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A posterior myocardial infarction (MI) can be difficult to see on a routine 12-lead ECG. In a patient with possible acute coronary syndrome (ACS), horizontal ST depression in leads V1–V3 may be a reciprocal clue to posterior injury—not a reassuring result. Clinicians may look for supporting anterior-lead features and record supplemental posterior leads V7–V9 when symptoms persist and the standard ECG is inconclusive. Possible heart attack symptoms require emergency care; do not wait for a particular ECG finding or another tracing before calling for help.
When should clinicians suspect a posterior MI?
Consider posterior MI when a patient is being evaluated for possible ACS and the clinical presentation is concerning, especially if the standard ECG shows horizontal ST-segment depression in V1–V3. This pattern can reflect reciprocal changes from injury on the posterior side of the heart, which a routine 12-lead ECG does not view directly.
Suspicion is stronger when the anterior-lead pattern also includes a dominant R wave in V2, upright T waves in the anterior leads, or a broad, prominent R wave. The 2023 European Society of Cardiology (ESC) ACS guideline describes ST depression in V1–V3, particularly with a positive terminal T wave, and/or ST elevation in V7–V9 as highly suggestive of posterior coronary occlusion, often involving the left circumflex artery. These are findings to interpret alongside the patient’s history, examination, and other tests—not a stand-alone diagnosis.
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| Finding | How clinicians use it |
|---|---|
| Horizontal ST depression in V1–V3 | A reciprocal-change clue that may indicate posterior injury, particularly in a patient with a concerning ACS presentation. |
| Dominant R wave in V2 | A supporting feature in the posterior-MI pattern described in the 2022 American College of Cardiology (ACC) chest-pain pathway. |
| Upright anterior T waves | Another feature that can support the posterior-MI pattern in the ACC pathway. |
| Broad, prominent R wave | A further supporting feature described in the ACC pathway; interpretation depends on the whole tracing and clinical context. |
| ST elevation in V7–V9 | May provide direct evidence of posterior ST elevation when supplemental leads are recorded. |
No single item in the table should be interpreted in isolation. The standard tracing, symptoms, clinical status, and any supplemental leads contribute to the assessment.
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What do posterior leads V7–V9 add?
V7–V9 are supplemental ECG leads that view the posterior chest. They can help clinicians investigate posterior STEMI when the routine 12-lead ECG is nondiagnostic and suspicion remains. The 2025 ACC/AHA ACS guideline highlights their use in selected suspected-ACS cases, including ongoing symptoms with a nondiagnostic ECG and ST depression in V1–V3. The 2023 ESC guideline likewise describes posterior leads as useful when symptoms continue and the standard ECG is inconclusive.
Placement described in the 2022 ACC pathway
- V7: left posterior axillary line.
- V8: tip of the left scapula.
- V9: left paraspinal region.
Place all three in the same horizontal plane as V6. In the posterior STEMI pattern described by the 2022 ACC pathway, ST elevation of at least 0.5 mm in one or more of V7–V9 is a confirmation criterion. Clinicians must interpret that threshold with the full ECG and clinical assessment; it does not replace them.
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How should clinicians interpret the standard and posterior ECG together?
The practical question is whether a concerning clinical presentation and the standard ECG leave a plausible posterior-occlusion pattern that warrants further assessment. Clinicians can consider the following findings together:
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- Whether the standard 12-lead ECG is diagnostic or remains inconclusive.
- Whether V1–V3 show reciprocal ST depression and whether supporting posterior-pattern features are present.
- Whether symptoms continue or the patient’s clinical status changes.
- Whether supplemental V7–V9 show ST elevation.
The 2025 ACC/AHA guideline states: “The absence of electrocardiographic evidence of ischemia does not exclude ACS.” A normal or nondiagnostic initial ECG therefore cannot, by itself, rule out ACS. Under clinical assessment, repeat ECGs may be appropriate when symptoms or the patient’s condition warrant them.
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The same 2025 guideline reports that a second and/or third ECG during EMS transport may identify up to 15% of additional STEMI cases that were not present on the first study. That figure concerns STEMI detection by serial prehospital ECGs generally; it is not a rate specific to posterior MI.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which symptoms require emergency help?
Heart attack warning signs can include central chest pressure, squeezing, fullness, or pain; discomfort in the arms, back, neck, jaw, or stomach; shortness of breath; cold sweat; nausea; lightheadedness; and unusual tiredness. Symptoms can vary and may be less obvious in some people. Symptoms alone do not reliably identify the location of an infarction.
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If you or someone nearby may be having a heart attack, call 911 in the United States or your local emergency number elsewhere. Do not wait for symptoms to worsen, for a repeat ECG, or for posterior leads to be recorded. The American Heart Association’s public guidance is to call 911 when heart attack warning signs occur: Warning Signs of a Heart Attack.
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Guidelines cited
- 2025 ACC/AHA/ACEP/NAEMSP/SCAI acute coronary syndromes guideline.
- 2023 ESC acute coronary syndromes guideline.
- 2022 ACC emergency-department chest-pain consensus pathway.
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