Posterior myocardial infarction behind anteroseptal ST depression — case discussion

Abbreviations

Scenario

Practice case (not a real patient). A 61-year-old woman presents to the emergency department with 45 minutes of ongoing central chest pain. She is haemodynamically stable. Her first 12-lead ECG shows 1 mm horizontal ST depression in V1–V3 with positive terminal T waves and no ST elevation in any standard lead. There is no LVH or bundle branch block, and no previous ECG is available.

1. The first ten minutes

• Record and read the ECG at once: ESC 2023 recommends 12-lead ECG recording and interpretation as soon as possible at the point of FMC, with a target of under 10 min (Class I, Level B).^[1] ACC/AHA 2025 in-hospital assessment: in suspected ACS, acquisition and interpretation of an ECG within 10 minutes is recommended to help guide management (COR 1, LOE B-NR); NHFA/CSANZ 2025: in people presenting with chest pain or other symptoms suggestive of ACS, record and assess an ECG for evidence of ACOMI within 10 min of first clinical contact (Consensus).^[2][4]
• Monitor her: ESC 2023 recommends continuous ECG monitoring and defibrillator capacity as soon as possible in suspected ACS with other ECG changes or ongoing chest pain (Class I, Level B).^[1] NHFA/CSANZ 2025 recommends continuous monitoring and defibrillator availability with ongoing ischaemic symptoms while assessment for ACOMI continues (strong recommendation, low certainty).^[4]

2. Reading the first ECG

• No lead shows ST elevation, so the classic cut-points (≥1 mm outside V2–V3; ≥1.5 mm in V2–V3 for a woman) are not met.^[1][2]
• Her ST depression still counts: if new or presumed new, 1 mm horizontal ST depression in V1–V3 meets the ACC/AHA 2025 Table 3 NSTE-ACS criterion of new or presumed new, usually dynamic, horizontal or down-sloping ST depression ≥0.5 mm in ≥2 contiguous leads.^[2] The Fifth UDMI (2026) likewise says new horizontal or down-sloping ST depression ≥0.5 mm and/or T-wave inversion >1 mm in two contiguous leads is suggestive of myocardial ischaemia.^[12]
• Do not stop there. ESC 2023: ST depression in V1–V3, especially when the terminal T wave is positive, is highly suggestive of posterior coronary occlusion, often of the left circumflex artery.^[1] ACC/AHA 2025: ST depression in V1-V3 could indicate an evolving posterior STEMI and should be managed with a high index of suspicion, with a posterior lead ECG if warranted.^[2]

3. Posterior leads

• ACC/AHA 2025 Table 3: posterior leads (V7–V9) should be obtained in patients with suspected left circumflex occlusion, particularly in the setting of isolated ST depression ≥0.5 mm in V1–V3.^[2] ESC 2023 recommends additional ECG leads (V3R, V4R and V7–V9) if total vessel occlusion is suspected and standard leads are inconclusive (Class I, Level B).^[1]
• The posterior ECG shows 1 mm ST elevation in V8 and V9. ESC 2023 says ST elevation in V7–V9 is highly suggestive of posterior coronary occlusion (often the left circumflex artery), and the Fifth UDMI (2026) says a posterior MI suggested by ST depression ≥1 mm in V1, V2 and/or V3 (particularly with a dominant R wave in V1 or V2) is confirmed by ST elevation in the posterior leads V7 to V9.^[1][12] Fourth UDMI (2018), not restated in the Fifth (history): it had recommended a 0.5 mm ST elevation cut-off in V7–V9 (≥1 mm in men under 40).^[5]

4. What this means for her

• Posterior MI is one of the ACOMI patterns NHFA/CSANZ 2025 names beyond the traditional ST elevation criteria, and recognising these patterns should prompt urgent consultation with cardiology services for consideration of reperfusion strategy.^[4] People with symptoms and ECG changes consistent with ACOMI require urgent reperfusion, and a clinical decision pathway should not be used (NHFA/CSANZ 2025, strong recommendation, very low certainty).^[4] ESC 2023: ST elevation is the most sensitive sign of ongoing coronary occlusion, but other ECG findings can suggest ongoing coronary occlusion (or severe ischaemia); if they are present, prompt triage for immediate reperfusion therapy is indicated.^[1]
• The choice and timing of reperfusion are taught in STEMI: reperfusion strategy.

5. If the posterior leads had been negative

• Her initial ECG would still show ST depression in V1–V3, and the 2021 AHA/ACC chest pain guideline says that if ST depression is identified on the initial ECG, management should follow the NSTE-ACS guidelines; that pathway is taught in NSTE-ACS management.^[3]
• Her symptoms are persisting, so record further ECGs: NHFA/CSANZ 2025 recommends recording and assessing additional ECGs if symptoms persist, change or recur, or there is diagnostic uncertainty (Consensus), and ESC 2023 recommends an additional 12-lead ECG with recurrent symptoms or diagnostic uncertainty (Class I, Level C).^[4][1] ACC/AHA 2025 says the timing of repeat ECGs should be guided by symptoms, especially recurrent chest pain, and any change in clinical condition.^[2]