What type of 12-lead Electrocardiogram (EKG) is best for capturing ST changes in a suspected posterior Myocardial Infarction (MI)?

Posterior Myocardial Infarction Detection on ECG

To best capture ST changes of a posterior myocardial infarction, additional posterior leads (V7-V9) should be recorded at the fifth intercostal space, with V7 at the left posterior axillary line, V8 at the left mid-scapular line, and V9 at the left paraspinal border. 1

Standard ECG Limitations in Posterior MI

The standard 12-lead ECG has significant limitations in detecting posterior wall myocardial infarction:

• Posterior wall ischemia is often "electrically silent" on standard 12-lead ECG
• Up to 7% of STEMIs are isolated posterior MIs that may be missed 2
• Left circumflex artery occlusions are frequently overlooked on standard ECG 1

Indirect Signs of Posterior MI on Standard 12-lead ECG

When reviewing a standard ECG, look for these indirect signs of posterior MI:

• ST depression in leads V1-V3
• Upright or tall T waves in V1-V3 (ST elevation equivalent)
• Tall, wide R waves in V1-V3
• R/S wave ratio greater than 1.0 in lead V2 3

Correct Technique for Posterior Lead Placement

1. Position the patient appropriately (sitting up or in lateral recumbent position)
2. Locate the fifth intercostal space on the posterior thorax
3. Place leads at the following positions:
   • V7: Left posterior axillary line
   • V8: Left mid-scapular line
   • V9: Left paraspinal border 1

Diagnostic Criteria for Posterior MI

• ST elevation ≥0.05 mV (0.5 mm) in leads V7-V9 is diagnostic
• For increased specificity, use a cut-point of ≥0.1 mV (1 mm) ST elevation, especially in men <40 years old 1, 4

Clinical Implications

• Early identification of posterior MI is crucial for proper triage and treatment
• Studies show that 97% of patients with ST elevation in V7-V9 have posterior wall motion abnormalities on echocardiography 5
• The circumflex coronary artery is typically the infarct-related artery in posterior MIs 5
• Posterior MIs are associated with significant mitral regurgitation in 69% of cases 5

Common Pitfalls

• Failure to consider posterior MI when ST depression is seen in V1-V3
• Not recording posterior leads when clinical suspicion is high
• Misinterpreting ST depression in V1-V3 as non-STEMI when it may represent posterior STEMI
• Studies show only 38% of doctors correctly identify potential posterior MI on ECG 2
• Only 20% of doctors correctly position posterior leads 2

When to Use Posterior Leads

Posterior leads should be strongly considered when:

• There is high clinical suspicion for acute circumflex occlusion
• Initial ECG is non-diagnostic but symptoms are concerning
• ST-segment depression is present in leads V1-V3, especially with positive T waves 1, 4

Remember that time is muscle - early diagnosis of posterior MI allows for appropriate reperfusion therapy, which significantly reduces morbidity and mortality.