What EKG findings are expected in leads visualizing an acute posterior wall myocardial infarction (MI)?

EKG Findings in Acute Posterior Wall Myocardial Infarction

ST segment depression in the inferior leads and V2-V4, with ST elevation in V7-V9 are the expected findings in leads that visualize an acute posterior wall myocardial infarction.

Electrocardiographic Manifestations of Posterior MI

Posterior wall myocardial infarction presents with distinctive ECG patterns that differ from other types of MI due to the anatomical location of the affected myocardium:

Primary Findings

• ST depression in anterior precordial leads (V1-V4): This is the hallmark finding and represents a "mirror image" of ST elevation that would be seen if leads were placed directly over the posterior wall 1, 2
• ST elevation in posterior leads V7-V9: When additional posterior leads are placed, ST elevation ≥0.5 mm is seen in these leads, confirming posterior wall involvement 1
• Upright (positive) T waves in leads V1-V3: Often accompanies the ST depression in anterior leads 1

Associated Findings

• ST depression in inferior leads (II, III, aVF): May be present when the posterior infarction extends to involve the inferior wall 2
• Dominant R waves in V1-V2: May develop as the infarction evolves, representing the mirror image of Q waves 1
• R/S ratio ≥1 in V1-V2: With concordant positive T waves in the absence of conduction defects 1

Diagnostic Criteria

The American College of Cardiology and American Heart Association guidelines specify the following criteria for posterior MI diagnosis:

• ST depression in V1-V3 with upright T waves suggests posterior wall STEMI 2
• Confirmation requires ST elevation in posterior leads V7-V9 1
• The diagnostic threshold for ST elevation in posterior leads is:
  • ≥0.5 mm in V7-V9 (standard criterion) 1, 2
  • ≥1.0 mm may be used for increased specificity, especially in younger men (<40 years) 1

Clinical Significance

Posterior MI is often underdiagnosed because:

1. Standard 12-lead ECG does not directly visualize the posterior wall 3
2. Physicians may misinterpret ST depression in V1-V3 as non-ST elevation MI rather than posterior STEMI 4
3. Many clinicians are unfamiliar with the need to obtain additional posterior leads 4

Important Considerations

• Posterior MI represents approximately 3-7% of all acute myocardial infarctions 5, 4
• The circumflex coronary artery is typically the infarct-related artery 3
• Patients with posterior MI should be treated as STEMI patients, eligible for reperfusion therapy 1, 2
• Failure to recognize posterior MI may result in inappropriate triage and treatment delays 5

Diagnostic Pitfalls

• Misdiagnosis: ST depression in anterior leads may be misinterpreted as non-STEMI or unstable angina 4
• Incomplete evaluation: Failure to obtain posterior leads (V7-V9) when ST depression is noted in V1-V3 4
• Incorrect criteria application: Using 1 mm rather than 0.5 mm as the threshold for ST elevation in posterior leads reduces sensitivity significantly 6

Recommendations

When evaluating a patient with suspected acute coronary syndrome:

1. Obtain a standard 12-lead ECG within 10 minutes of first medical contact 2
2. If ST depression is noted in V1-V3, especially with upright T waves, obtain additional posterior leads (V7-V9) 1, 2
3. Apply the appropriate diagnostic threshold (≥0.5 mm ST elevation in V7-V9) 1, 6
4. Consider posterior MI as STEMI equivalent requiring urgent reperfusion therapy 1, 2

By recognizing these distinctive ECG patterns and obtaining the appropriate additional leads, clinicians can improve the diagnosis and management of patients with acute posterior wall myocardial infarction.