What electrocardiogram (ECG) leads are used for STEMI (ST-Elevation Myocardial Infarction) posterior mirroring?

Posterior STEMI Mirroring: Leads V1, V2, and V3

Posterior STEMI manifests as ST-segment depression in leads V1, V2, and V3 on the standard 12-lead ECG—these anterior leads show "mirror image" reciprocal changes of the posterior ST elevation. 1

Understanding the Mirror Image Phenomenon

The posterior wall of the left ventricle is not directly visualized by standard 12-lead ECG leads. When posterior myocardial infarction occurs:

• ST-segment depression appears in leads V1, V2, and V3 as the reciprocal (mirror image) of posterior ST elevation 1
• This pattern is termed "posterior or posterolateral ischemia" and represents an ST-elevation equivalent requiring emergent reperfusion therapy 2, 3, 4
• The ST depression is typically horizontal with dominant R waves and upright T waves in the anterior leads 3, 4

Confirming the Diagnosis with Posterior Leads

When you suspect posterior MI based on ST depression in V1-V3, immediately obtain posterior leads V7, V8, and V9 for confirmation: 2, 3, 4

• V7: Posterior axillary line (same horizontal plane as V6) 2
• V8: Below the scapula 2
• V9: Paravertebral border 2

ST elevation ≥0.05 mV in at least one of V7-V9 confirms posterior MI and mandates STEMI management with emergent reperfusion 2, 3

Clinical Context and Culprit Arteries

Posterior MI with ST depression in V1-V3 can result from occlusion of either:

• Left circumflex artery (LCx) - most common cause 1, 5
• Right coronary artery (RCA) - when occurring with inferior MI 1

When posterior changes accompany inferior MI (ST elevation in II, III, aVF), also obtain right-sided leads V3R and V4R to assess for right ventricular involvement, which significantly impacts management 1, 2

Critical Pitfalls to Avoid

Up to 7% of all STEMIs are isolated posterior infarctions that are frequently missed because clinicians fail to recognize the reciprocal changes in V1-V3 or don't obtain posterior leads 6, 5

• Only 38% of hospital doctors correctly identified posterior MI in one study, with the majority unable to properly position posterior leads 6
• Posterior MI must be managed as STEMI with immediate reperfusion therapy—delays increase morbidity and mortality 2, 3, 4
• ST depression in V1-V3 can also represent anterior subendocardial ischemia, so clinical context and posterior lead confirmation are essential 3
• Never administer fibrinolytics based solely on ST depression in anterior leads—confirm with posterior leads first or proceed directly to angiography 3, 4

Practical Algorithm

1. Patient presents with chest pain and ST depression in V1-V3 → Suspect posterior MI 1, 3
2. Immediately obtain posterior leads V7-V9 (target <10 minutes) 2, 3
3. ST elevation ≥0.05 mV in V7-V9 → Confirmed posterior STEMI → Activate cath lab for emergent reperfusion 2, 3, 4
4. If inferior ST elevation also present → Add right-sided leads V3R-V4R to assess RV involvement 1, 2
5. If posterior leads unavailable or non-diagnostic but high clinical suspicion → Proceed directly to emergent coronary angiography 3, 4