Right-Sided & Posterior MI

Why this matters

Get the standard 12-lead first. It does not see well behind the heart, and it sees little of the right ventricle. Posterior occlusion can hide as ST depression in V1–V3. Crews may call that a non-ST-elevation myocardial infarction (NSTEMI) while the artery is closed. A right-ventricular infarct comes with many inferior STEMIs. Pause before nitroglycerin. It can drop the blood pressure in that patient.

Use posterior leads (V7–V9) when you suspect circumflex occlusion, especially isolated ST depression of 0.5 mm or more in V1–V3. Use a right-sided lead (V4R) with inferior patterns when your protocol includes it, especially before nitrates.

Right Ventricular Infarct

• It usually comes with an inferior STEMI. Think of a proximal right coronary artery.
• V4R: ST elevation in the right-sided V4 position is the classic field clue. Clean the skin, move V4, and label the tracing V4R.
• The clinical picture can be clear lungs, hypotension, and neck veins that are up. The right ventricle needs blood coming back. You do not need all three findings.
• Nitroglycerin drops blood returning to the heart. That can collapse the blood pressure in a right-ventricular infarct. See nitro cautions. Fluids are a protocol decision if they are hypotensive from right-ventricular failure and the lungs are clear. Do not give a large bolus to every inferior MI.

Posterior MI

• It is often the circumflex artery, or a dominant right coronary artery. The 12-lead shows ST depression in V1–V3, sometimes with tall R waves. Those tall R waves are posterior Q waves seen from the front.
• Place V7–V9 on the left back, at the same level as V6, from the posterior axillary line to just left of the spine. 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.

Clinical sequence

1. Get a standard 12-lead within about 10 minutes of first medical contact when ACS (a heart-attack workup) is possible. See the STEMI lesson.
2. If you see inferior ST elevation, add a right-sided lead if you are trained and protocol includes it. If you see anterior depression without anterior ST elevation, add posterior leads.
3. Transmit both tracings. Use STEMI-alert language: “inferior ST elevation with V4R elevation,” or “posterior ST elevation in V7–V9.”
4. Give aspirin if protocol allows it. Give nitroglycerin only after you have checked the right ventricle. Go to a PCI-capable hospital. Give oxygen if the saturation is under about 90%. Do not use a non-rebreather as a default when the saturation is already near 98%.

Common errors

• Calling isolated V1–V3 depression only demand ischemia, and skipping posterior leads.
• Giving nitroglycerin in a hypotensive inferior myocardial infarction without a right-sided lead.
• Forgetting to label V4R or V7–V9, so the emergency department reads it as a standard V4.

Practice

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) — U.S. EMS model language; protocol still wins
• 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 blood returning to the heart in RV infarct. This site does not diagnose STEMI.