8. Which of the following best describes the ECG findings of a true posterior myocardial infarction?

A. ST elevation in V5-V6 with R:S ratio < 1 in V1

B. ST depression in the inferior leads with R:S ratio > 1 in V1

C. ST depression in V1-V2 with R:S ratio > 1 in V1

D. ST depression in V1-V2 with R:S ratio < 1 in V1

E. None of the above

The ECG findings of a posterior wall MI are different than the typical ST elevation seen in other myocardial infarctions. A posterior wall myocardial infarction occurs when posterior myocardial tissue (now termed inferobasilar), usually supplied by the posterior descending artery (a branch of the right coronary artery in 80% of individuals), acutely loses blood supply due to intracoronary thrombosis in that vessel. This frequently coincides with an inferior wall MI due to the shared blood supply. The ECG findings on an acute posterior wall MI include:

1. ST segment depression (not elevation) in the septal and anterior precordial leads (V1 to V4). This occurs since these ECG leads will see the MI backwards (since the leads are placed anteriorly, but the myocardial injury is posterior).

2. The ratio of the R wave to the S wave in leads V1 or V2 is > 1.

3. ST elevation in the posterior leads of a posterior ECG (leads V7 to V9). Suspicion for a posterior MI must remain high, especially if inferior ST elevation is also present.

4. ST elevation in the inferior leads (II, III, and aVF) may be seen if an inferior MI is also present.

Below is an ECG example of a true posterior wall MI