What are the Modified Scarbosa criteria for predicting myocardial ischemia in an adult patient with Left Bundle Branch Block (LBBB)?

Modified Sgarbossa Criteria for LBBB

The Modified Sgarbossa criteria are a validated electrocardiographic tool for diagnosing acute myocardial infarction in patients with left bundle branch block, replacing the original third criterion (≥5 mm discordant ST elevation) with a proportional measurement (≥1 mm ST elevation with ST/S wave ratio ≤-0.25), which significantly improves sensitivity from 49% to 80% while maintaining 99% specificity. 1

The Three Modified Sgarbossa Criteria

The Modified Sgarbossa criteria consist of three ECG findings, any one of which indicates acute coronary occlusion requiring immediate reperfusion therapy:

1. Concordant ST Elevation

• ST-segment elevation ≥1 mm in leads with a positive (concordant) QRS complex 2, 3
• This criterion has 73% sensitivity and 92% specificity 2, 3
• This represents the most sensitive of the three criteria 2

2. Concordant ST Depression in V1-V3

• ST-segment depression ≥1 mm in precordial leads V1, V2, or V3 2, 3
• This criterion has 25% sensitivity but 96% specificity 2, 3
• Highly specific but less commonly present 2

3. Proportional Discordant ST Elevation (The Modified Component)

• ST-segment elevation ≥1 mm AND ST/S wave ratio ≤-0.25 1
• This replaces the original Sgarbossa criterion of ≥5 mm discordant ST elevation 1
• The modification significantly improves diagnostic accuracy compared to the original weighted criteria (sensitivity 80% vs 49%, P<0.001) 1
• Specificity remains excellent at 99% 1

Physiologic Rationale

The Modified Sgarbossa criteria work by identifying loss of the normal discordance pattern seen in LBBB:

• In uncomplicated LBBB, leads with predominantly negative QRS complexes (QS or rS) typically show ST elevation and positive T waves (appropriate discordance) 2
• Leads with large monophasic R waves normally show ST depression and inverted T waves (appropriate discordance) 2
• Loss of this normal discordance pattern (concordance) indicates myocardial injury or ischemia 2
• Real-time validation in patients with intermittent LBBB has confirmed that dynamic ST changes in narrow QRS beats correlate with similar ST changes during LBBB beats 4

Clinical Application and Treatment Algorithm

Immediate Management

Any patient with LBBB meeting any single Modified Sgarbossa criterion should receive immediate reperfusion therapy, preferably primary PCI with door-to-balloon time ≤90 minutes 2, 3:

• If PCI cannot be performed within 90 minutes of first medical contact AND symptom onset is <3 hours: administer fibrinolytic therapy (Level B recommendation) 3
• For symptom onset 3-6 hours in high-risk patients: consider fibrinolytic therapy if PCI unavailable within 90 minutes (Level C recommendation) 3

Diagnostic Approach

The American College of Emergency Physicians provides specific guidance:

• Patients with concordant ST deviations ≥1 mm (0.1 mV) toward the major QRS deflection require immediate reperfusion therapy (Level B) 2
• Patients with discordant ST deviations meeting the proportional criterion (≥1 mm with ST/S ratio ≤-0.25) in ≥2 contiguous leads require immediate reperfusion therapy 2, 1
• Serial ECGs should be obtained for patients not meeting criteria but with persistent symptoms to detect evolving ischemia 2, 5

Critical Clinical Considerations

High Specificity, Variable Sensitivity

• The Modified Sgarbossa criteria have superb specificity (>98%) and positive predictive value for acute coronary occlusion 6, 1
• Individual criterion sensitivity ranges from 19-80%, meaning absence of criteria does not exclude MI 2, 1
• The criteria are best used as a "rule-in" test rather than a "rule-out" test 6, 7

Common Pitfalls to Avoid

• Do not wait for cardiac biomarkers before initiating reperfusion therapy in patients meeting Modified Sgarbossa criteria 3, 6
• LBBB patients historically receive lower rates of reperfusion therapy despite having higher mortality rates 3
• The 2013 STEMI guidelines removed "new or presumably new LBBB" as a STEMI equivalent, making the Modified Sgarbossa criteria essential for identifying true acute coronary occlusion 6
• Measure the ST/S ratio carefully in the modified third criterion—this proportional measurement is what distinguishes it from the original criteria 1

Application Beyond Native LBBB

The Sgarbossa criteria have been validated in ventricular paced rhythms with similar diagnostic characteristics 2:

• Discordant ST elevation ≥5 mm: 53% sensitivity, 88% specificity 2
• Concordant ST elevation ≥1 mm: 18% sensitivity, 94% specificity 2
• ST depression ≥1 mm in V1-V3: 29% sensitivity, 82% specificity 2

Prehospital Recognition

Paramedics can successfully identify acute MI using the Modified Sgarbossa criteria to activate the catheterization laboratory, as demonstrated in case reports of successful early intervention 8

Superiority Over Original Criteria

The validation study comparing Modified to Original Sgarbossa criteria demonstrated:

• Modified criteria sensitivity: 80% vs Original weighted criteria: 49% (P<0.001) 1
• Modified criteria sensitivity: 80% vs Original unweighted criteria: 56% (P<0.001) 1
• Modified criteria specificity: 99% vs Original weighted criteria: 100% (P=0.5, not significant) 1
• Modified criteria specificity: 99% vs Original unweighted criteria: 94% (P=0.004) 1

The proportional measurement (ST/S ratio ≤-0.25) is superior to the absolute measurement (≥5 mm) because it accounts for the varying amplitude of QRS complexes across different leads and patients 1.