What's Hot in Clots Special Edition: The Fifth Universal Definition of MI

What Changed, Why It Matters, and Where Questions Remain

You might think this is just a long paper about definitions, i.e., “why do I care”? 

I can tell you that there are few documents in 2026 that I’ve found more relevant in cardiovascular and thrombosis practice, especially if you conduct clinical or research work related to myocardial infarction (MI).

Then & Now

A bit of history first from the dinosaur, yours truly…

Back in the old days, acute coronary syndromes (ACS) were defined using WHO criteria: an event qualified if two of the following three were present: classic symptoms, ECG changes, and supporting cardiac biomarkers. In subsequent years, the ACC and ESC collaborated across the Atlantic to further develop the definitions of MI, resulting in what became known as the “universal definition.” The AHA and World Heart Federation later became involved to provide a broader global perspective.

We then had iterative revisions. The penultimate version, the Fourth Universal Definition, included five types of MI:

• Type 1 – commonly related to coronary thrombosis in the setting of atherosclerosis

  
  

• Type 2 – often occurring when another stressor, such as anemia, sepsis, or tachycardia, created myocardial oxygen supply-demand mismatch

  
  

• Type 3 – sudden cardiac death with symptoms or signs suggestive of MI

  
  

• Type 4 – related to percutaneous coronary intervention (PCI), including several subtypes

  
  

• Type 5 – related to coronary artery bypass grafting (CABG). 

There were many good things about this framework, but it also had limitations. Many clinicians found it complex, and it created challenges in research studies.

So, in this context, I find the Fifth Universal Definition of Myocardial Infarction very timely and important. Here are a few highlights about the new document:

• Let’s celebrate lumping! Instead of the old five-tiered system, we now only have three groups: primary, secondary, and procedure-related myocardial infarctions.

• Primary MI is not just about standard plaque ruptures. The definition focuses on a primary coronary event. In other words, it could be from coronary thrombosis, spontaneous coronary artery dissection (SCAD), coronary spasms, or embolic events.

• Secondary MI occurs when another acute issue results in supply-demand mismatch in the backdrop of known epicardial coronary disease. Examples include anemia, hypotension, or severe hypertension.

Oh, that will be a relief for cardiology consult services. Or will it? To discourage clinicians from labeling every ICU patient with an elevated troponin as having an MI, a secondary MI now requires objective evidence. You actually need imaging indicating a new regional wall motion abnormality or confirmed obstructive epicardial coronary artery disease.

• The third group is related to cardiac procedures. If you’re not a primary operator for those, forget about this one!

• Sex-specific troponin cutoffs are mandatory. Using different 99th percentile upper reference limits for male and female patients is no longer optional. 

• A clear line is drawn between a MI and everyday wear-and-tear. The document provides helpful guidance on acute and chronic myocardial injury that does not fulfill the criteria for MI. If a patient has elevated but completely stable troponin values—which we see frequently in patients with chronic kidney disease or heart failure—think chronic myocardial injury, not active MI.

  
  

• I also like the forward-looking effort to align MI designations with ICD-11 codes. That should help us do better with future surveillance studies and EHR-based analyses.

Commentary

I hold great respect for the author group. This is an enormous document that must have taken hundreds of hours to prepare, and I find many elements incredibly helpful. That said, let me share a couple of areas that could potentially be improved in the future:

• Supply/demand mismatch only for (the old and also new) type II events? Really? The disconnect between supply and demand of oxygen (and nutrients) is a common denominator. Even for the old and also new forms of type I MI, there’s still a mismatch of supply and demand. In some of them, there’s an acute coronary reason and in some, there’s not. The description can improve further.

• Treatment decisions remain another challenge. Understanding the mechanism is helpful, but it may be even more useful to connect the mechanism and pathophysiology more directly with management. For example, we need aggressive antithrombotic therapy (and oftentimes PCI) for atherothrombotic primary MI and procedure-related MI related to stent thrombosis. In contrast, a coronary thrombotic event and SCAD may both fall under primary MI but they require very different management strategies.

All in all, I’m quite excited about this new definition of MI. What do you think?

Read the Fifth Universal Definition of Myocardial Infarction here.