It is difficult to determine if a patient with a left bundle branch block (LBBB) has an acute myocardial infarction (AMI) because ST segments are “appropriately discordant” with the terminal portion of the QRS. That means if the QRS complex is negative (or downgoing), the ST segment normally will be positive (or elevated). Similarly if the QRS complex is positive (or upgoing), the ST segment will be negative (or depressed).
PV Card: Sgarbossa’s Criteria
In 1996, Sgarbossa et al looked through the GUSTO-1 trial patients with LBBB and AMI. They derived 3 criteria which may help diagnose the “hidden” AMI. The criteria are:
1. ST elevation ≥ 1 mm concordant with QRS complex (most predictive of AMI of the 3 criteria)
2. ST depression ≥ 1 mm in lead V1, V2, or V3
3. ST elevation ≥ 5 mm where discordant with QRS complex
Use these criteria with caution though. None of these criteria are perfect. They are to help you risk-stratify. For instance, criteria #3 (ST elevation ≥ 5 mm) can exist in asymptomatic patients with LBBB because of concurrent left ventricular hypertrophy and high voltages.