What are the characteristic ECG changes in posterior myocardial infarction (MI)?

ECG Changes in Posterior Myocardial Infarction

The most characteristic ECG findings in posterior myocardial infarction include horizontal ST-segment depression in leads V1-V3, tall and upright T waves in these same leads, and the subsequent development of tall and wide R waves in leads V1-V2. 1, 2

Primary ECG Findings

• Horizontal ST-segment depression in leads V1-V3 is the most common initial finding in posterior MI 1, 2
• Tall, upright T waves in leads V1-V3 often accompany the ST depression 2, 3
• Development of tall and wide R waves (≥0.04 seconds) in leads V1-V2 with R:S ratio ≥1 in V2 occurs as the infarction evolves, representing the equivalent of pathological Q waves in the posterior wall 2, 4
• These changes represent a "mirror image" of the ST elevation and Q waves that would be seen if leads were placed directly on the posterior wall 1

Posterior Lead Recordings (V7-V9)

• Recording of posterior leads is strongly recommended in patients with suspected posterior MI, particularly when the initial ECG shows ST depression in V1-V3 1, 2
• Posterior leads should be placed at the fifth intercostal space: V7 at the left posterior axillary line, V8 at the left mid-scapular line, and V9 at the left paraspinal border 1, 2
• ST elevation ≥0.05 mV in leads V7-V9 confirms the diagnosis of posterior MI 1
• In men >40 years old, a higher cut-point of ≥0.1 mV is recommended for increased specificity 1, 2

Clinical Significance

• Posterior MI accounts for up to 7% of all STEMIs but is frequently missed using standard 12-lead ECG alone 5
• Isolated posterior STEMI is often overlooked because standard STEMI criteria focus on ST elevation rather than the reciprocal changes seen in V1-V3 5, 3
• Failure to recognize posterior MI can lead to delays in reperfusion therapy, resulting in increased morbidity and mortality 5, 1
• ST depression maximal in V1-V4 has been shown to have 97% specificity for occlusion myocardial infarction requiring emergent intervention 3

Associated Findings

• Posterior MI often occurs in conjunction with inferior or lateral wall MI 6, 2
• When ST depression in V1-V3 accompanies inferior ST elevation, consider posterior involvement 2, 6
• The presence of these combined patterns can help determine whether the occlusion is in the right coronary artery or circumflex artery 2

Diagnostic Challenges

• Standard 12-lead ECG is often insufficient to diagnose posterior MI, with studies showing that up to 50% of circumflex occlusions are missed by traditional STEMI criteria 3, 5
• Early anterior ST depression in inferior MI is not always specific for posterior infarction, as shown in studies using pyrophosphate scanning 6
• However, persistent anterior ST depression (>72 hours) has been associated with posterior infarction in 87% of cases 6

Common Pitfalls

• Mistaking posterior MI for anterior ischemia or non-STEMI, leading to delayed reperfusion therapy 5, 3
• Failing to record posterior leads (V7-V9) in patients with suspicious ST depression in V1-V3 1, 5
• Not recognizing that ST depression <1mm in V1-V3 can still represent posterior MI (34% of cases in one study had <1mm depression) 3
• Confusing posterior MI changes with other causes of ST depression such as LVH, electrolyte abnormalities, or medication effects 1

Diagnostic Algorithm

1. Identify horizontal ST depression in leads V1-V3 with upright T waves in a patient with chest pain 2, 3
2. Record posterior leads (V7-V9) to look for ST elevation ≥0.05 mV (≥0.1 mV in men >40 years) 1, 2
3. Look for associated inferior (II, III, aVF) or lateral (I, aVL, V5-V6) ST elevation that may accompany posterior MI 2, 6
4. Monitor for the development of tall R waves in V1-V2 (R ≥0.04 sec and R:S ratio ≥1 in V2) in serial ECGs 2, 4
5. Consider posterior MI as a working diagnosis even with minimal ST depression if the pattern is consistent, as studies show that 34% of posterior MIs have <1mm ST depression 3