Study note
MedicineCardiology

Acute myocardial infarction: diagnostic criteria and classification

THE CONTEXT
Acute myocardial infarction (MI) requires acute myocardial injury with evidence of acute ischaemia. The classification distinguishes atherothrombotic, supply–demand, fatal and procedure-related events.

Diagnostic criteria

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Acute MI requires acute myocardial injury with clinical evidence of acute myocardial ischaemia and a rise and/or fall in cardiac troponin, with at least one value above the 99th centile upper reference limit.
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At least one additional feature is required: ischaemic symptoms, new ischaemic ECG changes, pathological Q waves, imaging evidence of new loss of viable myocardium or a new regional wall motion abnormality with an ischaemic pattern, or a coronary thrombus identified by angiography or autopsy.

Types of myocardial infarction

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Type 1 MI results from acute atherothrombosis in the artery supplying the infarcted myocardium.
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Type 2 MI results from an imbalance between myocardial oxygen supply and demand unrelated to acute atherothrombosis.
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Type 3 MI is cardiac death with symptoms suggesting myocardial ischaemia and presumed new ischaemic ECG changes before cardiac troponin values become available or abnormal.
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Type 4a MI occurs during percutaneous coronary intervention. Type 4b involves stent thrombosis and type 4c involves restenosis; the source describes types 4b and 4c as consistent with type 1 MI.
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Type 5 MI occurs during coronary artery bypass grafting.

Procedure-related MI within 48 hours

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For patients with normal baseline troponin, the stated thresholds are above five times the 99th centile upper reference limit for type 4a MI and above ten times that limit for type 5 MI.
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An elevated troponin meeting the relevant threshold must be accompanied by at least one qualifying finding.
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Qualifying findings include new pathological Q waves, imaging evidence of presumed new loss of viable myocardium with an ischaemic pattern, or angiographic evidence of a procedural flow-limiting complication.
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New ischaemic ECG changes are an additional qualifying criterion for type 4a MI only.
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Flow-limiting complications include coronary dissection, occlusion of a major epicardial artery or graft, side-branch occlusion-thrombus, disrupted collateral flow or distal embolisation.
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