Right-Sided & Posterior MI
Objective: When the 12-lead looks inferior or has isolated anterior depression, add the right and posterior views your protocol allows — and do not dump preload in a right-ventricular infarct.
Why This Is Hard
The standard 12-lead is blind behind the heart and stingy on the right ventricle. Posterior occlusion hides as ST depression in V1–V3 and gets called “NSTEMI” while the artery is closed. Right-ventricular infarct rides with many inferior STEMIs and then someone sprays nitro. The 2025 ACC/AHA/ACEP/NAEMSP ACS guideline: posterior leads (V7–V9) when you suspect circumflex occlusion, especially isolated ST depression ≥0.5 mm in V1–V3. Right-sided leads (V4R) belong with inferior patterns when your protocol includes them — especially before nitrates.
- Inferior STE (II, III, aVF) — get V4R if protocol includes it
- ST depression V1–V3, especially with tall R — posterior leads V7–V9
- RV infarct: preload-dependent — nitro and morphine-style venodilation can crash BP
- Still a STEMI-alert / transmit / PCI destination problem
Right Ventricular Infarct
- Usually accompanies inferior STEMI (proximal RCA teaching picture).
- V4R: ST elevation in the right-sided V4 position is the classic field clue. Clear the skin, move V4, relabel the tracing.
- Clinically: clear lungs, hypotension, JVD — the “preload-dependent” triad when it is present. Do not require all three.
- Nitroglycerin drops preload. That can wreck RV infarcts. See nitro cautions. Fluids are a protocol call if they are hypotensive from RV failure without pulmonary edema — not a free-for-all bolus in every inferior MI.
Posterior MI
- Often circumflex (or dominant RCA). The 12-lead shows ST depression in V1–V3, sometimes with tall R waves (posterior Q waves seen from the front).
- V7–V9: left back at the same horizontal as V6, around the posterior axillary line to the left paraspinal area. ST elevation there is a posterior STEMI equivalent.
- Do not wait for the ED to “add posterior leads later” if your protocol lets you do it on scene and symptoms continue.
Street Sequence
- Standard 12-lead within 10 minutes of first medical contact when ACS is on the table (STEMI lesson).
- Inferior STE → right-sided lead if trained/protocol. Anterior depression without anterior STE → posterior leads.
- Transmit both. Use STEMI-alert language: “inferior STE with V4R elevation” or “posterior STE in V7–V9.”
- Aspirin if protocol. Nitro only after you have thought about the RV. PCI-capable destination.
Field Pitfalls
- Calling isolated V1–V3 depression “just demand ischemia” and skipping posterior leads.
- Nitro in a hypotensive inferior MI without a right-sided look.
- Forgetting to label V4R / V7–V9 on the printout so the ED thinks it is a standard V4.
Practice
60-second drill
On a training torso, move electrodes: V4R, then V7–V9. Time it. Say out loud what would make you do each set.
Related: 12-Lead STEMI Recognition, Nitroglycerin Cautions, Aspirin in ACS.
Sources & Further Study
Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.
- NASEMSO National Model EMS Clinical Guidelines (2022) — EMS companion: pulse, perfusion, and protocol still outrank a pretty strip
- 2025 ACC/AHA/ACEP/NAEMSP/SCAI ACS Guideline — posterior leads V7–V9 for suspected circumflex occlusion / isolated ST depression in V1–V3
- Right-sided V4R with inferior patterns when protocol includes it. Nitrates can drop preload in RV infarct. This site does not diagnose STEMI.
Education only. Apply learning within your protocols and medical direction. This is not clinical care guidance.
Check Your Understanding
Street decisions from this lesson only. After you check, the key is highlighted. Education practice — not a certification exam.