What is a Myocardial Infarction?

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MI image

Blockage of a coronary artery results in ischemia of the myocardium supplied by that vessel. In practice, this usually arises due to thrombosis of an atheromatous plaque in a coronary artery (yellow above). The clot formed in the lumen of the artery deprives the area of myocardium it supplies of oxygenated (arterial) blood. Ischemia of the affected region of myocardium  progresses to necrosis over a period of hours. In theory, therefore, myocardial infarction (MI) is a histological diagnosis requiring the demonstration of necrotic myocardium under the microscope. In reality, we have no way of obtaining a sample of myocardium to establish the presence of myocardial necrosis. In clinical practice we, therefore, rely on a combination of imprecise indicators of the presence of myocardial necrosis to diagnose MI.

In 2012, a new universal diagnosis of MI was accepted by the European and North American cardiology societies. The 'third universal definition of myocardial infarction':

A myocardial infarction is assumed to have taken place or to be taking place if there is evidence of A) myocardial necrosis in a clinical setting consistent with B) acute myocardial ischemia.

Evidence of A) (myocardial necrosis) is a rise to (or fall from) a cTn (cardiac troponin) level above the 99th centile. (ie a dynamic change in troponin levels with at least one 'high' value)

Evidence of B)  acute ischemia is at least one of the following

  1. Symptoms of ischemia
  2. New or presumed significant ST segment-T wave changes or new LBBB
  3. Development of pathological Q waves on the ECG
  4. Imaging evidence of new loss of viable myocardium or new regional wall abnormality (in most cases this refers to echocardiography findings)
  5. Identification of an intracoronary thrombus at angiography.

So, in clinical practice, MI is defined as A) plus one or more of B)

Note that there may be a retrospective element to this diagnosis. It may take time for the serum troponin level to rise. We may need to act before this and, therefore, should be able to recognise the possibility of evolving MI based purely on B) 1, 2 and 3. Proceeding to B) 5 with re-establishment of vessel patency may then be indicated and life-saving.

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