Posterior Myocardial Infarction (MI), EKG Findings, Inferior–Posterior Wall Patterns, False Negatives, and Post-MI Changes

Posterior Myocardial Infarction (MI):

• What is Posterior Myocardial Infarction (MI)?
• EKG Findings
• Inferior–Posterior Wall Patterns
• False Negatives
• Post-MI Changes

What is Posterior Myocardial Infarction (MI)?

A Posterior Myocardial Infarction (MI) occurs when the posterior wall of the left ventricle loses blood supply, usually due to blockage in the right coronary artery (RCA) or the left circumflex artery (LCx). Because the posterior wall lies on the back side of the heart, its electrical activity is not directly visualized by standard 12-lead EKGs. This makes posterior MI easy to miss if clinicians are not actively looking for indirect signs. Posterior MI often accompanies an inferior MI, and the two conditions commonly occur together.

Clinically, patients may experience classic ischemic symptoms such as chest pain, shortness of breath, diaphoresis, nausea, or epigastric discomfort. However, as posterior MI can be subtle on EKG, diagnosis frequently relies on a combination of clinical suspicion, reciprocal EKG changes, and confirmatory imaging such as cardiac biomarkers or emergent cardiac catheterization. Early recognition is crucial because delayed diagnosis increases the risk of complications such as arrhythmias, cardiogenic shock, and worsening myocardial damage.

EKG Findings

A 12-lead EKG does not directly view the posterior myocardium; instead, the clinician must identify reciprocal (mirror-image) changes in the anterior leads, particularly V1–V3. The classic EKG features of posterior MI include:

• ST depression in V1–V3 (mirror image of ST elevation)
• Tall R waves in V1–V3 (reciprocal of Q waves seen in MI)
• Upright T-waves or hyperacute appearance
• Dominant R wave in V2 (R/S ratio > 1)

To confirm posterior involvement, clinicians often place additional leads: V7, V8, and V9 on the patient’s back. ST elevation of ≥0.5 mm (≥1 mm in men <40 activation="" diagnosis.="" early="" enables="" faster="" findings="" in="" leads="" of="" old="" p="" posterior="" recognition="" reperfusion="" strongly="" supports="" the="" therapy.="" these="" years="">

Inferior–Posterior Wall Patterns

Posterior MI frequently occurs with inferior MI because they share blood supply from the RCA or LCx. When both inferior and posterior walls are involved, the EKG shows classic inferior MI patterns plus subtle or indirect posterior changes. Inferior MI typically presents with:

• ST elevation in leads II, III, and aVF
• Reciprocal ST depression in aVL

When combined with posterior involvement, the clinician may notice ST depression in V1–V3, tall R waves, and unusual anterior patterns. Recognizing this combination is important because patients with inferior–posterior MI have increased risk of complications such as right ventricular infarction, conduction abnormalities, bradycardia, and hypotension. Proper identification leads to tailored management, including cautious use of nitrates and appropriate fluid resuscitation.

False Negatives

Posterior MI is one of the most commonly missed MIs on EKG due to its subtle reciprocal patterns rather than obvious ST elevation. This makes false-negative interpretations a significant clinical challenge. Factors contributing to missed diagnosis include:

• Absence of direct posterior leads in the standard EKG
• Misinterpretation of ST depression as ischemia rather than a mirror image of ST elevation
• Tall R waves mistaken for right ventricular hypertrophy
• Normal early EKG in evolving MI

Because of these pitfalls, guidelines emphasize obtaining posterior leads (V7–V9) when posterior MI is suspected. If suspicion remains high despite a normal EKG, immediate further evaluation is recommended—such as repeated EKGs, troponin testing, bedside echo, or early cardiac catheterization. Rapid recognition is essential for minimizing damage and improving outcomes.

Post-MI Changes

After a posterior MI, characteristic long-term EKG changes may appear. Because the posterior wall is opposite the precordial leads, the changes appear as mirror-image patterns compared to typical anterior MI healing. Common post-MI findings include:

• Persistent tall R waves in V1–V3
• Persistent ST depression during the healing phase
• T-wave inversions appearing as late ischemic changes
• Resolution of acute injury patterns with evolving Q-wave equivalents

Imaging techniques such as echocardiography or cardiac MRI often reveal wall-motion abnormalities in the basal or mid-posterior left ventricle. Long-term management includes antiplatelet therapy, beta-blockers, statins, ACE inhibitors, lifestyle modification, and cardiac rehabilitation. Recognizing and following post-MI changes helps ensure proper monitoring and secondary prevention to reduce recurrent cardiac events.