Posterior MI ECG sign on standard leads?

• A. ST depression in V1-V3 with tall R waves; posterior MI best confirmed with posterior leads V7-9
• B. ST elevation in V1-V3
• C. ST depression in inferior leads
• D. Posterior MI cannot be detected on ECG

Answer: (A)

Posterior myocardial infarction often hides on the standard ECG because the affected area is on the back. When the posterior wall is ischemic, the electric vector moves away from the anterior chest leads, producing reciprocal changes that you can see in V1–V3: ST-segment depression paired with tall, upright R waves. This pattern is the mirror image of the ST elevation that would be seen if you were recording directly from the posterior wall. To confirm a posterior MI, you place posterior leads (V7–V9) on the back; ST elevation there verifies the infarct. So the characteristic finding on standard leads is ST depression in V1–V3 with tall R waves, with posterior leads V7–V9 used to confirm the diagnosis.