The anatomical location of a posterior myocardial infarction makes its diagnosis difficult for the Emergency Physician. Intimate knowledge of standard and posterior ECG manifestations of posterior myocardial infarctions is crucial to picking up on this potentially fatal pathology. This post aims to provide tips for evaluating patients for posterior myocardial infarction.

On standard ECG's, look for the following reciprocal changes in the anteroseptal leads: V1-V3
Dominant R in V2 (R/S > 1)
Horizontal ST depression
Tall broad R waves
These changes represent the anteroseptal leads representing electrical activity from anterior -> posterior resulting in reciprocal changes in these leads when the posterior myocardium is affected

If you suspect a posterior myocardial infarction, obtain and posterior ECG:
Lead placement: Pick up V4, V5, V6 and replace with V7, V8, V9
V7: left posterior axillary line
V8:  left tip of scapula
V9: Left paraspinal region
Look for ST elevations in V7, V8, V9 on your posterior EKG

Remeber your coronary artery anatomy! Posterior MI’s often co-exist with inferior or lateral STEMI. The PDA supplies the inferior and posterior wall of the heart. If you have a left dominant circulation, the PDA still supplies the inferior and posterior wall, but it now originates from the left circumflex artery. Posterior MI’s in conjunction with inferior or lateral MI’s indicate a larger infarct territory and increase in morbidity and mortality!

Standard ECG:

Posterior ECG:

• Standard ECG: ST elevation in II, III, aVF (inferior) + V5, V6 (lateral) with prominent ST depression in V1-V3 and R>S in V2
• Posterior ECG: ST elevation in V7-V9
• Don’t confuse with RV infarction- -> ST elevation in inferior leads (III > II) and in V1
  

References:
1. Oraii S, Maleki M, Abbas Tavakolian A, et al. Prevalence and outcome of ST-segment elevation in posterior electrocardiographic leads during acute myocardial infarction. J Electrocardiol 1999;32: 275-8
2. Posterior Myocardial Infarction - Life in the FastLane ECG Library." LITFL: Life in the Fast Lane Medical Blog. N.p., n.d. Web. 03 Dec. 2016
3. Morris, Francis, and William Brady. "ABC of Clinical Electrocardiography: Acute Myocardial Infarction-Part I." ABC of Clinical Electrocardiography: Acute Myocardial Infarction-Part I | The BMJ. BMJ, 2002. Web. 03 Dec. 2016.