A 62-year-old woman arrives at the emergency unit complaining of sudden, crushing central chest pain for the past 40 minutes, radiating down her left arm. Her ECG reveals marked ST-segment elevation in leads II, III and aVF. Which vessel is most likely occluded?

A. Posterior descending branch of the right coronary artery
B. Proximal left anterior descending artery
C. Left main coronary trunk
D. Obtuse marginal branch of the circumflex artery
E. High-lateral circumflex branch

Table of Contents

Detailed Explanation

1. Understanding the ECG leads involved
Leads II, III and aVF look at the inferior (bottom) aspect of the heart.
When these leads show ST elevation, it suggests the infarction is occurring in the inferior wall of the left ventricle.

2. Which artery supplies the inferior wall?
In most individuals (≈80%), the right coronary artery (RCA) gives rise to the posterior descending artery (PDA), which supplies the inferior portion of the heart.
→ Therefore, an inferior myocardial infarction is most commonly caused by RCA occlusion, specifically involving its posterior descending branch.

3. Why not the other options?

Left anterior descending (LAD) → Supplies anterior wall → Would cause ST elevation in V1–V4 (not inferior leads).
Left main stem → Supplies a huge myocardial territory → Would cause widespread ST elevation, not isolated inferior changes.
Circumflex branches → Supply lateral wall → Would cause elevation in I, aVL, V5–V6, not inferior leads.
High-lateral circumflex → Same reason as above.

Therefore, the correct answer is: A. Posterior descending branch of the RCA.

CHEAT SHEET FOR EXAM

ECG Territory → Likely Artery

ECG Leads	Infarct Area	Most Likely Artery
II, III, aVF	Inferior wall	RCA (PDA branch)
V1–V4	Anterior	LAD
V1–V6, I, aVL	Anterolateral	Proximal LAD
I, aVL, V5–V6	Lateral	LCX
V7–V9 (posterior leads)	Posterior MI	LCX > RCA

Key Points

Inferior MI = RCA unless the patient is left-dominant (less common).
Reciprocal changes often occur in opposite leads.
Posterior MI causes tall R waves in V1–V3 and ST depression; confirmed with V7–V9.
A new LBBB is a STEMI equivalent.
RCA occlusion may also cause bradycardia due to SA/AV node involvement.

FLASHCARDS (20 CARDS)

1. Q: Which ECG leads represent the inferior wall?
A: II, III, aVF.

2. Q: Most common cause of inferior STEMI?
A: RCA occlusion.

3. Q: What area does LAD supply?
A: Anterior wall and interventricular septum.

4. Q: ST elevation in V1–V4 suggests what?
A: Anterior MI due to LAD occlusion.

5. Q: What vessel usually supplies the AV node?
A: RCA.

6. Q: Inferior MI may cause what rhythm?
A: Sinus bradycardia or AV block.

7. Q: Lateral MI leads?
A: I, aVL, V5, V6.

8. Q: Lateral MI artery?
A: Left circumflex.

9. Q: Posterior MI ECG feature in V1–V3?
A: ST depression + tall R waves.

10. Q: Which artery gives PDA in right-dominant circulation?
A: RCA.

11. Q: Dominant circulation definition?
A: Artery giving rise to PDA.

12. Q: Left-dominant heart PDA origin?
A: Circumflex.

13. Q: What ECG change suggests left main occlusion?
A: Widespread ST elevation.

14. Q: Right ventricular infarction is associated with which MI?
A: Inferior MI due to proximal RCA occlusion.

15. Q: Which leads check RV involvement?
A: V3R, V4R.

16. Q: Reciprocal change in inferior MI?
A: ST depression in I and aVL.

17. Q: Why avoid nitrates in RV infarction?
A: Preload dependent → severe hypotension.

18. Q: Which coronary artery supplies SA node (most often)?
A: RCA.

19. Q: ST elevation in I, aVL suggests occlusion of which artery?
A: LCX or diagonal branches of LAD.

20. Q: Which artery causes posterior MI most commonly?
A: LCX.

MCQs FOR REVISION

1. ST elevation in II, III, aVF + bradycardia. Most likely artery?

A. RCA
B. LAD
C. Circumflex
D. Left main
Answer: A

2. Tall R waves in V1–V3 + ST depression. Diagnosis?

A. Anterior MI
B. Posterior MI
C. Lateral MI
D. Septal MI
Answer: B

3. Widespread ST elevation including I, aVL, V1–V6. Cause?

A. LAD occlusion
B. Left main occlusion
C. RCA occlusion
D. LCX occlusion
Answer: B

4. ST elevation in V5–V6, I, aVL suggests which artery?

A. LAD
B. RCA
C. LCX
D. PDA
Answer: C

5. In right-dominant circulation, which artery supplies the inferior wall?

A. LCX
B. LAD
C. RCA
D. Left main
Answer: C

SUMMARY

Inferior myocardial infarction is classically identified by ST-segment elevation in leads II, III and aVF. These leads visualise the inferior part of the left ventricle, which is usually supplied by the right coronary artery in right-dominant circulation. Therefore, RCA occlusion is the most common cause of inferior STEMI. Reciprocal ST depression may be seen in leads I and aVL. Inferior MI may present with bradycardia because the RCA often supplies the SA and AV nodes. LAD occlusion instead produces anterior ST elevation in V1–V4. Circumflex occlusion typically causes lateral ST elevation in I, aVL, V5 and V6, not inferior changes. Left main occlusion leads to widespread ST elevation and is far more extensive. Posterior MI produces tall R waves and ST depression in V1–V3, confirmed by ST elevation in V7–V9. Understanding the ECG–artery correlation is crucial for rapid diagnosis and reperfusion therapy planning. The RCA’s posterior descending branch is the specific vessel supplying most of the inferior wall. Therefore, in a patient with acute chest pain and inferior ST elevation, RCA occlusion is overwhelmingly the most likely culprit vessel.

Myocardial infarction.