Author: Manuel Celedon, MD (Health Sciences Clinical Instructor & Rapid Medical Evaluation [RME] Physician, Harbor-UCLA Medical Center; HEAL Physician) // Editors: Manpreet Singh, MD (@MPrizzleER) and Alex Koyfman, MD (@EMHighAK)

INTRODUCTION

Isolated or true posterior myocardial infarction (PMI) is a rare entity occurring in about 4% of all ST-elevation myocardial infarctions (STEMIs) (1). The standard ECG lead placement cannot directly illustrate what is occurring in the posterior heart. As a result, it is difficult to diagnose true PMIs. Isolated PMI is an indication for emergent reperfusion therapy and the absence of ST-segment elevation in the standard 12-lead ECG means the diagnosis is often missed. Luckily, most  PMIs occur in conjunction with an inferior or lateral STEMI, due to shared blood supply, making them less likely to be missed (2-4). Posterior infarction is associated with 15-20% of STEMIs. Isolated PMIs are often misjudged and undertreated as NSTEMIs because the damage is occurring in the ‘blind spot’ of the standard electrocardiogram. The clinical presentation of true posterior MIs are not much different from other myocardial infarctions; however, the lack of ST-elevation on the standard ECG can lead to diagnostic delay. The majority of these patients have a stenosis or occlusion of the right coronary artery (RCA). It is important to recognize acute PMI because patients with inferior or lateral MI who also have PMI have a larger infarct region, lower resultant ejection fraction, as well as higher morbidity and mortality (1,5). Additionally, patients with isolated PMI often do not receive reperfusion treatment if a STEMI is not suspected, due to the lack of classical ST-segment elevation (2).

ELECTROCARDIOGRAM

Making the diagnosis of PMI using the standard ECG is elusive because the lack of specific leads that directly represent this area (6). One has to re-think the meaning of certain findings on the standard ECG and what they represent. For example, we are conditioned to think that ST-segment depressions represent subendocardial or non-transmural ischemia. However, in the right precordial leads (V1-V3) your differential for ST-segment depression should include right ventricular hypertrophy with strain and posterior STEMI in addition to anterior subendocardial ischemia. Similarly, the differential for tall R-waves in leads V1-V3 should include the possibility that they represent posterior Q-waves or that upright T-waves may actually be posterior T-wave inversions. It is important to rethink the approach to the ECG and to maintain a high index of suspicion for PMI when the following findings are seen on the standard ECG:

1. ST-segment depression (horizontal >> downsloping or upsloping) in leads V1-V3 (see figure 1)
2. Prominent upright T-waves in leads V1-V3
3. Combination of horizontal ST-segment depression with
4. Prominent R-waves in leads V1-V3
5. R/S ratio > 1 in lead V2
6. Co-existing acute inferior and/or lateral myocardial infarction

Figure 1

USE THE POSTERIOR LEADS

Logically, it makes sense that if we cannot visualize the posterior heart via the standard ECG we should devise a direct way to evaluate the posterior heart, and this is where the posterior leads play a role. The extra posterior leads (V7-V9) significantly increase the detection of posterior myocardial injury when compared to the standard 12-lead ECG (6). This is accomplished by placing lead V8 on the left side of the back at the tip of the scapula, then by placing leads V7 and V9 on either side of that (see figure 2). ST-segment elevation in the posterior leads of greater than 0.5mm to 1 mm suggest posterior STEMI (7). A cutoff of 0.5mm ST-segment elevation is used by some because of the greater distance between the infarcted area and the leads in PMI (8).  It is important to use this 15-lead ECG in the subgroup of patients that is more likely to have PMI to help improve time to recognition.

Figure 2

SCRUTINIZE THE ANTERIOR LEADS

Make sure to place close attention to the anterior or right precordial leads. Amend your approach to the standard ECG to include increased scrutiny of leads V1-V3. Ask yourself if those ST-segment depressions in leads V1-V3 could represent posterior ST-segment elevations in PMI. If you see horizontal ST-segment depressions and upright T-waves in leads V1-V3, you should think this is a posterior MI until proven otherwise.

ECG Examples:

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2.

3.

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. Boden E, Kleiger R, Gibson R, Schwartz D, et al. Electrocardiographic evolution of posterior acute myocardial infarction: Importance of early precordial ST-segment depression. Am J Cardiol 1987;59:782-7.
3. Brady W, Hwang V, Sullivan R, et al. A comparison of 12- and 15-lead ECGs in ED chest pain patients: impact on diagnosis, therapy and disposition. Am J Emerg Med 2000;18:239-43.
4. Zalenski R, Rydman R, Sloan E, et al. Value of posterior and right ventricular leads in comparison to the standard 12-lead electrocardiogram in evaluation of ST-segment elevation in suspected acute myocardial infarction. Am J Cardiol 1997;79:1579-85.
5. Matetzky S, Freimark D, Chouraqui P, et al. Significance of ST segment elevations in posterior chest leads (V7 to V9) in patients with acute inferior myocardial infarction: application for thrombolytic therapy. J Am Coll Cardiol 1998;31:506-11.
6. Rich M, Imburgia M, King T, et al. Electrocardiographic diagnosis of remote posterior wall myocardial infarction using unipolar posterior lead V9 Chest 1989;96:489-93.
7. Brady W. Acute posterior wall myocardial infarction: electrographic manifestations. Am J Emerg Med 1998;16:409-13.
8. Wung S, Drew B. New electrocardiographic criteria for posterior wall acute myocardial ischemia validated by a percutaneous transluminal coronary angioplasty model of acute myocardial infarction. Am J Cardiol 2001;87:970-4.
9. Figure 1 website: http://ecg.utah.edu/img_index
10. Figure 2 website: http://1.bp.blogspot.com/-VLlbr8Iof-E/UYz9TudYuII/AAAAAAAACjk/fdbzROm2e8E/s1600/post+leads-MI.gif