Can myocardial infarction be rapidly identified in emergency department patients who have left bundle-branch block?

Fibrinolytic therapy is recommended for patients who have chest pain and left bundle-branch block (LBBB). However, the presence of baseline ECG abnormalities makes early accurate identification of acute myocardial infarction (AMI) difficult. The predictive ability of clinical and ECG variables for identifying patients with LBBB and AMI has not been well studied. We sought to determine the prevalence and predictors of myocardial infarction among patients presenting to the emergency department with LBBB on the initial ECG who were evaluated for myocardial infarction.

All patients presenting to the ED were prospectively risk stratified on the basis of clinical and historical variables. ECGs from patients with LBBB were compared retrospectively with previously published criteria for identification of AMI. The ability of a new LBBB to predict AMI was also determined.

Twenty-four (13%) of the 182 patients with LBBB had AMI. Clinical and historical variables were similar in patients with and without AMI. A new LBBB had a sensitivity of 42% and a specificity of 65%. The presence of concordant ST-segment elevation or depression had specificities and positive predictive values of 100%; however, sensitivities were only 8% and 17%, respectively. The best diagnostic criterion was the presence of concordant ST-segment elevation or depression on the ECG or an initially elevated creatine kinase MB (sensitivity, 63%; specificity, 99%).

ECG criteria for identifying patients with AMI and LBBB identify only a small minority of patients with AMI. Treating all patients with LBBB and chest pain with fibrinolytics would result in treatment of a significant number of patients without AMI.