Posterior Occlusive Myocardial Infarction on Chronic Right Bundle Branch Block

After experiencing syncope, a woman in her 80s with several cardiovascular risk factors presented to the emergency department with shock. The initial troponin was 3,872 ng/L, and the presenting electrocardiogram (ECG) showed a known right bundle branch block (RBBB), but with ST-segment depressions in V1-V2 out of proportion to the patient's baseline RBBB-related secondary repolarization abnormality. After 3 days of hypotension refractory to antibiotics, left heart catheterization was pursued and demonstrated 100% occlusion of the second obtuse marginal artery, confirming the patient had experienced posterior occlusive myocardial infarction (OMI). This case highlights one ECG pattern consistent with posterior OMI in patients with chronic RBBB.