What is the differential diagnosis for a patient presenting with suspected myocardial infarction (MI)?

Differential Diagnosis of Myocardial Infarction

When a patient presents with suspected MI, you must immediately distinguish it from five potentially lethal mimics that require fundamentally different management: aortic dissection, acute pericarditis, acute myocarditis, spontaneous pneumothorax, and pulmonary embolism. 1

Life-Threatening Mimics Requiring Immediate Differentiation

Aortic Dissection

• Look for: Tearing chest pain radiating to the back, pulse differential between arms, blood pressure differential >20 mmHg between arms, widened mediastinum on chest X-ray 1
• Critical pitfall: Administering thrombolytics or anticoagulation to aortic dissection is catastrophic 1
• Diagnostic approach: Emergency echocardiography is recommended as the initial imaging modality in the emergency setting for suspected aortic dissection 1

Acute Pericarditis

• Look for: Pleuritic chest pain relieved by sitting forward, pericardial friction rub, diffuse ST elevation (not confined to vascular territories), PR depression 1
• Key distinction: ST elevation in pericarditis is concave upward and lacks reciprocal changes seen in MI 1
• Echocardiography indication: Recommended in patients with suspected pericardial disease, including effusion or effusive-constrictive process 1

Acute Myocarditis

• Look for: Recent viral illness, diffuse ST changes, elevated troponin without coronary distribution pattern, preserved wall motion on echo or diffuse hypokinesis 1
• Diagnostic approach: Echocardiography recommended for evaluation of chest pain in patients with suspected myopericarditis 1

Pulmonary Embolism

• Look for: Sudden dyspnea, pleuritic chest pain, tachycardia, hypoxemia, right ventricular strain pattern on ECG (S1Q3T3), elevated troponin from right ventricular strain 1
• Echocardiography role: Recommended for detecting echocardiographic signs and guiding therapeutic approach in patients with known pulmonary embolism 1
• Critical distinction: Troponin elevation from PE represents Type 2 MI (supply-demand mismatch) rather than acute coronary occlusion 2

Spontaneous Pneumothorax

• Look for: Sudden sharp chest pain, dyspnea, decreased breath sounds unilaterally, hyperresonance to percussion, absent lung markings on chest X-ray 1
• Echocardiography indication: Recommended for detection in patients with severe deceleration injury or chest trauma 1

Other Cardiac Conditions Mimicking MI

Takotsubo Cardiomyopathy

• Look for: Postmenopausal woman with emotional/physical stress, apical ballooning on echo, ST elevation that resolves quickly, troponin elevation disproportionate to ECG changes 1
• Diagnostic approach: Echocardiography recommended for evaluation of chest pain in patients with suspected Takotsubo cardiomyopathy 1

Acute Valvular Dysfunction

• Look for: New murmur, acute heart failure symptoms, specific valve pathology on echocardiography 1
• Echocardiography indication: Recommended in patients with cardiac murmurs and symptoms suggestive of heart failure, myocardial ischemia/infarction, or clinical evidence of structural heart disease 1

Non-Cardiac Conditions That Elevate Troponin

Type 2 Myocardial Infarction (Supply-Demand Mismatch)

• Causes include: Severe anemia, hypotension, hypertensive emergency, tachyarrhythmias, severe hypoxemia, sepsis 1, 2
• Key distinction: Troponin elevation without acute coronary thrombosis; requires treating the underlying condition rather than emergent catheterization 1, 2
• Diagnostic challenge: Differentiating Type 1 MI (acute coronary occlusion) from Type 2 MI requires clinical context, ECG evolution, and often coronary angiography 2

Acute Decompensated Heart Failure

• Look for: Volume overload, elevated BNP/NT-proBNP, pulmonary edema, known cardiomyopathy 1, 3
• Troponin interpretation: Mild troponin elevation common in ADHF without acute coronary syndrome 3

Renal Failure

• Look for: Chronically elevated troponin at baseline, lack of rise-and-fall pattern, absence of acute ECG changes 1

Diagnostic Algorithm for Suspected MI

Within 10 Minutes of Presentation

1. Obtain 12-lead ECG 1, 4
2. Assess for STEMI criteria: ST elevation ≥0.1 mV in two contiguous leads (≥0.2 mV in V2-V3 for men ≥40 years, ≥0.25 mV for men <40 years, ≥0.15 mV for women) 1, 4
3. Check for lethal mimic features: Pulse differentials, pericardial friction rub, unilateral breath sounds 1

If Initial ECG Non-Diagnostic

• Serial ECGs at 15-30 minute intervals in symptomatic patients 1, 4
• Consider posterior leads (V7-V9) if suspecting circumflex occlusion 4
• Consider right-sided leads (V3R-V4R) if suspecting RV involvement with inferior MI 4

Cardiac Biomarker Interpretation

• Troponin timing: First detectable at 2-4 hours, 100% sensitivity at 10-24 hours for cTnI 1
• Rise-and-fall pattern required: Single elevated troponin without dynamic changes suggests chronic elevation or non-ischemic injury 1
• Do not wait for troponin results to initiate reperfusion therapy in STEMI 4

Role of Echocardiography

• Class I indication: Evaluation of acute chest pain with non-diagnostic ECG and cardiac enzymes when resting echo can be performed during pain 1
• Class I indication: Evaluation of chest pain in patients with known underlying cardiac disease 1
• Class I indication: Evaluation of chest pain with hemodynamic instability unresponsive to simple measures 1

Critical Pitfalls to Avoid

ECG Interpretation Errors

• LBBB or ventricular pacing obscures ST changes: Apply modified Sgarbossa criteria rather than standard STEMI criteria 5
• Posterior MI presents as ST depression in V1-V3: Obtain posterior leads to confirm 4
• 40% of acute coronary occlusions do not meet STEMI criteria: Maintain high suspicion with ongoing symptoms despite non-diagnostic ECG 6

Biomarker Misinterpretation

• AST is NOT recommended for MI diagnosis due to lack of cardiac specificity 7
• Troponin elevation alone does not equal MI: Requires clinical context of ischemia 1, 2
• Less than 25% of patients with ischemic chest pain have acute MI: Avoid overdiagnosis 1

Management Errors

• Do not give thrombolytics without excluding aortic dissection 1
• NSTEMI-OMI patients have significant delays in catheterization despite similar angiographic findings and complications as STEMI 6
• Maintain low threshold for urgent angiography when clinical suspicion high despite non-diagnostic ECG 5