What are the diagnostic criteria for myocardial infarction (MI)?

Diagnostic Criteria of Myocardial Infarction

Myocardial infarction (MI) is diagnosed when there is evidence of myocardial necrosis in a clinical setting consistent with acute myocardial ischemia. 1

Primary Diagnostic Criteria

The diagnosis of acute MI requires the following:

• Detection of a rise and/or fall of cardiac biomarker values (preferably cardiac troponin) with at least one value above the 99th percentile upper reference limit (URL) 1

AND at least one of the following:

• Symptoms of myocardial ischemia 1
• New or presumed new significant ST-segment–T wave changes or new left bundle branch block (LBBB) on ECG 1
• Development of pathological Q waves on ECG 1
• Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality 1
• Identification of an intracoronary thrombus by angiography or autopsy 1

ECG Criteria for Myocardial Ischemia

ST-Segment Elevation

• New or presumed new ST-segment elevation at the J point in two or more contiguous leads 1:
  • ≥0.2 mV (2 mm) in leads V1, V2, or V3 1
  • ≥0.1 mV (1 mm) in other leads 1
  • In men <40 years: ≥2.5 mm in leads V2-V3 2
  • In men ≥40 years: ≥2 mm in leads V2-V3 2
  • In women: ≥1.5 mm in leads V2-V3 2

Non-ST Elevation Changes

• New or presumed new ST-segment depression 1
• T-wave abnormalities (new or presumed new symmetric inversion of T waves ≥1 mm in at least two contiguous leads) 1
• Hyperacute T-waves (tall and peaked) may be seen during very early phases of acute MI 1, 2

Classification of MI Types

Type 1: Spontaneous MI

• Related to atherosclerotic plaque rupture, ulceration, fissuring, erosion, or dissection with resulting intraluminal thrombus 1

Type 2: MI Due to Oxygen Supply/Demand Imbalance

• Condition other than CAD contributes to imbalance between myocardial oxygen supply/demand 1
• Examples: coronary endothelial dysfunction, coronary artery spasm, coronary embolism, tachy/bradyarrhythmias, anemia, respiratory failure, hypotension, severe hypertension 1, 3

Type 3: MI Resulting in Death When Biomarkers Unavailable

• Cardiac death with symptoms suggestive of myocardial ischemia and presumed new ischemic ECG changes or new LBBB 1
• Death occurs before blood samples could be obtained or before cardiac biomarker could rise 1

Type 4: MI Related to PCI

• Type 4a: PCI-related MI - defined by elevation of cTn values >5× 99th percentile URL in patients with normal baseline values 1
• Type 4b: Stent thrombosis-related MI - detected by coronary angiography or autopsy with biomarker elevation 1

Type 5: MI Related to CABG

• Defined by elevation of cardiac biomarker values >10× 99th percentile URL in patients with normal baseline cTn values 1
• Plus either new Q waves/LBBB, angiographic evidence of graft/native coronary occlusion, or imaging evidence of new loss of viable myocardium 1

Special Considerations

MINOCA (MI with Non-Obstructive Coronary Arteries)

• Meets AMI criteria but has non-obstructive coronary arteries on angiography (no stenosis ≥50% in any major epicardial vessel) 1
• Requires exclusion of alternate diagnoses such as myocarditis, pulmonary embolism, and sepsis 1

Reinfarction

• May present diagnostic difficulties when cardiac troponin is persistently high 1
• Consider using biomarkers with shorter time course (CK-MB or myoglobin) to clarify timing 1

Imaging in MI Diagnosis

• Echocardiography can detect regional or global systolic/diastolic dysfunction, increased wall thickness, pericardial effusion, or intracavitary thrombi 1
• Cardiovascular magnetic resonance (CMR) can identify edema, hyperemia, irreversible injury (necrosis/scar), and regional wall motion abnormalities 1
• Imaging techniques are particularly valuable when ECG is inconclusive or to identify non-ischemic causes of chest pain 1

Pitfalls and Caveats

• A normal ECG does not rule out the diagnosis of MI, as some patients with myocardial necrosis may not exhibit ECG changes 1
• Certain conditions can confound ECG interpretation, including bundle branch block, left ventricular hypertrophy, and Wolff-Parkinson-White syndrome 1
• Elevated troponin levels indicate myocyte injury but are not disease-specific and can result from both ischemic and non-ischemic mechanisms 1, 3
• In patients with reinfarction, sequential samples of biomarkers with shorter time courses may be needed to clarify timing 1
• Careful clinical correlation is essential, as biomarker elevation can occur in non-ischemic conditions such as myocarditis, Takotsubo syndrome, and sepsis 1, 3