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The Society for Cardiovascular Angiography and Interventions () has released a new definition for myocardial infarction (MI) following coronary revascularization aimed at identifying only those events likely to be related to poorer patient outcomes.

In the new criteria -- published as an expert consensus document in Catheterization and Cardiovascular Interventions and the Journal of the American College of Cardiology -- creatine kinase-myocardial band (CK-MB) is the preferred cardiac biomarker over troponin, and much greater elevations are required to define a clinically relevant MI compared with the universal definition of MI proposed in 2007 and revised in 2012.

Also, the new definition uses the same biomarker elevation thresholds to identify MIs following both percutaneous coronary intervention (PCI) and coronary artery bypass grafting (CABG), whereas the universal definition has different thresholds for events following the two procedures.

"What we've really tried to emphasize in this classification scheme is the primary link between biomarker elevations and prognosis," according to , of and the in New York City, one of the authors of the document.

"In the universal definition of MI, they even acknowledged that their criteria were arbitrary," Stone said in an interview. "We've tried to reduce the arbitrariness of the cutoff values that we selected so that the researcher, academician, clinician, hospital administrator, etc., can be confident that these levels that we're recommending are the ones that are associated with a worse prognosis for patients suffering periprocedural complications."

The Change

The existing universal definition for MI defines events following PCI according to an increase in cardiac troponin to greater than five times the 99th percentile upper reference limit (URL) within 48 hours when baseline levels are normal, with confirmation by electrocardiogram (ECG), imaging, or symptoms.

For CABG-related MI, the increase must be more than 10 times the 99th percentile URL within 48 hours when baseline levels are normal, with confirmation by ECG, angiography, or imaging.

But, Stone and colleagues wrote, the relationship between that degree of troponin elevation after a revascularization procedure and prognosis is not as strong as the association between a CK-MB elevation and patient outcomes.

Using a small elevation in troponin to define a post-procedure MI could find myocardial necrosis that is unlikely to be associated with poor clinical outcomes, which could have far-reaching implications, they wrote.

"Widespread adoption of an MI definition not clearly linked to subsequent adverse events such as mortality or heart failure may have serious consequences for the appropriate assessment of devices and therapies, may affect clinical care pathways, and may result in misinterpretation of physician competence," they wrote.

To address that issue, the expert panel convened by SCAI sought to define clinically relevant MI after PCI or CABG.

A clinically relevant MI is defined in the new document based on an increase of at least 10 times the upper limit of normal in the level of CK-MB within 48 hours after a revascularization procedure when baseline levels are normal.

When the CK-MB level is not available, then an increase in troponin I or T of at least 70 times the upper limit of normal can be used to define a clinically relevant MI, according to the authors.

However, if an ECG shows new pathologic Q-waves in at least two contiguous leads or a new persistent left bundle branch block, then the thresholds can be lowered to at least five times and at least 35 times the upper limit of normal for CK-MB and troponin, respectively.

Further guidance is provided for identifying clinically relevant post-procedure MIs when the cardiac biomarker levels are elevated at baseline.

Dueling Definitions

Co-chairman of the Task Force for the Universal Definition of Myocardial Infarction, , of in Auckland, New Zealand, noted some limitations of the new definition, including the lack of a requirement for ischemic symptoms.

"Ischemic symptoms have always been a basic tenet of the diagnosis of MI, and it should be no different for a [PCI-related] MI," he wrote in an accompanying editorial.

In addition, with the use of such large elevations in biomarker levels in the new definition, "there will be very few PCI-related events identified, and an opportunity to improve patient outcomes may be lost," he wrote.

Troponin should remain the preferred biomarker over CK-MB, White argued, pointing to variability in and analytical issues with CK-MB assays, the need for sex-specific cutoffs for CK-MB levels, the need for higher thresholds of CK-MB to determine abnormalities because all individuals have circulating levels of the biomarker, and the reduced sensitivity and specificity of CK-MB.

Also, he said, CK-MB is becoming increasingly unavailable at medical centers.

"With CK-MB becoming obsolete, troponin will become the gold standard, and CK-MB will no longer have a role in defining PCI injury and infarction in clinical practice," White wrote.

Stone admitted that troponin ultimately might be preferable to CK-MB because of its greater specificity, although the evidence does not yet support it.

"I think there's a general desirability to move to troponins, although when you look at the data that's out there it's much stronger correlating CK-MB elevations to subsequent prognosis," he said. "I think a lot of the troponin elevations are just noise or troponins are just too sensitive."

Room for Both?

White noted in his editorial that "the rationale for the SCAI definition has been well articulated by its authors and may be appropriate in an individual trial, but it should not supplant the universal definition of MI," he wrote.

When asked whether the new definition would replace the universal definition, Stone said there is a place for both sets of criteria.

"We would propose the clinically relevant definition be the one that is used to make most substantial decisions right now, [such as] trade-offs between efficacy and safety for new drugs and devices, in judging hospital systems and physicians, etc.," he said. "But I do think there's value in both, and they will both continue to evolve over time as new data becomes evident."

From the American Heart Association:

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