How to diagnose a posterior wall myocardial infarction (MI)?

Diagnosing Posterior Wall Myocardial Infarction

Posterior wall MI should be diagnosed using posterior leads (V7-V9) at the fifth intercostal space, with ST elevation ≥0.05 mV (or ≥0.1 mV in men >40 years old) considered diagnostic. 1

Standard ECG Findings Suggestive of Posterior MI

• ST depression in leads V1-V3, especially when terminal T waves are positive (ST elevation equivalent) 1
• Horizontal ST depression with upright T waves in precordial leads 1
• Absence of diagnostic ST elevation in standard 12-lead ECG despite ongoing ischemic symptoms 1

Posterior Lead Placement and Interpretation

1. Lead placement:

   • V7: Left posterior axillary line at the fifth intercostal space
   • V8: Left mid-scapular line at the fifth intercostal space
   • V9: Left paraspinal border at the fifth intercostal space 1

2. Diagnostic criteria:

   • ST elevation ≥0.05 mV in leads V7-V9 is diagnostic
   • Specificity increases with ST elevation ≥0.1 mV (recommended for men >40 years old) 1
   • Pathologic Q waves may appear in leads V7-V9 in 75% of patients with confirmed posterior MI 2

Clinical Importance

• Posterior MI accounts for 15-21% of all acute MIs, often occurring with inferior or lateral wall involvement 1
• Isolated posterior MI occurs in approximately 4% of all AMI patients 2
• Left circumflex coronary artery is typically the culprit vessel (identified in 94% of catheterized patients) 2, 3
• Patients with posterior MI have:
  • Higher peak creatine kinase levels (larger infarct area)
  • Lower left ventricular ejection fraction
  • Higher incidence of adverse clinical events (reinfarction, heart failure, death) 4

Diagnostic Algorithm

1. Initial assessment:

   • If standard 12-lead ECG shows ST depression in V1-V3 with upright T waves in a patient with ischemic symptoms, suspect posterior MI 1

2. Additional ECG leads:

   • Immediately obtain posterior leads (V7-V9) 1
   • Consider right-sided leads (V3R-V4R) if inferior MI is also suspected to identify right ventricular involvement 1

3. Interpret findings:

   • ST elevation ≥0.05 mV in V7-V9 confirms posterior wall involvement 1
   • Use higher threshold (≥0.1 mV) for men >40 years old 1

4. Additional diagnostic testing:

   • Obtain cardiac biomarkers (troponin) 5
   • Consider echocardiography to assess for posterior wall motion abnormalities (present in 97% of posterior MI cases) 2

Clinical Pitfalls to Avoid

• Missing the diagnosis: Posterior MI is often overlooked because standard 12-lead ECG lacks direct visualization of the posterior wall 1, 2
• Delayed treatment: Without posterior leads, these patients may be misclassified as having unstable angina or non-ST elevation MI, potentially delaying appropriate reperfusion therapy 3
• Failing to recognize the significance: ST depression in V1-V3 may be misinterpreted as non-specific changes rather than reciprocal changes of posterior wall ST elevation 1
• Underestimating severity: Posterior MI is associated with larger infarct size and worse outcomes compared to isolated inferior MI 4

Treatment Implications

• Patients with confirmed posterior MI should receive the same reperfusion therapy as those with ST-elevation MI 1
• Early identification is crucial for proper triage and treatment 2
• Patency of the infarct-related artery in posterior MI results in improved left ventricular ejection fraction at discharge 4

By systematically evaluating patients with suspected posterior MI using posterior leads, clinicians can identify these often-missed infarctions and provide appropriate reperfusion therapy, potentially improving outcomes in this high-risk population.