A new definition of Myocardial Infarction: What’s Changed

The Fifth Universal Definition of Myocardial Infarction makes a major change to the way we classify MI.

• Primary MI
• Secondary MI
• Procedure-related MI

But the most important message is not the new terminology. It is that an elevated troponin does not diagnose myocardial infarction.

• Troponin elevation tells us that myocardial injury has occurred.
• Arise or fall in troponin, this indicates acute myocardial injury.
• To diagnose myocardial infarction, there must also be evidence that the injury is caused by myocardial ischaemia.

That distinction is particularly important in Emergency Medicine, where elevated troponin is commonly seen in patients with sepsis, pulmonary embolism, heart failure, anaemia, tachyarrhythmias and other critical illnesses. A patient with sepsis and a dynamic troponin rise does not automatically have a “Type 2 MI”. They may simply have acute myocardial injury.

Primary vs Secondary MI

Primary MI occurs when the problem originates in the coronary circulation. This includes not only plaque rupture and coronary thrombosis, but also:

• spontaneous coronary artery dissection
• coronary embolism
• coronary vasospasm.

A useful way to think about it is:

Secondary MI occurs when another acute illness causes a significant imbalance between myocardial oxygen supply and demand, resulting in genuine myocardial ischaemia.

Examples include:

• severe anaemia,
• hypoxaemia,
• shock,
• hypotension and
• sustained tachyarrhythmia.

The key change is that a physiological stressor plus an elevated troponin is no longer enough. There should be convincing evidence that myocardial ischaemia has actually occurred.

Procedure-related MI has also changed

The old PCI- and CABG-related MI categories have been replaced by Procedure-related MI.

Importantly, the traditional troponin thresholds of >5 times the 99th percentile after PCI and >10 times after CABG are no longer used as defining criteria.

A large post-procedural troponin rise alone does not establish MI.

The Bottom Line

The Fifth Universal Definition asks us to stop asking:

“How high is the troponin?”

and start asking:

“Why is the myocardium injured, is the injury truly ischaemic, and what caused it?”

That distinction between myocardial injury and myocardial infarction may be the most important clinical message in the entire document.

Read the full Literature Rounds review on EM Mastery for the detailed changes, practical examples and implications for Emergency Medicine.