In a patient with ongoing ischemic chest pressure, horizontal ST depression in V1–V3 can be easy to file as a nonspecific anterior abnormality and move on. That can miss injury on the opposite side of the left ventricle. The practical question is not whether anterior ST depression proves posterior infarction—it does not—but when it should prompt a posterior ECG and how to read the result.
Why the standard ECG can hide posterior injury
The standard 12-lead ECG has no electrodes directly over the posterior left ventricle. Leads V1–V3 view the heart from the front; injury currents directed toward the back may therefore appear as reciprocal ST depression in those anterior leads.
That mirror-image explanation is useful, but it is not a diagnosis by itself. ST depression can reflect other forms of ischemia or repolarization change, so interpret its distribution, shape, comparison with prior ECGs, and the patient’s symptoms together.
Use the pattern to decide when to add leads
With ongoing ischemic symptoms and an inconclusive standard ECG, isolated ST depression of at least 0.5 mm in V1–V3 is a reason to record posterior leads—especially when circumflex occlusion is suspected. That threshold is a trigger to look further, not proof that the posterior wall is infarcting.
The 2026 Fifth Universal Definition describes posterior infarction as suggested by ST depression of at least 1 mm in V1, V2, and/or V3, particularly when the R wave is larger than the S wave in V1 or V2; ST elevation in posterior leads V7–V9 is the corroborating ECG finding. These thresholds serve different purposes: a lower threshold can prompt additional leads, while the fuller precordial pattern increases suspicion.
| Finding | How to use it |
|---|---|
| New, isolated ST depression maximal in V1–V3 during ischemic symptoms | Consider posterior leads; do not label the finding diagnostic on its own. |
| Dominant R wave or R/S ratio greater than 1 in V1 or V2 | Supportive reciprocal clue, not a required feature; it can also have other explanations. |
| ST depression across many leads, or a pattern that fits LVH or a conduction abnormality | Consider broader ischemia or another cause rather than assuming a posterior location. |
A tall R wave in V1 or V2 can be understood as the mirror image of a posterior Q wave. But it may reflect an older infarct or another cause, and its absence does not rule out acute posterior injury. Do not make R-wave dominance a prerequisite for obtaining posterior leads.
Place V7–V9 accurately—and read them in context
Position V7, V8, and V9 in the same horizontal plane as V6, across the left posterior chest:
- V7: left posterior axillary line.
- V8: left midscapular line.
- V9: left paraspinal border.
If electrodes are moved from their standard positions, make sure the tracing is identified and interpreted as a posterior-lead ECG—not as standard V4–V6. Correct labeling matters when the tracing is communicated to cardiology or compared with prior ECGs.
Posterior-lead ST elevation may be small: a commonly used J-point threshold is 0.5 mm. Read the measurement with the neighboring leads, symptoms, and the whole ECG; do not simply apply the usual 1-mm threshold for most standard leads. New posterior-lead elevation in a compatible presentation supports a posterior STEMI pattern and warrants urgent ACS and reperfusion assessment.
A positive posterior ECG adds evidence; a negative one does not erase the concern. Posterior changes can be subtle or evolve over time. If symptoms continue or the clinical picture changes, repeat ECG assessment and continue the ACS evaluation rather than treating one negative posterior tracing as an exclusion test.
Make the next move match the uncertainty
The useful sequence is: recognize the distribution, add leads when the presentation warrants it, then interpret the standard and posterior tracings together. A posterior ECG is a localization tool, not a substitute for clinical assessment or serial evaluation.
For residents, the key distinction is between a clue that prompts a test and a pattern that supports a diagnosis. V1–V3 depression can be that first clue; posterior-lead elevation can provide the confirming ECG evidence. Neither R-wave dominance nor a single isolated ST measurement should carry the whole conclusion.
Practical takeaways
- In suspected ACS, isolated ST depression of at least 0.5 mm in V1–V3 can prompt posterior leads; it does not establish posterior MI.
- V7–V9 sit across the left posterior chest in the same horizontal plane as V6.
- Posterior-lead ST elevation may be subtle; a commonly used threshold is 0.5 mm at the J point, interpreted in context.
- A dominant R wave in V1 or V2 is supportive, not required or specific.
- A negative posterior tracing does not rule out evolving ischemia when symptoms or other ECG findings remain concerning.
