What is a Myocardial Infarction?

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The 4th Universal Definition (2018)

Abbreviations: URL: upper reference limit, MI: myocardial infarction, PCI: Percutaneous coronary intervention, hs: high sensitivity, cTn: cardiac troponin, ACS acute coronary syndrome. AMI: acute myocardial injury. MI: myocardial infarction

A ‘myocardial Infarction’ (MI) is necrosis of myocardial tissue secondary to inadequate myocardial arterial blood supply. In clinical practice, we usually lack definitive evidence of necrosis (myocardial histology) and, on occasion, we also have no definitive evidence of loss of blood supply (angiography etc). Hence, the need for accepted clinical criteria to establish the diagnosis of MI.

In 2018, a collaboration between the major global cardiology societies altered the clinical definition of myocardial infarction and introduced the concept of ‘acute myocardial injury’. Results from high sensitivity cardiac troponin assays (hs-cTn) play a key role in the new definitions.

hs-cTn assays are laboratory-based assays capable of measuring the minute levels of cardiac troponin isoforms (T or I) present in the plasma of normal individuals. These assays also demonstrate a high degree of reproducibility for measurements at the 99th percentile URL.

Acute Myocardial Injury: Definition

‘Detection of an elevated plasma cardiac troponin level above the 99th percentile URL’. The injury is considered ‘acute’ if there is evidence of a dynamic change in plasma troponin concentration during the presentation. The dynamic change in troponin level may be either a) a rise in troponin concentration to a level above the 99th percentile URL or b) a significant fall in troponin from an abnormally high concentration detected at presentation*.

*A fall of >20% from the high level (i.e. above the 99th percentile) is considered a significant change.

The 99th percentile URL: 99% of individuals in a normal 'reference' population will have a plasma concentration of cardiac troponin below this value.

Myocardial Infarction: The 4th Universal Definition

‘The term myocardial infarction should be used when there is an acute myocardial injury with clinical evidence of acute myocardial ischemia.’

Evidence of acute myocardial ischemia in type I myocardial infarction:

1. Symptoms of myocardial ischemia
2. New ischemic ECG changes
3. Development of pathological Q waves on the ECG
4. Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality in a pattern consistent with an ischemic etiology
5. Identification of a coronary thrombus by angiography or at autopsy

Note that the definition mandates a dynamic change in cTn levels, that is rise or fall in cTn concentration to or from a value above the 99th percentile URL in order to diagnose acute myocardial infarction.

Based on the underlying cause of inadequate myocardial blood supply, five types of acute MI are recognised.

Type I: Type I MI is caused by atherothrombotic disease. In most of these cases, hypoperfusion is due to acute thrombotic occlusion of a coronary artery secondary to disruption (erosion or rupture) of an atheromatous plaque.

Type II: In type II MI, mismatch between myocardial perfusion and myocardial tissue need is not caused by coronary artery atherothrombosis. Blood flow through the coronary arteries may be compromised by many conditions unrelated to atherothrombosis, for example, coronary artery spasm or dissection. Equally, myocardial perfusion may be reduced as part of a global reduction in cardiac output from whatever cause.

Type III: In these cases, the patient dies in the acute phase of the infarct before a rise in serum biomarkers of myocardial cell death becomes detectable. Diagnosis is established at autopsy.

[Criteria 1 to 4 above are taken to indicate the presence of ischemia in a Type II or Type III MI]

Type IV: MI related to PCI.

Type V: MI related to coronary artery bypassing grafting.

Thygesen K et al. Fourth Universal Definition of Myocardial Infarction (2018). Circulation 2018; 138: e618 November 13, 2018.

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