Which new EKG finding is highly suspicious for STEMI?

A new left bundle branch block (LBBB) is highly suspicious for ST-segment elevation myocardial infarction (STEMI) because it can indicate significant ischemia or obstructive coronary artery disease. LBBB can mask or alter the typical EKG findings associated with STEMI, particularly the elevation of the ST segment. When there is a new onset of LBBB, especially in the context of chest pain or other cardiac symptoms, it raises the clinician's suspicion for a myocardial infarction, as it reflects a change in the heart's electrical conduction that can occur due to acute ischemia.

In patients presenting with chest pain, the recognition of a new LBBB, along with a history that includes risk factors for coronary artery disease, prompts further investigation and possible intervention even if classic ST-segment elevation is not present. This is why new LBBB is treated with a high degree of concern in the emergency setting.