Acute myocardial infarction (MI) is often difficult to detect when LBBB is present on ECG. A large clinical trial of thrombolytic therapy for acute MI (GUSTO-1) evaluated the electrocardiographic diagnosis of evolving acute MI in the presence of LBBB. Among 26,003 North American patients who had an acute myocardial infarction confirmed by enzyme studies, 131 (0.5%) had LBBB. A scoring system, now commonly called Sgarbossa criteria, was developed from the coefficients assigned by a logistic model for each independent criterion, on a scale of 0 to 5. A minimal score of 3 was required for a specificity of 90%.
Three criteria are included in Sgarbossa's criteria:
ST elevation ≥1 mm in a lead with a positive QRS complex (ie: concordance) - 5 points
ST depression ≥1 mm in lead V1, V2, or V3 - 3 points
ST elevation ≥5 mm in a lead with a negative (discordant) QRS complex - 2 points
≥3 points = 90% specificity of STEMI (sensitivity of 36%)
Validation and usefulness
A high take-off of the ST segment in leads V1 to V3 is well-described with uncomplicated LBBB, such as in the setting of left ventricular hypertrophy. In a substudy from the ASSENT 2 and 3 trials, the third criteria added little diagnostic or prognostic value.
A Sgarbossa score of ≥3 was specific but not sensitive (36%) in the validation sample in the original report. A subsequent meta-analysis of 10 studies consisting of 1614 patients showed that a Sgarbossa score of ≥3 had a specificity of 98% and sensitivity of 20%. The sensitivity may increase if serial or previous ECGs are available.
Other methods for detecting AMI in patients with LBBB
Several other studies have evaluated the usefulness of different ECG findings in diagnosing acute MI when LBBB is present. Wackers et al. correlated ECG changes in LBBB with localization of the infarct by thallium scintigraphy. The most useful ECG criteria were:
Serial ECG changes — 67 percent sensitivity
ST segment elevation — 54 percent sensitivity
Abnormal Q waves — 31 percent sensitivity
Cabrera's sign — 27 percent sensitivity, 47 percent for anteroseptal MI
Initial positivity in V1 with a Q wave in V6 — 20 percent sensitivity but 100 percent specificity for anteroseptal MI
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