 In this prospective observational study the investigators will identify a consecutive series of adult patients with LBBB presenting to the ED with suspicion of ACS. The investigators will collect data including demographics, cardiac risk factors, initial ECG measurements, lab and radiographic results, procedure results, and clinical outcomes such as 30-day death or myocardial infarction MI. The investigators will analyze the data using a cohort study design to calculate odds ratios between ECG characteristics and the outcomes of interest. 