You’re flying through a question block when an ECG pops up with ST elevations. Easy, right? Not always. On USMLE-style vignettes, ST elevation is a pattern-recognition trap: the right diagnosis often depends on which leads, what the ST segment looks like, and what else is (or isn’t) happening—reciprocal changes, PR depression, Q waves, troponins, symptoms, and timing. This post breaks down a classic STEMI vignette and—more importantly—shows you how to dismantle every distractor so you don’t get baited on test day.
Tag: Cardiovascular > ECG Interpretation
The Clinical Vignette (USMLE Style)
A 58-year-old man with hypertension, hyperlipidemia, and a 30–pack-year smoking history presents with 30 minutes of crushing substernal chest pain radiating to the left arm. He is diaphoretic and nauseated. BP 92/58 mm Hg, HR 52/min. Lungs are clear. ECG shows:
- ST-segment elevations in leads II, III, and aVF
- Reciprocal ST depressions in leads I and aVL
Question: Which artery is most likely occluded?
Correct Answer: Inferior STEMI → RCA (Most Common)
Why it’s RCA
ST elevation in II, III, aVF localizes to the inferior wall. The inferior wall is supplied most commonly by the right coronary artery (RCA) (right-dominant circulation is most common). A classic extra clue:
- ST elevation in lead III > lead II suggests RCA rather than LCX.
- Bradycardia and hypotension can occur due to RCA involvement of:
- SA node (often) and AV node (often) → sinus bradycardia, AV block
- Right ventricle → preload-dependent hypotension
What to do clinically (Step 2-relevant)
- This is a STEMI: the management is immediate reperfusion (PCI preferred if available quickly; otherwise fibrinolysis if no contraindications).
- Inferior MI + hypotension + clear lungs should make you think: right ventricular infarct.
- Get right-sided ECG leads (V4R).
- Treat RV infarct hypotension with IV fluids.
- Avoid nitrates (and often diuretics) because RV infarcts are preload dependent → nitrates can cause severe hypotension.
Lead Localization Cheat Sheet (High Yield)
| Territory | ECG leads | Typical artery |
|---|---|---|
| Septal | V1–V2 | LAD (septal branches) |
| Anterior | V3–V4 | LAD |
| Lateral | I, aVL, V5–V6 | LCX (or diagonal LAD) |
| Inferior | II, III, aVF | RCA (most common) or LCX |
| Posterior | ST depression V1–V3 + tall R waves; ST elevation V7–V9 | RCA or LCX |
Now the Real Skill: Why Each Distractor Is Wrong (and When It’s Right)
Below are common answer choices that show up with ST elevation questions. The key is learning the “signature” features that separate them from a STEMI.
Distractor 1: Left Anterior Descending (LAD) Occlusion
Why it’s wrong here
LAD occlusion classically causes anterior wall MI:
- ST elevations in V1–V4 (sometimes extending to V5–V6, I, aVL if large)
But our ECG is inferior (II, III, aVF) with reciprocal depression in I/aVL—a classic inferior STEMI pattern.
When LAD is the right answer
Think LAD if you see:
- V1–V4 ST elevation (anteroseptal MI)
- Possible new LBBB in the right clinical context (though Step exams increasingly emphasize not using “new LBBB = STEMI” as a blanket rule)
- Large anterior infarcts → higher risk of pump failure, cardiogenic shock, ventricular arrhythmias
High-yield complication:
- Ventricular septal rupture (3–5 days post MI) → harsh holosystolic murmur + acute heart failure.
Distractor 2: Left Circumflex (LCX) Occlusion
Why it’s wrong here (most of the time)
LCX can supply the inferior wall in left-dominant systems, but RCA is more common. In many vignettes, the exam wants you to choose RCA unless there are features pointing toward LCX.
When LCX is the right answer
LCX is more likely if you see:
- Inferior ST elevations but lead II ≥ lead III (a soft clue)
- Isolated lateral MI (I, aVL, V5, V6)
- Posterior MI findings (which are easy to miss):
- ST depressions in V1–V3
- Tall R waves in V1–V2 (posterior Q-wave equivalent)
- Confirm with posterior leads V7–V9 showing ST elevation
High-yield pearl: Posterior MI can look like “anterior ischemia” because it shows up as ST depression in V1–V3. Always consider posterior MI if the ST depression is horizontal and paired with tall R waves.
Distractor 3: Acute Pericarditis
Why it’s wrong here
Pericarditis can cause ST elevation, but the shape and distribution are different.
Pericarditis ECG pattern:
- Diffuse, concave (“smiley”) ST elevations across many leads
- PR depression (especially in limb leads)
- No reciprocal ST depressions (except sometimes in aVR and V1)
Our vignette has regional ST elevation (inferior leads) plus reciprocal depression (I, aVL), which is much more consistent with STEMI.
When pericarditis is the right answer
Clues include:
- Sharp chest pain worse with inspiration and better leaning forward
- Recent viral illness
- Pericardial friction rub
- Possible pericardial effusion
High yield:
- ST/T ratio in V6: pericarditis often has relatively large ST elevation compared with T wave (students use this as a supportive clue, not a primary one).
Distractor 4: Benign Early Repolarization
Why it’s wrong here
Early repolarization can have ST elevation, but it’s usually:
- Seen in young, healthy patients
- Stable over time
- Most prominent in precordial leads
- Often has J-point notching (“fishhook”)
- No reciprocal changes
- Patient is typically not diaphoretic with crushing chest pain
Our patient has classic ACS symptoms and reciprocal changes—don’t let early repol bait you when the clinical story screams MI.
When early repol is the right answer
- Asymptomatic or non-cardiac symptoms
- No troponin rise
- No dynamic ECG changes on serial ECGs
Step strategy: If you’re deciding between early repol and STEMI, ask:
- Is there reciprocal depression?
- Is there a convincing ACS story?
- Are changes dynamic?
Distractor 5: Left Ventricular Aneurysm (Old MI with Persistent ST Elevation)
Why it’s wrong here
LV aneurysm is a classic “persistent ST elevation” trap. It typically occurs weeks to months after a transmural MI.
ECG clues:
- Persistent ST elevation in the same leads as a prior MI
- Often accompanied by deep Q waves
- Patient’s symptoms are not acute crushing chest pain with diaphoresis (unless they’re having something else too)
When LV aneurysm is the right answer
- History of prior MI
- Stable ECG pattern (no evolution)
- Possible complications: mural thrombus → embolic stroke; ventricular arrhythmias
High-yield:
- LV aneurysm increases risk of ventricular arrhythmias and thromboembolism.
Distractor 6: Hyperkalemia (ECG Changes)
Why it’s wrong here
Hyperkalemia doesn’t usually cause regional ST elevations in a coronary distribution. It causes:
- Peaked T waves
- Prolonged PR
- Widened QRS
- Eventually sine-wave pattern → cardiac arrest
If you see ST elevation confined to inferior leads with reciprocal depression, think coronary occlusion first—not electrolytes.
When hyperkalemia is the right answer
- ESRD, missed dialysis, ACE inhibitor + spironolactone, tumor lysis, rhabdo
- Bradyarrhythmias + wide QRS + peaked T waves
The “Reciprocal Changes” Rule (Test-Winning)
Reciprocal ST depression strongly supports STEMI over pericarditis/early repol.
Common reciprocal patterns:
- Inferior STEMI (II, III, aVF) → reciprocal depression in I, aVL
- Lateral STEMI (I, aVL, V5–V6) → reciprocal depression in II, III, aVF
- Anterior STEMI may show reciprocal depression in inferior leads
Step 2 Management Nuggets (High Yield)
Immediate actions in suspected STEMI
- Aspirin ASAP
- Reperfusion (PCI preferred)
- High-intensity statin
- Anticoagulation (e.g., heparin) as part of ACS protocol
- Oxygen only if hypoxic (don’t reflexively give to everyone)
- Pain control: morphine is not first-line; use selectively
Inferior MI specifics: watch for RV infarct
Suspect RV infarct if:
- Inferior STEMI + hypotension
- Clear lungs
- JVP may be elevated
Do:
- Right-sided leads (V4R)
- IV fluids
Avoid:
- Nitrates
- Diuretics
- Other preload-reducing agents if hypotensive
Rapid-Fire Self-Check: If You See ST Elevation, Ask These 6 Questions
- Which leads are elevated? (localize territory)
- Is there reciprocal depression? (supports STEMI)
- What’s the ST shape? (convex “tombstone” often MI; concave diffuse suggests pericarditis/early repol)
- Any PR depression? (pericarditis)
- Any Q waves/persistent pattern? (old MI/LV aneurysm)
- Does the clinical story fit ACS? (don’t ignore symptoms)
Take-Home Summary
- Inferior STEMI = ST elevation in II, III, aVF with reciprocal depression in I, aVL → RCA occlusion most likely.
- Lead III > lead II is a helpful clue toward RCA.
- Inferior MI can involve the AV node → bradycardia and heart block.
- Hypotension + clear lungs in inferior MI = think right ventricular infarct → fluids, avoid nitrates.
- The distractors are beaten by distribution + reciprocity + ST shape + clinical context.