What are the indications for a posterior electrocardiogram (EKG)?

Indications for Posterior Electrocardiogram (EKG)

The primary indication for a posterior EKG is suspected posterior wall myocardial infarction in patients with ischemic chest pain who have ST-segment depression in the anterior precordial leads (V1-V3) with upright T waves. 1

Primary Indications

• Suspected posterior wall STEMI when standard 12-lead ECG shows:

  • ST-segment depression in leads V1-V4 (especially with horizontal ST depression)
  • Upright T waves in leads with ST depression
  • Prominent R waves in leads V1-V2 (R/S ratio > 1 in V2)
  • Normal or non-diagnostic standard 12-lead ECG in a patient with ongoing chest pain suggestive of ischemia 1

• Risk stratification in patients with inferior STEMI to assess for posterior wall involvement, which indicates larger infarct size and higher risk of complications 1

Clinical Scenarios Warranting Posterior EKG

1. Ongoing chest pain with non-diagnostic standard ECG - Approximately 4% of acute MI patients show ST elevation isolated to posterior leads V7-V9 that would be "hidden" from standard 12-lead ECG 1, 2

2. Suspected circumflex artery occlusion - Left circumflex artery is the most common culprit vessel in posterior MI and may not produce ST elevation on standard ECG 2, 3

3. Inferior STEMI - To assess for concomitant posterior involvement, which affects prognosis and management decisions 1, 4

4. ST depression in V1-V4 with upright T waves - This pattern strongly suggests posterior wall injury rather than anterior subendocardial ischemia 1, 5

Technique for Posterior EKG

• Place leads V7, V8, and V9 on the posterior chest wall:
  • V7: Posterior axillary line at the same horizontal level as V6
  • V8: Posterior scapular line at the same horizontal level as V6
  • V9: Left paraspinal border at the same horizontal level as V6

Clinical Significance

Posterior STEMI is diagnostically important because:

1. It qualifies patients for acute reperfusion therapy (fibrinolysis or primary PCI) 1

2. It identifies high-risk patients - Studies show patients with posterior MI have:

   • Higher frequency of cardiovascular risk factors
   • More in-hospital complications
   • Higher rates of mitral regurgitation (69% of patients) 2, 4

3. It changes management - Without posterior leads, these patients might be misclassified as non-STEMI or unstable angina, potentially delaying appropriate reperfusion therapy 1, 2

Diagnostic Criteria for Posterior STEMI

• ST elevation ≥1 mm in at least two contiguous posterior leads (V7-V9) 2, 4
• This finding has been shown to identify patients with enzymatically confirmed acute posterior wall MI 2

Common Pitfalls to Avoid

1. Missing the diagnosis by not obtaining posterior leads when anterior ST depression with upright T waves is present

2. Misinterpreting isolated ST depression in V1-V3 as non-STEMI when it may represent posterior STEMI "mirror image"

3. Delaying reperfusion therapy in patients with suspected posterior STEMI - Current guidelines recommend fibrinolytic therapy for patients with ECG findings consistent with true posterior MI and symptom onset less than 12 hours 1

4. Failing to recognize that posterior MI often occurs with inferior or lateral MI but can also occur in isolation 5

The timely acquisition of posterior leads in appropriate clinical scenarios can significantly impact diagnosis, risk stratification, and management decisions in patients with suspected acute coronary syndromes.