Historical Perspective
Recognition of LBBB in AMI dates back to 1917 in an account by Oppenheimer and Rothschild. Early descriptions of BBB in AMI reported an incidence of 10% to 15% and a mortality rate of 42% to 63%. Patients with BBB tended to be significantly older and to have an increased frequency of hypertension, congestive heart failure, previous myocardial infarction, and cardiogenic shock. It is, therefore, difficult to discern if historical studies documenting the significantly increased mortality risk (approximately 2-fold) in BBB were confounded by age and comorbid conditions. Additionally, studies included patients with both LBBB and right BBB, recorded ECG data at widely varied time points, lacked discrimination between new and old infarction, and had limited diagnostic resources to confirm AMI at presentation.
For more than 60 years, clinicians have recognized that a diagnosis of AMI in the setting of LBBB is especially challenging. Because left ventricular activation occurs much later in LBBB and the initial septal activation advances from right to left (opposite of the normal situation), septal Q waves indicative of an AMI are absent. Additionally, secondary ST-T wave abnormalities that occur in LBBB obscure the recognition of injury currents in ischemia and infarction. Despite the suggestion of multiple criteria for diagnosis of AMI in the setting of LBBB, it generally was believed that clinicians remained largely blind to ECG changes in patients with LBBB. In 1996, Sgarbossa et al. published an analysis from the GUSTO-1 (Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries-1) trial evaluating multiple different ECG criteria that had been suggested as being potentially useful for the diagnosis of AMI in the setting of LBBB. Of these, ST-segment elevation concordant with the major QRS deflection in any lead and ST-segment depression in leads V1, V2, or V3 had odds ratios for AMI of 25.2 (95% confidence interval [CI]: 11.6 to 54.7) and 6.0 (95% CI: 1.9 to 19.3), respectively; discordant ST-segment elevation of 5 mm or more in any lead had a weaker association (odds ratio: 4.3, 95% CI: 1.8 to 10.6). When combined, these 3 ECG criteria yielded a sensitivity and specificity of approximately 78% and 90%, respectively. Subsequent validation studies have confirmed that ST-segment concordance criteria are highly specific for AMI, but generally have reported much lower sensitivities than the initial study. Currently, the Sgarbossa criteria are used most extensively to diagnose AMI in the setting of a known chronic LBBB. Although data exist to support use of the Sgarbossa criteria in new or indeterminate-age LBBB, current guidelines do not specifically include this application of the criteria in their recommendations.