Which statement describes the significance of a new left bundle branch block in chest pain?

• A. New left bundle branch block in the setting of chest pain can indicate STEMI.
• B. Left bundle branch block always rules out STEMI.
• C. Existing LBBB with chest pain confirms myocardial infarction.
• D. Left axis deviation.

Answer: (D)

A new left bundle branch block seen with chest pain is a red flag for an acute coronary event because it can mask or mimic the typical ST-elevation patterns used to diagnose STEMI. In practice, a new LBBB in this setting is treated as a potential STEMI until proven otherwise, prompting urgent evaluation and often reperfusion treatment. This is why the statement that new LBBB can indicate STEMI best fits the scenario. The idea that LBBB always rules out STEMI is not correct, because MI can occur with LBBB and be difficult to diagnose on ECG. Saying existing LBBB with chest pain confirms MI isn’t reliable, since MI can occur without a new LBBB, and left axis deviation isn’t the main significance here.