Scenario: This 12-lead electrocardiogram (ECG) was obtained from a 36-year-old African American man who was evaluated at an emergency department for dizziness and chest pressure. The patient had no significant medical history and denied using any prescription or illicit drugs. He had been evaluated for similar complaints at the same setting several weeks earlier, and his ECG and bloodwork were unremarkable at that time. The current cardiac workup showed mild cardiomegaly on the chest x-ray, with slight anemia and leucopenia that were not seen previously. Other tests, including cardiac enzymes and urine toxicology, were negative.

This ECG shows sinus bradycardia with a morphological QRS pattern of right bundle branch block (RBBB). There is also a left axis deviation (negative R in lead aVF and positive R in lead I). This is called a bifasicular block (RBBB plus left anterior fascicle block of...

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