23-05: Acute Posterior Myocardial Infarction

ECG Findings

• ST-segment depression in leads V₁-V₃

• Upright T waves in V₁-V₃

• Tall R waves with an R-wave/S-wave ratio greater than 1 in lead V₁-V₃, most often seen in V₂-V₃, developing over the course of hours

Pearls

1. Note that some authors refer to “posterior” infarction as “inferolateral” infarction, based on echocardiographic data that show that the involved portion of the left ventricle in question is actually inferolateral rather than truly posterior in location.

2. This diagnosis is often missed as the posterior portion of the left ventricle has no ECG electrodes directly overlying it. This means that posterior MI will only reflect as reciprocal changes that are most common in V₁-V₃. Instead of ST elevations, one will find ST depressions, and instead of Q waves, one will see tall R waves.

3. Posterior involvement may be confirmed with posterior leads. V₇ is located in the left posterior axillary line; V₈ is located at inferior tip of left scapula; V₉ is positioned between V₈ and the spine, all in the same horizontal plane as V₆.

4. Frequently, an inferior MI is also present with a posterior MI, since the right coronary artery serves both areas.