Posterior Myocardial Infarction

• A true posterior MI (PMI) involves the L circumflex artery; if the artery is dominant (10% of pts), occlusion will also cause an inferior MI
• Posterior MIs represent about 15-20% of all acute MI (usually accompanied by lateral or inferior MI); a true posterior MI is even less common (approx 3%)
• EKG criteria: ST-segment depression in V1-V3, prominent R waves in V1-V3, R/S wave ratio >1.0 in lead V2, upright T-waves in V1-V3, often coexisting inferior or lateral MI. (if using posterior leads, V7-V9, ≥1mm ST elevation)
• Patients with isolated PMI often have a delayed diagnosis due to lack of the classical ST-segment elevation
• A study by Huey et al in JACC 20 yrs ago found that 40 pts LCx-related infarcts, ST-segment elevation was present in only 48% and ST-segment depression in only 45%; 38% of these patients had no ST-segment deviation whatsoever.
• A 1999 study by Matetzky et al in JACC, examined 33 patients with acute MI based on cardiac enzymes, none of whom had ST-segment elevation on the standard 12-lead ECG. In all of these cases, the posterior leads revealed ST-elevation. 20/33 pts underwent coronary angiography, and all were diagnosed with LCx occlusion. In this study, 61% of pts had ST-depression in leads V1 to V3, 9% had a positive R-waves in V1, and 44% had a positive R-waves in V2.
• use of right ventricular precordial leads can be beneficial in evaluating a patient with suspected inferoposterior or RV infarction, although this will not reliably distinguish between RCA or LCx involvement

 

(Victoria Kelly MD, 9/7/10)