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

Study objectives: Fibrinolytic therapy is recommended for patients who have chest pain and left bundle-branch block (LBBB). However, the presence of b aseline ECG abnormalities makes early accurate identification of acute myoc ardial infarction (AMI) difficult. The predictive ability of clinical and E CG variables far identifying patients with LBBB and AMI has not been well s tudied. 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. Methods: All patients presenting to the ED were prospectively risk stratifi ed on the basis of clinical and historical variables. ECGs from patients wi th LBBB were compared retrospectively with previously published criteria fo r identification of AMI. The ability of a new LBBB to predict AMI was also determined. Results: Twenty-four (13%) of the 182 patients with LBBB had AMI. Clinical and historical variables were similar in patients with and without AMI. A n ew LBBB had a sensitivity of 42% and a specificity of 65%. The presence of concordant ST-segment elevation or depression had specificities and positiv e predictive values of 100%; however, sensitivities were only 8% and 17%, r espectively. The best diagnostic criterion was the presence of concordant S T-segment elevation or depression on the ECG or an initially elevated creat ine kinase MB (sensitivity, 63%; specificity, 99%). Conclusion: ECG criteria for identifying patients with AMI and LBBB identif y only a small minority of patients with AMI. Treating all patients with LB BB and chest pain with fibrinolytics would result in treatment of a signifi cant number of patients without AMI.