Sgarbossa Criteria for Diagnosing AMI in Left Bundle Branch Block
The Sgarbossa criteria consist of three specific ECG findings that identify acute myocardial infarction in patients with left bundle branch block, with high specificity (>90%) justifying immediate reperfusion therapy when any criterion is present. 1, 2
The Three Sgarbossa Criteria
The original criteria assign points to create a diagnostic score, though any single criterion meeting threshold is clinically significant:
ST-segment elevation ≥1 mm concordant with the QRS complex (sensitivity 73%, specificity 92%) - This is the most sensitive criterion and carries the highest diagnostic weight 1, 3
ST-segment depression ≥1 mm in leads V1-V3 (sensitivity 25%, specificity 96%) - Highly specific but less commonly present 1, 3
ST-segment elevation ≥5 mm discordant with the QRS complex (sensitivity 19%, specificity 82%) - The least sensitive but still clinically useful when present 1, 3
Physiologic Basis
LBBB normally produces secondary ST-T wave changes that are discordant to the QRS complex - meaning leads with predominantly negative QRS complexes show ST elevation, while leads with large R waves show ST depression. 1, 4 When this expected discordance is lost (concordance develops) or becomes excessively discordant, it suggests acute myocardial injury superimposed on the baseline LBBB pattern. 1
Clinical Application and Treatment Implications
Any patient with LBBB meeting Sgarbossa criteria should receive immediate reperfusion therapy, preferably primary PCI with a door-to-balloon time ≤90 minutes. 5, 2 If PCI cannot be achieved within this timeframe, fibrinolytic therapy should be administered, particularly when symptom onset is <3 hours. 5, 2
The American College of Emergency Physicians provides Level B recommendations for:
- LBBB with concordant ST-segment deviations ≥0.1 mV (1 mm) toward the major QRS deflection 5
- Discordant ST-segment deviations ≥0.5 mV (5 mm) away from the major QRS deflection in 2 or more contiguous leads 5
Modified Sgarbossa Criteria
A 2012 revision replaced the absolute 5 mm discordant ST elevation threshold with a proportional ST/S ratio ≤-0.25, which significantly improved sensitivity from 52% to 91% while maintaining 90% specificity. 6 This modified criterion measures ST-segment elevation relative to the depth of the S wave, making it more applicable across varying QRS amplitudes. 6
Extension to Ventricular Pacing
The Sgarbossa criteria have been validated in patients with right ventricular pacing, showing similar diagnostic characteristics:
- ST elevation ≥5 mm discordant with QRS (sensitivity 53%, specificity 88%) 1
- ST elevation ≥1 mm concordant with QRS (sensitivity 18%, specificity 94%) 1
- ST depression ≥1 mm in V1-V3 (sensitivity 29%, specificity 82%) 1
Critical Limitations and Pitfalls
The major limitation is low sensitivity - the criteria cannot exclude AMI when absent, as they identify only 48.7% of LBBB patients with confirmed elevated cardiac biomarkers. 3, 7 Therefore:
- Never use negative Sgarbossa criteria to rule out AMI - serial ECGs, high-sensitivity troponins, and clinical correlation remain essential 2, 4
- Patients with new or presumably new LBBB are 5 times more likely to have AMI than those with chronic LBBB 7
- Echocardiography showing focal wall motion abnormalities can provide additional diagnostic information when ECG interpretation is challenging 4
Risk Stratification Value
Beyond diagnosis, Sgarbossa scores predict mortality - patients with scores ≥3 have significantly higher 30-day mortality (23.5% vs 7.7%) and 1-year mortality (33.7% vs 20.2%) compared to those with scores <3. 3 This validates their utility not just for diagnosis but for risk stratification in this high-risk population. 3