggestive of ischemia
2. Apply modified Sgarbossa criteria:
   • Look for concordant ST elevation ≥1 mm in any lead
   • Check for ST depression ≥1 mm in leads V1-V3
   • Calculate ST/S ratio in leads with discordant ST elevation (ratio ≤-0.25 is positive)
3. If any criterion is positive: Activate cardiac catheterization lab immediately 2, 3
4. If criteria are negative but clinical suspicion remains high:
   • Obtain serial ECGs to detect dynamic changes
   • Check cardiac biomarkers (troponin)
   • Consider urgent angiography if symptoms persist despite medical therapy 1

Special Considerations for Precordial Leads

The precordial leads deserve special attention in paced rhythms:

• ST depression in V1-V3 is highly specific (82%) for AMI in paced rhythms 1
• Consider obtaining posterior leads (V7-V9) if suspecting posterior wall involvement 1, 5
• ST elevation ≥0.05 mV in V7-V9 can help identify posterior MI that might be missed on standard leads 5

Clinical Pearls and Pitfalls

• Don't wait for troponin elevation before activating the cath lab if modified Sgarbossa criteria are positive - early reperfusion saves myocardium 1
• The unweighted modified Sgarbossa criteria (where any positive criterion is considered diagnostic) provides better sensitivity without sacrificing specificity 2, 4
• Excessive discordance (ST/S ratio ≤-0.25) is the most sensitive criterion and should not be overlooked 4
• Beware of false negatives: The absence of Sgarbossa criteria does not rule out AMI - clinical judgment remains important 1
• Consider reprogramming the pacemaker in non-dependent patients to allow evaluation of intrinsic rhythm, but do not delay invasive investigation 1

The modified Sgarbossa criteria represent the most validated tool for diagnosing AMI in patients with paced rhythms and should be applied promptly to identify patients who would benefit from urgent reperfusion therapy.