What are the differential diagnoses and management strategies for patients presenting with symptoms and ECG (electrocardiogram) changes suggestive of a STEMI (ST-Elevation Myocardial Infarction), but without clear evidence of a heart attack, and how can STEMI mimics such as pericarditis, pulmonary embolism, and myocarditis be differentiated and treated?

STEMI Mimics: Recognition and Management

Immediate Diagnostic Approach

When a patient presents with ST-segment elevation, obtain a 12-lead ECG within 10 minutes and immediately assess for true STEMI versus mimics by looking for reciprocal ST depression, which strongly indicates coronary occlusion rather than a mimic. 1

The most critical initial step is distinguishing true STEMI from conditions that produce ST elevation without coronary occlusion 2, 3. The ECG remains the cornerstone for this differentiation, but specific features separate ischemic from non-ischemic causes 3, 4.

Key ECG Features That Distinguish STEMI from Mimics

Reciprocal ST Depression (Most Specific for True STEMI)

• In anterior ST elevation: ST depression ≥0.025 mV in lead II occurs in 40% of STEMI patients but in 0% of non-ischemic cases 4
• In inferior ST elevation: ST depression ≥0.025 mV in lead I occurs in 83% of STEMI patients but in 0% of non-ischemic cases 4
• Reciprocal changes are the single most reliable ECG feature for confirming coronary occlusion 4

Features Suggesting Non-Ischemic ST Elevation

• PR depression in chest leads: Present in 38% of non-ischemic cases versus only 12% of STEMI 4
• ST depression in aVR: Associated with non-ischemic diagnosis in multivariable analysis 4
• Absence of terminal QRS distortion: Terminal QRS distortion occurs in 40% of STEMI but only 7% of non-ischemic ST elevation 4
• Concave ST segments: More typical of pericarditis, though convex ST elevation can occur in 22% of STEMI 4

Common STEMI Mimics and Their Distinguishing Features

Acute Pericarditis

• Widespread ST elevation in multiple non-contiguous leads (not confined to a single coronary territory) 3
• PR depression particularly prominent in chest leads (V2-V6) and limb leads 4
• Absence of reciprocal ST depression (except in aVR and V1) 4
• ST elevation typically concave upward rather than convex 3
• Troponin may be mildly elevated but typically lower than expected for the degree of ST elevation 5

Critical pitfall: Regional or focal pericarditis can mimic STEMI with localized ST elevation and elevated troponin, particularly in young patients 5, 6. Consider this diagnosis in younger patients without traditional cardiac risk factors presenting with chest pain and ST elevation 5.

Myocarditis/Myopericarditis

• Can present with focal ST elevation mimicking STEMI, particularly in younger patients 5
• Troponin elevation may be significant 5
• Major risk: Administering thrombolytics can cause catastrophic complications including intracranial bleeding, myocardial rupture, and hemorrhagic tamponade 5
• Consider in patients <40 years with recent viral illness, absence of traditional cardiac risk factors, and diffuse or regional ST changes 5

Acute Pulmonary Embolism

• ST elevation in V1-V4 can occur in massive or submassive PE 7
• Syncope occurs in 67% of PE cases presenting with ST elevation (much higher than typical STEMI) 7
• Right ventricular dysfunction on echocardiography present in 90% of cases 7
• Concomitant deep venous thrombosis found in 90% when assessed 7
• S1Q3T3 pattern, right axis deviation, or new right bundle branch block may be present 7
• In-hospital mortality 17% in this presentation, indicating high-risk PE 7

Management decision: If clinical features suggest PE (syncope, pleuritic chest pain, hypoxia, tachycardia out of proportion to ST elevation), obtain CT pulmonary angiography before cardiac catheterization 7.

Early Repolarization/Benign Early Repolarization

• Notching or slurring at the J point 3
• ST elevation typically concave upward 3
• Stable over time on serial ECGs (does not evolve like STEMI) 3
• Most prominent in precordial leads V2-V4 3
• No reciprocal ST depression 4

Left Ventricular Aneurysm

• Persistent ST elevation weeks to months after prior MI 3
• Pathologic Q waves in same leads with ST elevation 3
• ST elevation does not evolve on serial ECGs 3
• History of prior MI typically present 3

Takotsubo Cardiomyopathy

• Typically presents with anterior ST elevation or diffuse ST changes 4
• More common in postmenopausal women with emotional or physical stressor 4
• Troponin elevation typically mild relative to degree of wall motion abnormality 4
• Coronary angiography shows no obstructive disease 4

STEMI Confounders (Cannot Reliably Diagnose or Exclude STEMI)

Left Bundle Branch Block (LBBB)

• New or presumed new LBBB with clinical suspicion of ongoing ischemia should be treated as STEMI 2, 1
• Concordant ST elevation (ST elevation in leads with positive QRS) is the best indicator of acute MI with LBBB 1
• When in doubt with LBBB and ongoing symptoms, proceed to emergent angiography 2, 1

Left Ventricular Hypertrophy Pattern

• Baseline ST elevation in precordial leads makes STEMI diagnosis challenging 3
• Compare to prior ECG if available 2
• Rely more heavily on clinical presentation and troponin evolution 2

Ventricular Paced Rhythm

• Cannot reliably diagnose STEMI from ECG alone 3
• Proceed based on clinical suspicion and troponin measurements 2

Algorithmic Approach to ST Elevation

1. Obtain 12-lead ECG within 10 minutes of first medical contact 1

2. Assess for reciprocal ST depression:

   • Present → High probability of true STEMI, activate catheterization lab 4
   • Absent → Proceed to step 3 4

3. Evaluate for mimic features:

   • PR depression in chest leads + widespread ST elevation → Pericarditis 4
   • Young patient + recent viral illness + no risk factors → Consider myocarditis 5
   • Syncope + hypoxia + RV dysfunction → Consider PE, obtain CT pulmonary angiography 7
   • Stable ST elevation + prior MI + Q waves → LV aneurysm 3

4. Check for confounders (LBBB, LVH, paced rhythm):

   • If present with ongoing symptoms → Treat as STEMI 2, 1

5. Obtain high-sensitivity troponin immediately and repeat at 1-3 hours 2

6. When diagnosis remains uncertain despite above evaluation:

   • Proceed to emergent coronary angiography if hemodynamically unstable or high clinical suspicion 2
   • Consider echocardiography to assess wall motion and RV function 7
   • Obtain posterior leads V7-V9 if posterior MI suspected 1

Critical Management Principles

Do not delay reperfusion therapy in true STEMI while ruling out mimics - the cost of missing STEMI exceeds the cost of unnecessary catheterization 2, 1. However, recognize that thrombolytic therapy in myocarditis can be catastrophic 5.

When to Activate Catheterization Lab Immediately

• ST elevation meeting STEMI criteria with reciprocal ST depression 4
• New or presumed new LBBB with ongoing ischemic symptoms 2, 1
• Hemodynamic instability regardless of ECG pattern 2
• ST depression in ≥8 leads with ST elevation in aVR (suggests left main or multivessel disease) 1

When to Pursue Alternative Diagnosis First

• Young patient (<40 years) without risk factors, recent viral illness, widespread ST elevation without reciprocal changes → Consider myocarditis, avoid thrombolytics 5
• Syncope + hypoxia + tachycardia with right precordial ST elevation → Obtain CT pulmonary angiography for PE 7
• Widespread ST elevation + PR depression without reciprocal changes → Likely pericarditis 4

Role of Troponin

• Troponin elevation does not distinguish STEMI from mimics (elevated in myocarditis, PE, pericarditis) 2, 5
• Serial troponins help confirm myocardial injury but should not delay reperfusion in suspected STEMI 2
• Magnitude of troponin elevation relative to ST elevation may provide clues (disproportionately low troponin suggests mimic) 5

Special Populations and Presentations

Posterior MI Recognition

• Isolated ST depression ≥0.05 mV in V1-V3 with positive terminal T waves represents posterior STEMI equivalent 1
• Obtain posterior leads V7-V9 showing ST elevation ≥0.05 mV to confirm 1
• Treat as STEMI with immediate reperfusion 1

Right Ventricular Infarction

• In inferior MI, obtain right precordial leads V3R-V4R 2
• ST elevation in these leads indicates RV involvement affecting management 2

Patients Presenting >12 Hours After Symptom Onset

• Reperfusion therapy indicated if clinical or ECG evidence of ongoing ischemia 2
• In stable patients >12 hours without ongoing ischemia, benefit of PCI is uncertain 2